tapr psr #144 winter 2020 president’s corner · for direction and swr, activity is infrequent for...

TAPR PSR #144 Winter 2020

TAPR is a community that provides leadership and resources to radio amateurs for the purpose of advancing the radio art.

President’s CornerBy Steve Bible, N7HPR

HamSCI, the Ham Radio Science Citizen Investigation, is a platform for the publicity and promotion of projects that are consistent with the following objectives:• Advancescientificresearchandunderstandingthroughamateurradioactivities.• Encouragethedevelopmentofnewtechnologiestosupportthisresearch.• Provideeducationalopportunitiesfortheamateurcommunityandthegeneralpublic.HamSCI serves as a means for fostering collaborations between professional researchers and

amateurradiooperators.Itassistsindevelopingandmaintainingstandardsandagreementsbetweenallpeopleandorganizationsinvolved.HamSCIisnotanoperationsorfundingprogram,norisitasupervisoryorganization.HamSCIwasstartedbyham-scientists,whostudyupperatmosphericandspacephysics.These

scientists recognized that projects such as the Reverse Beacon Network, WSPRNet, PSKReporter, DX Cluster, ClubLog, and more are generating big data sets that could provide useful observations oftheEarth’sionosphereandrelatedsystems.Becauseofthis,HamSCI’sinitialfocusisonthesefieldsofresearch.SoundslikesomethingTAPRshouldbeinvolvedwith.Well,weare.TAPRhasahandinthePersonalSpaceWeatherStation(PSWS)projectthatultimatelyaimsto

create a small, multi-instrument system that can make ground-based measurements of the space environment.Theobservationsfromthisprojectwillnotonlybeusefultotheownerofthesystem,butalsoaggregatedintoacentraldatabaseforspacescienceandspaceweatherresearchpurporses.Initialworkfocusesonthedevelopmentofascientific-gradeHFradioreceiver,aswellasthe

necessarysoftwareandnetworkinfrastructure.TAPR’sisworkingonvariousaspectsoftheproject:• ScottyCowling,WA2DFI–TangerineSDRhardwaredesignandcheerleading.• JohnAckermann,N8UR–GPSandtiming• DaveWitten,KD0EAG–Magnetometerboard

President’s Corner 01NOTICE 02TAPR.ORG – The Next Generation 02TAPR Members to Attend 2020 HamSCI Workshop 03On the Air with YT7MPB 04Donate to TAPR 04DCC Wrap-Up 05AX.25 + FEC = FX.25 08TAPR Wear Available 12Automatic Tuner for Small Magnetic Loop Antennas 13Write Here! 29On the Net 29The Fine Print 30Our Membership App 31

TAPR PSR #144 Winter 2020PAGE 2


TAPR.ORG – The Next GenerationBy John Ackermann, N8UR

We’re pleased to introduce you to the new TAPR.ORGwebsiteandon-linestore.We’vecompletely rebuilt the site using a modern content management system and e-commerce platform, hosted at a well-respected provider thatprovidesautomaticsecurityupdates.

We’ll be updating the front page with new posts regularly, so plan to stop by even if you’re not shopping!There’salotmorecontenttocomeaswe

continue to migrate content from the old site to the new, but for now the on-line store is fully functionalandloadedwithallcurrentTAPRproducts.Wehaveremovedsomeveryoldproductsfrom the store; if you need one of those, please contact us and we’ll let you know if we still have it available.We’realsointegratingourmembershipmanagementintothenewsystem.Allnewmemberships

and renewals are available as products within the store, and there will be special members-only featuresonthesite.Finally,we’llbechangingthewaythePacket Status Register (PSR)isdistributedandwillbe

discontinuingthecurrentmailinglists.BeforethenextPSR is published, we’ll send a note to those listsexplainingthechanges,andhowtoaccessPSRgoingforward.Ofcourse,theremaybeafewglitchesaswegetthenewsiterolling.Ifyouhaveanyproblems,

or suggestions, please let us know!###

• TomMcDermott,NE5G–RFboard,designdocumentation and proofreading• DaveLarsen,KV0S–TangerineSDR

webmastering,weeklyTeamspeaksessionmoderating and programming• BillEngelke,AB4EJ–Buildingtheweb-

based agitator• NathanielFrissell,W2NAF–Scienceteam

and HamSCI meeting leaderTAPRpeepswillbeattendingtheHanSCI

WorkshopattheUniversityofScranton,March20-21,whichyoucanreadaboutonpage3.

Hope you can join us there!73, SteveBible,N7HPR,PresidentTAPR

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NOTICEMembership-onlyaccessofPSR will begin

withthenextissueofthenewsletter.Inotherwords,onlyTAPRmemberswillbeabletoviewthenewsletterwhenitispublished.(Youcanfinda membership application on page 31 or visit our membership webpage.)

Past issues of PSR will still be available on TAPR’swebsite and membership-only issues willappeartherewhentheyarenolongercurrent.

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https://tapr.org/?page_id=5261

https://tapr.org/?page_id=2587



TAPR Members to Attend 2020 HamSCI WorkshopMarch 20-21, 2020, University of Scranton

By Nathaniel A. Frissell, W2NAF

TAPRmemberswillattendthe3rdAnnualHam Radio Science Citizen Investigation (HamSCI)workshopMarch20-21,2020attheUniversity of Scranton in Scranton, Pennsylvania as part of a team meeting for the HamSCI PersonalSpaceWeatherStationproject.HamSCIis a collective of hams and professional space science researchers working together for the mutualbenefitofthehamradiohobbyandtheadvancement of the understanding of ionospheric andspacescience.StartingJanuary1,2020,TAPRformally

became a participant in the National Science Foundation(NSF)-fundedprojecttodevelopaPersonalSpaceWeatherStation(PSWS)incollaborationwithHamSCIscientists.TAPR’smain role is the development of a new software definedradio(SDR)receiverwithspecificationsthat meet the needs of scientists to remotely sensetheionosphere.ThiscollaborationleddirectlytoTAPR’slatestproject,theTangerineSDR.Additionally,TAPRmembersare contributing to the development of PSWS ground magnetometer, PSWS software and databasesandothercriticalPSWSsystems.

At the HamSCI workshop, all aspects of

thePSWSwillbediscussed.LeadPrincipalInvestigator(PI)fromtheUniversityofScranton,Dr.NathanielFrissell,W2NAF,willpresenttheoverarching PSWS goals and objectives, as well asprovideanoverallstatusreportontheproject.TangerineSDRchiefengineer,ScottyCowling,WA2DFI,willpresentthelatestdesignsand

statusfortheTangerineSDRdataengine(DE).TomMcDermott,N5EG,willdiscusshisdesignandprogressfortheRFboardmodules,andJohnAckermann, N8UR, will present on the precision timingmodules.DavidWitten,KD0EAG,hasbeen leading the development of the ground magnetometer(GMAG)hardware,andplansto



provideandupdateandhardwaredemonstration,alongwithGMAGPIfromNewJerseyInstituteofTechnologyDr.HyominKim,KC2MCR,andbeta-testerFrankieBonte,KE8GTT.BillEngelke,AB4EJ,oftheUniversity of Alabama is a lead of the control software and database team, andwillprovideupdatesonthesesystems.Finally,theCaseWesternReserveUniversityteamledbyKristinaCollins,KD8OXT,Dr.DavidKazdan,AD8Y,andJohnGibbons,N8OBJwilldiscussthelowcostversionofthePSWS.AndDr.PhilErickson,W1PJE,ofMITHaystackObservatorywillpresentmethodsofionosphericremotesensingusingPSWScapabilities,includingFT8basedtechniquesandsignalsreceivedfromWWV.

In addition to serving as a team meeting for the PSWS project, the 2020 HamSCI workshop also seeks to strengthen ties between the HamSCI and auroralsciencecommunitieswiththeoverarchingmeetingtheme“TheAuroralConnection.”

Saturday morning will feature tutorial talks on optical aurora and citizen science, natural radio emissions from the aurora and using the auroral modepropagationasahamradiooperator.ThesetalkswillbegivenbyNASAScientistandAurorasaurusleadandfounder,Dr.ElizabethMacDonald,DartmouthradiophysicistDr.JimLaBelle,andhamradiooperatorDavidHallidy,K2DH,respectively.TheFridaynightbanquetwillfeature,TimDuffy,K3LR,theworld-

famouscontesterandCOOandgeneralmanagerofDXEngineering.Contributed talks, demos, and posters will also be given by members of the hamradioandsciencecommunities,aswellasthegeneralpublic.Tolearnmore about the 2020 HamSCI workshop, submit an abstract, or register for themeeting,visithttp://hamsci.org/hamsci2020.

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On the Air with YT7MPB

Miroslav‘Misko’Skoric,YT7MPB,hasrecentlystartedlisteningforpactorcontactsat20m,40m,and80mbands.IfanybodycanhearYT7MPB-8node(basedonLinBPQsoftware)callingCQPACTORorhisforwardingpartners,newconnectsarewelcomed.Aswell,possibleforwardingoverpactor1-4and/orrobustpacket.YT7MPB-8isusuallyactivefrom1330UTCto1730UTC.DuetoheavyQRNinthelocalareaandnot-so-perfectantennas,withoutanyautomatictuningfordirectionandSWR,activityisinfrequentfornow.FromFebruary3to20,MiskoYT7MPBwillvisitIndia.Ifareciprocallicenseis

grantedontime,possibleVHFpacket,APRSexperimentswillbeperformed.###

Donate to TAPRTAPRisnowparticipatingintheAmazonSmileprogram!When you shop using the AmazonSmile program, Amazon makes

adonationtoTAPRequalto0.5%ofthepriceofyoureligibleAmazonSmilepurchases.AmazonSmileisthesameAmazonyouknow.Sameproducts,same

prices,sameservice.BookmarktheTAPRAmazonSmileProgramlink:

https://smile.amazon.com/ch/86-0455870Thatlinktakesyoutoaspecialloginportalwhereyouenteryour

normal Amazon credentials and get redirected at the same Amazon home pageexcepttherewillnowbeanoticethatyouaresupportingTAPR.OtherwaystodonatetoTAPR:

http://www.tapr.org/tapr_donate.html###



DCC Wrap-UpBy Stana Horzepa, WA1LOU

The2019ARRL-TAPRDigitalCommunicationsConference(DCC)ishistory,butyoucanreadthepublishedDCCpapers(listedbelow)ontheTAPRwebsiteundertheLibrary>DigitalCommunicationsConferencePapersmenu.

DCC Papers

PSAT2DTMFExperimentAPRStt–Touchtone®DigitalCommunicationsUsinganyRadioforDataExchangebyBobBruninga,WB4APRExtendingD-STARwithCodec2byAntonyChazapisIPV6forAmateurRadiobyDanielEstévez,EA4GPZ/MØHXMSynchronizationinFT8byMikeHasselbeck,WB2FKOWSPRinaneducationalProjectbyAnthonyLeCren,F4GOHPortableAudioFrequency-ShiftKeyingSensorsusingaHamshield

minibyNolanPearce,KE8JCTbyStephenS.Hamilton,KJ5HY,andKateJDuncan,KB2ZOOAnFPGALearningExperience:SPIInterfacetoMax10FPGAby

GregoryRaven,KF5NModulation–DemodulationSoftwareRadiobyAlexSchwarz,

VE7DXWHowtoKillPacket-Radio&APRS?CometoSerbia!(Part2)by

Miroslav“Misko”Skoric,YT7MPBGPSWatchTechnologybyDarrylSmith,VK2TDS

DCC Prize Winners

As usual, a wide array of prizes were awarded to DCC attendees at

theSaturdayeveningbanquet.Theprizeswenttothefollowingluckywinners:KenwoodTH-D74:Kathy,WB0UDNIcomID51APlus2:MaxYarterMFJ-1270X-TNC-XPacketController:Brian,N8WRLMFJ-1270DGX-Digi:Brian,N8WRLRowetel.comSM1000FreeDVAdaptor:Al,WA1VHDARRLgiftcertificates:George,K9TRV,Jason,KC5HWB,John,

WA8TON,Jonathon,KC3EFX,Mike,KG4DSG,Richard,K3PLRDXEngineeringgiftcertificate:John,W9DDDGreatScottGadgetsGreatFETOne:Greg,KF5NHamRadioDeluxesoftwareCD:Bob,AC8YVHROgiftcertificates:Corey,KB9JHU,Matt,WS8UTAPRgiftcertificates:Brannon,KF5WVW,Dan,AD0CQ,Garrett,

KD2SAK,Mark,N2OJO,Roland,W9HPXTAPRGPSKit(1995):Chris,KF7TUPWestMountainRadiocertificate:Dave,KD0EAG

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WA2DFI photo



DCC Photos by K9TRV & WA2DFITop: N7HPR, K9TRV, N8WRL, WB0UDN, N7HPR, K0PFXMiddle: WB9QZB, K3PLR, AC8ESBottom: KA9Q, KB9JHU, AC8YV, W9DDD, WA2DFI, KF7TUP, Max Yarter, N7HPR



Digital Communications Conference

Make your reservations now for three days of learning and enjoyment at the Charlotte Airport Hotel. The Digital Communications Conference schedule includes technical and introductory forums, demonstrations, a Saturday evening banquet and an in-depth Sunday seminar. This conference is for everyone with an interest in digital communications— beginner to expert.

September 11-13 Charlotte, North Carolina

2020 ARRL/TAPR

Call Tucson Amateur Packet Radio at: 972-413-8277, or go online to www.tapr.org/dcc

Charlotte Airport Hotel

Untitled-1 1 01/09/2020 12:04:10 PM



AX.25 + FEC = FX.25By John Langner, WB2OSZ

Whatcanyoudoifyourradiosignalisn’tquitestrongenoughtogetthroughreliably?Movetohigherground?Getabetterantenna?Morepower? Send data very very slowly with narrow bandwidth?Sometimesthosearenotoptions.Anotherwaytoimprove

communication reliability is to add redundant information so the messagewillstillgetthroughevenifpartsofitaremissing.

In the data communications world, adding additional information, sothereceivingendcanfixuperrors,isknownasForwardErrorCorrection(FEC).Thisisverycommon.Itisusedinallsortsofradiocommunication,QRcodes,DSL,anddiskdrives.ItisevenusedwithCD’sandDVD’stoworkarounderrorscausedbyscratches.

AX.25

TheAX.25frame,usedbyAPRSandAmateurPacketRadio,isnotverytolerantoflowqualityradiolinks.A16bit“FrameCheckSequence”(FCS)iscomputedfromtheearlierparts.ThereceivingendrecalculatestheFCSanddiscardstheframeifthereisamismatch.

Allittakesisasinglebadbittoruintheentireframe.Traditionalconnected mode packet has automatic acknowledgment of received framesandrequeststoresendanythataremissing.APRSdoesnotusethisfeaturesotheinformationisjustgoneandnobodyknows.

FX.25

Backaround2006,theStensatGroupcameupwithacleverwaytoaddForwardErrorCorrectiontoAX.25whilemaintainingcomplete

backwardcompatibilitywithexistingequipment.Atransmissionstartswithcarefullychosen64bitpatternstoidentifytheformatofthedatatofollow.DependingontheCorrelationTag,therearecertainnumbersofbytes

forthe“data”and“check”parts.TheReed-Solomonalgorithmisusedtofixanyerrors.Thenumberofbytesthatcanberepairedisonehalfthenumberofcheckbytes.



ThesendercanchooseanappropriatecorrelationtagdependingonamountofspacerequiredfortheAX.25frameandthedesiredstrengthoferrorcorrection.Thereceiverneedstobeabletoprocessanyofthem.FX.25hasbeenusedforseveralAmateurRadioSatellitesandallofthe

receptionsoftwarehasbeenorientedtowardthatspecializedapplication.NowthatFX.25hasbeenintegratedintoageneralpurposeTNC,manynewpossibilitiesarenowopen.Herearetheresultsofsomeexperimentstoseehowwellitactually

works.

Bit Error Rate

“BitErrorRate”(BER)isacommonmeasurementofhowwelladigitalcommunicationchannelperforms.Itissimplydefinedasthenumberofincorrectbitsdividedbythetotalnumberofbits.Errorscan be caused by many factors such as natural random noise, man-madeinterference,multi-path,fading,lowqualitymodems,incorrectadjustments(e.g.transmitaudiolevel),anddistortionaddedbytheradios(e.g.limitedbandwidth,pre-emphasis&de-emphasis).Itiseasytocomputetheimpactofdifferentbiterrorrates.Ifthe

probability of any bit getting corrupted is B then the probability of it gettingthroughcorrectlyis(1-B).TheprobabilityofNbitsgettingthoughcorrectlyis(1-B)N.Let’scomputetheprobabilityofan80byte(640bit)framegettingthroughwithdifferenterrorrates.

Nowwewilltryitexperimentally.GenerateathousandAPRSpacketsandmeasurehowmanyarereceived.Thereceivingendhasanoptiontointroduce a given Bit Error Rate, after the demodulator, using a random numbergenerator.

Theexperimentalresultsarequitesimilartothetheoreticalexpectation.Lossofonly1bitin1000(i.e.BER=10-3)causesalmosthalfoftheframestobelost.



FX.25 Improvement for APRS

Thesametestwillbeperformedwith1000framesagain,butthistimeusingFX.25.Thesenderhasachoiceof16,32,or64checkbytes.Onceagain, we introduce a controlled Bit Error Rate at the receiving end and countthenumberofframesreceivedcorrectly.

FX.25keepsgoingstronglongafterregularAX.25iscompletelyuseless.

FX.25 Improvement for traditional Packet Radio

ConnectedmodeusesthesameAX.25framesbutinadifferentway.Rather than a broadcast to everyone, it is used as a link between two specificstations.ThesendingTNCassignssequencenumberstotheinformationframes.Thereceivingendacknowledgeswhathasbeenreceivedandasksforfill-insofmissingpieces.



ConnectedmodeAX.25packetradioisoftenused,withapplicationslikeOutpostPMandWinLinkExpress,foremergencycommunications.Temporaryantennasandlossofinfrastructuremakereliablecommunicationsmoredifficult.Anythingthatcanextendrangeandimprovereliabilitycouldhaveasignificantimpactonpeople’slives.Thistestuses100frameseachwith128informationbytes.Thetotalamountofapplicationdatatransferred,dividedbytheelapsed

time will give us an effective throughput rate, measured in bytes per secondhere.Astheerrorrateincreases,moreretriesarerequired,andthethroughputdecreases.Atsomepoint,itjustgivesupbeforereachingtheend.Wegivethisascoreof0.Undergoodconditions,FX.25isslowerbecauseithasmoreoverhead.

Whenconditionsdeteriorate,FX.25showlittleornodegradationlongafterAX.25iscompletelyuseless.OnecouldimagineasystemwheretheFX.25transmissionisenabled

ordisabledbasedontheerrorratedetectedonthereceivingend.Itwouldnothavetobesymmetrical.MaximumthroughputmightbeachievedbyalowpowerstationtransmittingFX.25andahighpowerstationsendingregularAX.25.

FX.25 Quick Start Guide

Ifyougotthisfar,youareprobablywondering,“HowcanIexperimentwiththis?”The“DireWolf”softwareTNCnowhasFX.25builtinsoitiseasyto

usewithallyourfavoriteAPRSandPacketRadioapplications.Someofyoumightnotbefamiliarwiththissoftware.DireWolfisamodernsoftwarereplacementfortheold1980’sstyle

TNCthatusedspecialhardware.Itusesthecomputers“soundcard”foraudioinandout.Themodemisimplementedwithdigitalsignalprocessing(DSP)software.Withoutanyadditionalsoftware,itcanperformasanAPRSGPS

Tracker,Digipeater,InternetGateway(IGate),andanAPRSttgateway.ItcanalsobeusedasavirtualTNCforotherapplications.ItisfreeopensourcethatrunsonWindows,Linux(including

RaspberryPi),MacOSX,andBSDunix.It’snowveryeasytogetstartedwithFX.25:

Reception

FX.25receptionisalwaysenabledsoyoudon’tneedtodoanythingspecial.WatchingforthespecialcorrelationtagsequencesaddsaninsignificantamountofadditionaloverheadwhenonlyregularAX.25isinuse.

Transmission

Fortransmit,simplyspecify–X(uppercase)onthecommandlinefollowedby16,32,or64,forthedesirednumberofcheckbytes.Thespecificcorrelationtagwillbepickedautomaticallybasedontheframesize.Currentlythissingleoptionappliestoallradiochannels.

At the time this is being written, the new functionality is available only inthedevelopment(“dev”)branch.Nottoofarinthefuture,itshouldbeinrelease1.6.That’saboutallyouneedtoknowtogetstarted.

Summary

FX.25hasbeenusedformorethanadecadewithsomeAmateurRadio



satellites.Receivingimplementationswereorientedtowardthatveryspecializedapplication.NowthatFX.25iseasytouse,integratedintoageneralpurposeTNC,manyotherAX.25basedapplicationscangainthebenefitswhileretainingcompletecompatibilitywitholderequipment.

Recommended Reading

Wikipedia

https://en.wikipedia.org/wiki/FX.25_Forward_Error_Correction

FX.25 FEC extension to AX.25

http://www.stensat.org/docs/FX-25_01_06.pdf

Dire Wolf documentation

https://github.com/wb2osz/direwolf/tree/dev/doc

Most relevant related documents:

• AX.25+FEC=FX.25(similartothisbutlonger)• AX.25Throughput:Whyis9600bpsPacketRadioonlytwiceasfast

as 1200?• A Better APRS Packet Demodulator, part 1, 1200 baud

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PersonalizedLand’sEndclothingwiththeTAPRlogoandyournameandcallsignarenowavailablefromtheTAPRStoreathttp://business.landsend.com/store/tapr/SelectfromtheMen’sorWomen’scatalog.(Tomakeshoppingeasier,

thereare“TAPRRecommendedShirts”intheMen’scatalogincludingtwostylesofpoloshirts,eachavailablewithorwithoutpockets.)Thelogoisavailableinthreecolors--red,blue,andwhite.Thename/

callsignmonogramthreadwillmatchthelogocolor.(Werecommendthatyouusethewhitelogowithdarkcoloredshirts.)Pricesareveryreasonable,forexample,afteraddingthelogoand

monogram,ameshpocketshirtis$39.85plusshippingandsalestaxwhereapplicable.Processingtimeis5-7days.

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TAPR WearAvailable

https://en.wikipedia.org/wiki/FX.25_Forward_Error_Correction

http://www.stensat.org/docs/FX-25_01_06.pdf

https://github.com/wb2osz/direwolf/tree/dev/doc

http://business.landsend.com/store/tapr



Automatic Tuner for Small Magnetic Loop Antennas©

By Andrew Cornwall, VE1COR / KB1RSE

Accordingtothe2010UnitedStatesCensus80.7%ofthepopulationlivesin‘urban’regions.Urbanresidentsaccountforover90%ofthepopulationinsevenstates.Atrendtowardurbanizationhashappenedthroughoutthecountry.Thereisconsiderableleewayinthedefinitionofanurbanarea,yettheimplicationisthatanincreasingnumberofU.S.hamsliveinapartmentsorinhouseswithlotstoosmallforafullsizeHFantenna.ManyU.S.hamswithsufficientlandresideinneighbourhoodssubjecttohomeownersassociationrulesthatdonotallowfullsizeHFantennas1.Increasingurbanizationisalsooccurringinothercountries.AccordingtoUnitedNationsdatafor2018,55%oftheworld’spopulation now lives in urban areas2.Theimplicationforhamsisthereisagrowingneedforasmallfootprint,compact,capableHFantenna.Suchanantennamightbeanautotunedsmallmagneticloop.Thisarticleisaboutmakingaremoteautomatictunerforsmall

magneticloopantennas.Thearticleexplainsinsomedetailhowtheautotunerfunctionsandisused.Implicitinthisdescriptionishowtomakeone.Idonotprovideapartslisthereandcataloguenumbers.Fortunately,circuitcomponentsarestandardelectronicsasidentifiedinthecircuitdiagram.

A small magnetic loop antenna is relatively compact, easily portable, and does not have radials; it is directional, has some gain, and is suitableforuseoutdoors,and(withinsafeantennaproximitypowerlimits)indoorsoronabalcony3.Theantennaconsistsoftwoloops,onewithintheother,interconnectedbyinductance.Themuchsmallerloop,calledthecouplingloop,connectstoatransmitter/receiver.Thelarger,transmittingloop(italsoreceives),aboutametreindiameterforpopularHFbands,isconnectedinparallelwithavariablecapacitorcomprisinganLCcircuit.Tuninghappens,withmaximumantennaoutput,whenthe

variablecapacitoristurneduntiltheresonatefrequencyoftheLCloopmatchesthetransmittingfrequency.Atresonancethevariablecapacitorissubjecttoveryhighvoltages,uptoabout900voltswhentheantennaistransmitting5watts,and4,200voltsat100watts.Thecouplingloopdoesnotexperiencehighvoltages.Tuningforoptimumtransmittingconcurrentlyoptimizestheantennaforreceiving.TherearenumerousInternetsitescoveringsmallmagneticloop

antennadesignandconstruction.Further,thereareseveralsmallmagneticloopantennadesigncalculators.ThemostaccuratecalculatorformypurposesisKI6GD-GlennSperry’sMagneticLoopAntennaCalculator,LoopCalc.exe,whichcanbedownloadedfrom http://www.iw5edi.com/software/magnetic-loop-calculator.Picture1showsthetypeofsmallmagneticloopantennaIuse.The

autotunerisintheboxbelowtheantenna.FormyantennasIemploygoodqualitycoaxialcabletomakethetransmitterandcouplingloops.Itoperatesonthe40,30,and20metrebands.Otherbandsmay be accessed by changing the transmitting loop size and/or the rangeofthevariablecapacitor.Theantenna’svariablecapacitorisahighvoltageairvariablecapacitorwhichcanrotatecontinually360degrees4.Betteramateurradiosmallmagneticloopantennashaveatransmitting loop made of copper pipe or tubing, which is superior because of lower electrical resistance resulting in better performance andlowerQ.(Theautotunerdescribedhereshouldworkwellwithsuchatransmittingloop.)Alsoacostlyvacuumvariablecapacitor5 may be employed that can withstand much higher voltages, allowing forgreatertransmissionpower.Nevertheless,coaxbasedsmallmagnetic loop antennas with air variable capacitors are fairly popular andquiteeffective.Theyarealsoeasytoconstruct.

http://www.iw5edi.com/software/magnetic



Picture 1. Small Magnetic Loop Antenna with Autotuner



Table1providesthecharacteristicsoftheantennaIusedforthisarticle.Exceptforportableoperationsmallmagneticloopantennasare

rarelyfoundinhams’antennarepertoires.Theproblemistheantenna’sextremelyhighQmakingitdifficulttoaccuratelytune.Graph1illustrates the minute turning range of a traditional air variable capacitor toachieveanywherenearmaximumoutput.Theonlysignificanttransmitted signal occurs within a two degree portion of the 180 degree (half)rotationofavariablecapacitor,andsuccessfultuningiswithinless than a one-half degree range6.(Atleastthisisthesituationwithmysmallmagneticloopantenna.)

Thereisanotherinherentdifficultywithsmallmagneticloopantennas.Iftheantennaisnotequippedwithremotetuningtheradiooperatorhas to be able to reach the antenna’s variable capacitor to tune by hand, limitingwheretheantennacanbesetupandpossiblyexposingtheoperatortosignificantRFenergy.TherearecommercialandDIYdevicesthatmakeremotetuningpossible.Theseoftenutilizeasteppermotoror geared down DC motor at the antenna to turn the variable capacitor forminimumSWRatthetransmitter.Onetypehasahandheldmotorcontrol.AnotherisalogicbasedtunerthatautomaticallysearchesforminimumSWR.



I wanted to make a remote automatic tuner that would be precise, easy touse,andinexpensive.OnceIsettledontheuniquestrategyforremoteautomatictuning,thedesignoftheautotunerwasobviousandextremelysimple.Itisalsoinexpensivecostingabout$50,notincludingmetalboxandpartsforthesmallmagneticloopantenna.WithmydesignIconfidentlyautotuneasmallmagneticloopantennaremotelywithinafractionofadBofbestpossible.Thereisalimitationtotheautotuner’sperformance,however,andthisisnotedlater.

I have constructed several autotuners out of varied parts for the stepper motor, stepper motor driver, and small magnetic loop antenna air variable capacitors.Theyalldidanacceptablejob.Forguidance,however,specificmodelsofsteppermotorandvariablecapacitorandtheirsuppliersarementionedinthearticle.Aprintedcircuitboardisavailablewhichfacilitatesassemblingthecircuit.

Unique Tuning Objective

Usually antenna tuning strives to achieve the lowest SWR measured atthetransmitterortheantennafeedpoint.Myautotuner’sobjective,however, is to achieve the highest possible output at the antenna from the powerdeliveredtoit.Maximumtransmittedpowermightnotoccuratthe antenna’s lowest SWR, because the impedance of an optimally tuned smallmagneticloopantennamaynotbe50ohms,typicaltransmitteroutputandtransmissionlineimpedances.TuningtheantennaforlowestSWRdoesnotguaranteeabsolutelyhighestantennaradiation.Fortunately,inmyexperience,thehighestradiatedpowerofasmallmagneticloopantennaisconsistentwithacceptablylowSWR.Ifthiswere not the situation high SWR at the antenna can be managed by addingatraditionalantennatunerprobablyatthetransceiver. Picture 2. Autotuner - Sampling Antenna Below Variable Capacitor

Protector



Picture 2 shows the autotuner and variable capacitor mounted together onaplatformthatclampstothemastofthesmallmagneticloopantenna.ThevariablecapacitorishousedinawhiteshieldmadefromPVCpipe,andtheautotunerisinthemetalbox.(Theextraholesintheboxrelatetoaformerautotuner.)Picture3showstheplacementofpartsinsidethebox,andFigure1showsthecircuitoftheautotuner.Thecircuithasfivesections: sampling antenna, wide band receiver, microcontroller, stepper motordriver,andsteppermotor.Theyaredescribedbelow.

Sampling Antenna

Theperceivedsensitivityofthesamplingantennaincreaseswithfrequency.Thisismostlybecausesmallmagneticloopantennasofagivensizearemoreefficientathigherfrequencies,asshowninTable1fortheprototypeantenna.Thesamplingantennahasoneimportantlimitation.Itcanpickup

a signal from a nearby relatively high power transmitter and mislead the autotuner7.AlthoughinfrequentIhavehadtheembarrassmentof demonstrating the autotuner at a transmission laden hamfest with unsatisfactoryresults.Computermonitoringoftheautomatictuningprocess or autotuner manual mode, described later, can reveal where outsideRFislikelytothrowoffautotuning.Also,extraneousRFinterference can be reduced by using a shorter sampling antenna in conjunctionwithhighertuningpower,e.g.5watts.

Receiver

Thereceiverisverywideband,consistingoffour1N34ARFdiodesinabridgerectifier.A0.002uFcapacitorsuppressesnon-DCoutputfromthereceiver.Indeed,thecombinationofsamplingantennaandreceivercompriseafieldstrengthmeter.Linkingthereceivertothemicrocontrollerisa4N35optocoupler

which provides a non-inverting path, signal isolation, and limits the voltagetothemicrocontrollerinputto5volts,itsmaximumsafelevel.OutputfromthebridgerectifierisconnectedtotheLEDinsidetheoptocoupler.Theoptocouplerfunctionsinthelinearmode,wherecurrent passing through its output transistor varies in conjunction with theamountofcurrentgoingtotheinternalLED.ToomuchcurrentattheLED results in transistor saturation - this is the optocoupler’s switching

Picture 3. Inside the Autotuner Box



Figure 1. Autotuner Circuit



mode.Theoptocoupler’slinearmodesensitivity(beforesaturation)varies with the value of the output transistor emitter resistor, switchable betweenR1andR2inthecircuitdiagram.Voltageacrosstheemitterresistorisappliedtopin18ofthemicrocontroller.Ahighvaluefortheresistorresultsinhighsensitivitybutalsoearlysaturation.Conversely,alow value decreases the optocoupler’s sensitivity but allows for a greater rangeforlinearoperation.InthecircuitR1is3.0K-ohmfor‘HighSensitivity’andR2is1K-ohmfor‘LowSensitivity’,resultinginascalerangeof3:1.Bychangingthesensitivitysettingtheautotunermayoperate with a tuning signal that is neither too high nor too low, covering severalbandsgiventhelengthandpositionofthesamplingantenna.Thereistheopportunityforfurtherautotunersensitivityadjustment

atthetwopointgapatX3inthecircuitdiagram.Placingaresisterherereduces the current going from the diode bridge to the optocoupler LED.Aresistormaybeneedediftuningwattageisrelativelyhighandcannotbeaccommodatedbyshorteningormovingthesamplingantenna.Otherwisetheendsofthegapareconnected.

Even when the autotuner is turned off the receiver via the sampling antennaisexposedtothefullpowerbeingtransmittedbythesmallmagneticloopantenna.Thereisthepossibilitythattransmittinghighpower will cause the current handling capacity of the 1N34A bridge rectifierdiodes,50mA,andthe4N35optocouplerinputLED,60mA,tobeexceeded,destroyingthem.ThecircuitprovidesforanoptionalSPST,normallyopen,relaytodisconnectthesamplingantennawhentheautotunerisnottuningornotinmanualmode.Itislikely,however,thattransmitting high power will arc the variable capacitor before creating sufficientcurrenttodamagethereceiverdiodesoroptocouplerLED.Aprocedure for estimating how much full transmitting power the autotuner

can tolerate is described in a short article that may be downloaded from mywebsite.Ifarelayisnotinstalledthentherewouldbenoconnectionsat X1 on pin 17 of the microcontroller, and the gap in the sampling antennaconnection,atX2,wouldbeclosed.Theautotunerprogramassumesthatarelayispresent,ifnotthereisnoeffect.

Microcontroller

ThebrainsoftheautotunerisaninexpensivePicaxe18M2+microcontroller.Picaxemicrocontrollersareentirelyself-containedandsimpletoimplement.Supportcomponentsconsistofonlytworesistors.Thebuiltinoscillatorcanrunupto32MHz.Mostpinsareprogrammable for input or output, with many having the input option of10bitanaloguetodigitalconversion.ThereisaresidentBASICinterpreterhavingawiderangeofbuilt-infunctions,including5and10bitintegermathematics.ProgramcreationinBASICanduploadingittothe18M2+isdonebymeansofthePicaxeDevelopmentProgram;versionsofwhicharedownloadablefreetoworkwithWindows,Mac,andLinux.Forprogrammingandserialdatacommunicationsthe18M2+connects to a computer via a serial to USB port adaptor cable made for thePicaxe.APicaxechipcanbereprogrammedmorethan100,000times-necessaryforexperimentation.ThecircuitdiagramshowsthedigitalI/Opinassignmentsofthe

18M2+connectingtotheA4988steppermotorcontrollermodule,threetuningstatusLEDs,5voltstatusbuzzer,andoptional5voltsamplingantennarelay.TheLEDs,buzzer,andrelaymusteachdrawlessthanthe20mA,theperpinI/Ocurrentlimitofthe18M2+.Inthecircuitdiagramthere is also an optional capability at pin 4, with pair of connections at X4 and a 10 K ohm pull-up resistor, for adding a turn-limit sensor to



accommodatean180degreeturnvariablecapacitor;amodifiedprogramisneededforthis.

Stepper Motor Driver

TheautotuneremploysanA4988modularsteppermotordriverthatpowersandcontrolsatwophasebipolarsteppermotor.InputvoltagetotheA4988canrangefrom8to30volts;myautotunerpowersourceisapproximately12volts.TheA4988canprovideuptooneamp(twoampswithexternalheatsink)toeachofthetwophasecoilsofthesteppermotor.TheA4988hasanon-boardadjustablecurrentlimitingcontroltotailorpowertotherequirementsofthesteppermotor.Drivingasteppermotorcanbecomplex,andtheA4988module

handlesthistask.Specificpinsonthemodulereceiveinstructionsfromthemicrocontroller.Movingastepoccurswhenthe‘Step’pinreceivesapulse.Otherpinsaresethighorlowtocontrolsteppermotordirection,fractionalstepsize,andmotorpoweronoroff.

Stepper Motor

Theautotunersteppermotoristwophasebipolartypethatiscommonlyavailable.Thesteppermotorturnsthesmallmagneticloopantenna’svariablecapacitor.Asthenameimplies,asteppermotormovesindiscreteincrements.Themotorcanstart,stop,andgoforwardandbackward.ThesteppermotorreceivespowerfromtheA4988driverthroughfourwires,twoforeachphasecoil(inthecircuitdiagramthesearelabelledW1andW2).ForthisprojectIexperimentedwithsteppermotorshavingfull-step

turningincrementsof1.8degrees,0.9degrees,andagearedsteppermotorwiththeequivalentofabout0.25degrees.Thetuningincrement

ofthesteppermotorcanbefurtherreducedbyhalf-stepping,quarter-stepping,eighth-stepping,andsixteenth-steppingwithpotentiallyreducedmotortorque.Ihavehadthemostsuccesswith0.9degreesteppermotors.Theremainderofthisarticlegenerallyassumestheuseofa0.9degreesteppermotor,buta1.8degreesteppermotormaybeeasilyaccommodatedbyasmallchangeintheprogram.Thepresentautotunerhasa0.9degreeNEMA17cubesteppermotor,‘Steperonline’model17HM15-0904S,shownmountedtotheboxinPicture3.ThisandcomparablesteppermotorsmaybeobtainedfromAmazon.comorEbay,andotheron-linesources..Twophasebipolarsteppermotorshavethreepowerinput

specifications:voltage,ampsperphase,andresistanceperphase.Forexamplethepresentautotuner’ssteppermotorspecificationsare5.4voltsperphase,0.9ampsratedcurrentperphase,and6.0ohmsphaseresistance.Myexperienceisthatvoltageisnotanissuewhenamperageisatorbelowthesteppermotorphasecoilrating.Theturningforceprovidedbythesteppermotorisrelatedtothe

amperageitdraws.ThevariablecapacitorsI’veusedarefairlyeasytoturnbyhand(i.e.abletoturnthebareshaftwithfingerswithoutaknob)andcanberotated(evenwithpartialstepping)byasteppermotordrawing0.9amps.

Although the stepper motor turns in very small increments the chances ofthesteppermotorpositioningthevariablecapacitoratexactlymaximumradiatedpowerisstatisticallyhitormiss.Nearresonancethereare substantial changes in radiated power per increment of the stepper motor,asdisplayedinGraph1.Whilelandingthevariablecapacitorexactlyonmaximumradiatedpowerisnevercertain,usingpartial



stepping improves the odds that near miss tuning yields an acceptable result.Sixteenth-stepping,with3200incrementsperone-halfturnofthevariablecapacitor,isemployedbythepresentautotuner.Thesuccessofthisstrategyisevidentinthetestresultsshownattheendofthisarticle.However, with earlier autotuners I have had reasonably satisfactory tuningresultswithevenquartersteptuning.

Microcontroller Automatic Tuner Logic

Theautotunerhastwomodesofoperation,‘manual’and‘autotuning’.In both modes the microcontroller can send data through its serial port (convertedtoUSBbythePicaxeconnectingcable)toaconnectedcomputer8.Forthemanualmodetobeusefulacomputermustbeconnectedto

theautotuner.Manualmodeisengagedbylettingtheautotunerrunwithout any tuning power to the antenna during the reconnaissance sweep,describedbelow.Afternotfindinganysignaluponwhichtotargettuning,theautotunerrevertstomanualmode.Powertothesteppermotoristurnedoffandthevariablecapacitorcanbeturnedbyhand.Thenmeasurementsofanyradiatedpoweraresenttothecomputeraboutthreetimespersecond.Thisdataisusefulforunderstandingthenatureofthesmallmagneticloopantenna.Withoutatransmittedtuningsignal,manualmodecanbeusedtoassessambientRFenergyfromnearbytransmitters.

If a computer is connected while in autotuning mode, autotuning information on levels of antenna radiated power and variable capacitor positioniscommunicatedaftereachincrementofthesteppermotor.(Insignificantmeasurements,lessthan‘10’,arenotsenttothecomputer.)Thisinformationisimportantforunderstandingautotuningandfor

experimenting.AutotuningdataisthebasisofGraph1.Whenautomatictuninginthefieldcomputerdataarenotusuallyneeded,andtheautotunerishappytoworkwithoutacomputerconnection.Thetaskofthemicrocontrollerprograminautotuningmodeisto

maximizethesmallmagneticloopantenna’sradiatedpoweratagivenfrequency.Whilealowleveltuningsignalisbeingtransmittedtotheantenna and the stepper motor incrementally turns the variable capacitor, themicrocontrollertrackspowerbeingradiateduntilthemaximumisachieved.Thisdoesnotseemcomplicated,butI’vetriedseveralautotunealgorithmswithvariedsuccess.ForthepresentcombinationofautotunerandsmallmagneticloopantennaIusethe‘directmethod’-myterminology.

Direct method autotuning starts with a reconnaissance sweep of the entireturningrange(i.e.afullturn)ofthevariablecapacitortoidentifyaprobablemaximumsignallevel.Nextoneormoreactivetuningsweepscommence where the objective of the autotuner is to turn the variable capacitoruntilasignallevelisproducedthatisclosetoorexceedsthereconnaissancemaximum.Becausethereconnaissancesweepcouldhavebeenveryluckyinencounteringthemaximum,itsvaluemaynotbematchedduringactivetuning.Theinitialactivetuningobjective,therefore, is to turn the variable capacitor until a signal level is achieved thatisatleast95%ofthereconnaissancemaximum.Occasionallya95%valueisnotencounteredwhereuponanewactivetuningsweepisattemptedusinga90%threshold.Beforegivingupathirdandforthactivetuningsweepwillbeattemptedusing85%and80%thresholds.If tuning is not achieved after the fourth tuning sweep there is likely somethingwrongthatdoesnotinvolveautotuneroperation.Perhapsthere is a loose antenna connection, or there may be a high power



ham rig nearby adding its transmitted signal to the autotuner sampling antenna.Theautotunerhastodealwithvariationsintransmittedtuningpower

when the transmitter’s foldback circuit responds to large changes inSWR.EncounteringhighSWRmaycausethefoldbackfunctiontoreducetransmittertuningpower.AstuningapproachesantennaresonanceSWRdeclinessignificantlyandthefoldbackfunctionallowstuningpowertoincreasetothespecifiedlevel.Togivethefoldbackcircuit time to catch up to declining SWR, the stepper motor slows downturningthevariablecapacitor.Inthepresentprogram,tospeedup the tuning process, the stepper motor turns at half-step until antenna resonanceisapproachedthenslowsdowntosixteenthstep.Theautotunervisuallyandaudiblycommunicatesitsoperatingstatus

bymeansofabuzzerandthreeLEDsonthebox.Table2outlinestheBuzzerandLEDindications.

Mechanical and Shielding Considerations

I suggest two mechanical shortcuts that I found helpful with prototype construction.First,evidentinPicture2,isfasteningthetwoendsofthetransmitting loop to the variable capacitor by 1/4 inch stainless steel bolts extendingfromthevariablecapacitorprotectivehousing,bymeansofsmallhoseclamps.Thecoaxbraidattheendsofthetransmittinglooparestrippedforaninch,orso,andtinnedtomakeafirmerconnection.Althoughnoteleganttheconnectionisphysicallyandelectricallysecure.Also,thetransmittingloopiseasytosetupandtakedown.Thesecondshortcut,inPicture4,isusingtwoshortlengthsof1/4

inchPVC,reinforced,flexiblehosetoattachthesteppermotortothevariablecapacitorbymeansofa1/4inchaluminumroddriveshaft.

Picture 4. Connection Between Variable Capacitor and Stepper Motor



Thehoseistightlyclinchedtotheshaft’softhe stepper motor, variable capacitor, and rod byfoursmallhoseclamps.Theshaftsofthestepper motor, variable capacitor, and ends of the rod are roughened to enhance holding power.Ateachendoftherodthereisabouta3 mm gap inside the hose to electrically isolate thesteppermotorfromthevariablecapacitor.Ifthe variable capacitor turns relatively easily and the four hose clamps remain tight there is no slippagenortwistingevidentwiththislinkage.Furthertheflexiblehosecanaccommodatethe somewhat different diameters of the shafts of the variable capacitor, aluminium rod, and stepper motor; and overcome minor misalignment.Althoughmoreexpensivethanthejuryrigged

approach described above, stepper motor (CNC)shaftcouplersworkwelltoconnectthesteppermotorandvariablecapacitor.Acouplermustbespecifiedtoconformtothediametersoftheshaftsitisconnecting.Ifelectricallyconductive(i.e.madeofmetal)thentwo couplers are needed, one at the stepper motor and the other at the variable capacitor with a non-conductive drive shaft in between to electrically isolate the variable capacitor from theautotuner.Ihaveusedone-quarterinch,stiff



plasticrodforadriveshaft.Theautotuner’scircuitrymustelectricallyfloatinsidethemetal

box.Thereisnoelectricalnorpowerconnectionbetweenthecircuit’scomponentsandthebox.ThemetalboxshieldstheautotunerelectronicsfromthepotentiallysignificantRFfieldsurroundingthesmallmagneticloopantenna.Shieldingalsooccursatthetransmissioncablefromthesamplingantennatotheautotunerreceiver.(InitiallyIusedRG-174coax,buttheouterbraidbecamepartofthesamplingantenna.)Iusea transmission cable made of shielded audio cable with the two inner wirescarryingthesignalfromthetwolegsofthesamplingantenna.Thecable’sshieldisgroundedtothemetalbox.(Thesteppermotorismountedtotheboxbutthemotor’scoilsareinsulatedfromit.)Themethodofwiringtheelectronics and stepper motor is not critical,

althoughaprintedcircuitsimplifiesassembly.

Autotuner Platform - Rigidity Rules

A lesson I’ve learned in building four autotuners with different variable capacitor and autotuner mounting strategies is the variable capacitor and autotunerhavetobemutuallymountedonaveryrigidstructure.Ifthereisanyflexingatorquewillbuildupduringtuningbetweenthevariablecapacitorand stepper motor which results in the variable capacitor continuing to turn slightlywhenautotuningiscompleted.PreviouslyI’vefinagledthisproblembyprogramming the microcontroller to turn the stepper motor backwards a small amountasautotuningfinishes.Theprototypeautotunernowemploysa3/4inchplywood(16”x6”).Therigiditythisprovidesallowsboththesteppermotorandvariablecapacitortostoptheinstanttuningiscomplete.

Variable Capacitor Considerations

Small magnetic loop antenna air variable capacitors able to withstand highvoltagecanbeexpensivetopurchasenew.Thisisanissuewith

smallmagneticloopantennasregardlessoftuningmechanism.Myprimarysourceforhighvoltagevariablecapacitorshasbeenhamfleamarkets.The27-127pFvariablecapacitorusedwiththesmallmagneticloopantennaforthisarticleisafleamarketfind;platespacingsuggestsitmaybeabletowithstand90watts.PreviouslyIextensivelyusedanavailable10-260pFhighvoltagevariablecapacitor,doublethecapacitancerangewarrantedforthedesiredbands.Thismadetuningtwiceasdemanding(i.e.foreachsteppermotorincrementthechangeincapacitanceisdouble).DespitetheaddedchallengetheautotunerconsistentlytunedwithinonedB(80%)ofoptimum.Imentionmyvariablecapacitorexperiencetogivehearttosmallmagneticloopantenna builders who face a compromise when using less than ideal variablecapacitors.RecentlyIsuccessfullytriedavariablecapacitorfromRFParts

(www.rfparts.com),model73-175-23,14pF-145pF,3.2Kv($60US).BasedonLoopCalc.execalculations,the3.2Kvvoltageratingallowsmysmallmagneticloopantennatohandleabout65wattson40meters,and50wattson30metersand20meters.Ihadtoreducethevariablecapacitor’sturningresistancebyslightlylooseningthescrew(withlockingnut)holdingtherotorshaftrearpivotpoint.Onceturningresistancewaslessened,the‘Steperonline’model17HM15-0904Ssteppermotor,specifiedpreviously,readilyturnedthevariablecapacitor.

Operating the Autotuner

A small magnetic loop antenna is attached to a transceiver with ordinary50ohmcoaxtransmissioncable.ItendtoinsertanRFisolatorinthetransmissionlineneartheantenna.A12voltDCpowersource(asmallgelcellwilldo)withaswitchattheoperator’sposition,connects

www.rfparts.com



totheautotunerbytwoconductorhookupwire.ThecoaxandDCpowerwiremaybeaslongasneededtoachieveasatisfactoryantennaposition.If computer connection to the autotuner is desired it is done by means ofthePicaxeserialtoUSBadaptercable(atthecomputer)andlengthsofstereoheadphoneextensioncableshaving1/8inchfemaleandmaleendconnectors.I’vechainedtogetherover30ft.ofheadphoneextensioncables.Ifthereisatraditionaltunerbetweenthetransceiverandthesmall magnetic loop antenna, it should be in by-pass mode while the autotuner is actively tuning and in manual mode9.Myautotuneronaveragedrawssomewhatoveroneampwhentuning;

while‘on’atothertimes(e.g.manualmode)thedrawislessthan20ma.Autotuningcommenceswhen12voltpowerisswitchedon.The

autotuner beeps once as a reminder to commence transmitting a small, constanttuningsignal,whichmaybeanAMorCWcarrier,ortonemodulatedSSB.Whiletuningprogressesthetransceiver’sSWRindicatordipsandrisesseveraltimes.Thisisnormal.AttheendoftuningtheSWRmetermostlikelywillberelativelylow.Thetimetocompleteautotuningwithoneactivetuningsweepisabout50seconds.Subsequentactivetuningsweeps,ifneeded,addanother20to30secondseach.Thetuningsignalshouldnotinterferewithotherhams,becauseoftheantenna’shighQitiseffectivelytransmittingonlyafewseconds.Oncetuningisdone,thereisafinalbeep,thesteppermotorstops turning, and the red and blue LEDs light up steadily indicating that tuningwassuccessful.Table2referstotheLEDandbuzzerindicationsfornon-successfultuningoutcomes.Afterwards12voltpowertotheautotunercanbeturnedoff.Turningtheautotuneroffbeforetuningisfinisheddoesnoharm.

Veryinfrequentlytheautotunermissesacceptabletuningwithinfouractivetuningsweeps.Whenmiss-tuningoccurs,tuneagain.Repeatedfailureconfirmsthatthereissomethingwrongoperationallywiththeautotuner, the small magnetic loop antenna, or the antenna’s connection withthetransceiver.Inmanualmodethesetupisthesameasautotuningexceptacomputer

mustbeattachedtotheautotuner.Manualmodeisactivatedbyturningon12voltpowerbutnottransmittinganysignaltotheantenna.Afterareconnaissance sweep without any radiated signal the autotuner goes into manual mode and starts providing continual radiated signal data to the computer.ManualmodestatusisindicatedbytheyellowLEDturnedon.Twelvevoltpoweriskeptonaslongasmanualmodeoperationisdesired.Nopowergoestothesteppermotorandtheantenna’svariablecapacitorcanbeturnedbyhand.

Autotuning Results

ToillustratethecapabilityoftheautotunerforthisarticleIconductedatotaloffifteentrails.ThesmallmagneticloopantennawassetupinthedrivewayofmyhomeinNovaScotia.AnMFJ-915RFisolatorwasinsertedinthecoaxialcableneartheantenna.Myoperatingpositionwas25feetawayinthegaragewhichhadACpower,andaYaesuFT-897transceiverandalaptopcomputeronamakeshifttable.Theafternoonwassunnywithambienttemperatureof68deg.F.Table3presentsthetuningresultsoffifteentrials.Therewerefive

consecutivetrailsinthreegroupsforfrequenciesinthe40,30,and20metrebands.Thecriteriaforthefrequencieswerethattheywerenotspecialuse,notforCWcode,andnotbusy.TheYaesuFT-897transmittedaCWtoneat5watts,thelowestpossibleoutput10.



Threeparameterswererecordedforeachtrial:SWR,number of tuning sweeps, and percentage of highest signalachieved.SWRwasmeasuredbyanMFJ-813QRP,SWRmeter.Thenumberoftuningsweepsisthecountof360degreeactivetuningturnstheautotunertookto(nearly)matchthehighestsignallevelfoundinthattrial’sreconnaissancesweep.Asistypicalonlyonetuningsweepwasneeded.“PercentofHighestSignalfortheFrequencyGroup”

compares, in percentage terms, the signal level measured at the conclusion of each trial with the highest signal occurring amongallfivetrialsinthefrequencygroup.Thehighestgroup signal level is that encountered collectively among the fivetrialreconnaissancemaximumsandthefiveautotunedresults.Thetheoryisthatthehighestamongthesetenvaluesreasonablyestimatesthemaximumsignalpossibleforthefrequencygroup.AlsoshowninparenthesisinTable3isthepercentageof‘local’maximumpowerachievedforjustthattrail.TheresultsinTable3areselfevident.Theindividualtrial

signal values are very close to the probable highest signal valuefortheirfrequencygroup.Thelowestare92%forthesecondtrialinthe40metrebandand92%forthefourthtrailtheinthe20metreband;thesetrailsareboldedinthetable.Toputthispercentageintoperspective,92%isonly0.36dB(lessthanone-halfdB)belowthehighestprobablesignalvalue.Regardingeachtrail’slocalmaximum,theautotunedpercentageis96%orhigher.Theseautotuningresults



aretypical.Tuninganyantennatothisdegreewouldbeconsideredasuccess.TherelativelylowSWRsareencouragingbyanystandard,buttheyrelatetothisparticularantenna.Itiscomforting,nevertheless,toknow that after autotuning with this small magnetic loop antenna no further signal treatment is needed to protect the transceiver from high SWR.

Where to Obtain Autotuner Picaxe Programs and Additional Information

Iamaddingtomywebsiteprogramfilesandadditionalinformationtocomplementthedescriptioninthisarticle.Therearetwoprogramfiles.Oneisacircuittesterthatverifiesallfunctionsofthecircuitareoperatingcorrectly.Thiscanberunbeforethecircuitisinstalledintheautotuner.Thesecondprogramcontrolstheautotuner.PicaxeprogramsmaybeopenedinthePicaxeDevelopmentProgramoranytexteditortolookat(andedit)thecode.PicaxeBASICiseasytofollowandthecodetendstobeselfdocumenting.Otherinformationonthewebsiteincludesa circuit parts list, assembly tips involving the printed circuit board, how to estimate highest tolerable transmitting power, and some project history.Ileavemanydetailsofautotunerassemblytothereader.Myplatform,

3/4”plywoodboardabout6”x18”,issuitableforthevariablecapacitorsandautotunerboxesthatIhaveused.Theshortcutsdescribedabovehave facilitated construction, and the way of mounting the autotuner onthemastofasmallmagneticloopantennahasworkedwell.Otherthanarequirementforrigiditybetweenthesteppermotorandvariablecapacitor,thereisnothingvitalaboutmyapproaches.Whattheyaccomplishcanbeachievedinotherways.Indeedmostaspectsofthe

autotunerarestillamenabletoexperimentation.

Availability of a Printed Circuit Board

TosimplifyassemblingtheautotunercircuitIhaveaprintedcircuitboard,showninPicture3.TheboardwasdesignedtoprofessionalstandardsbyEdThompson,thenastudentattheInstituteofTechnologyCampusoftheNSCC(NovaScotiaCommunityCollege).Ed’sabilityand diligence are commendable, and he recommended changes that significantlyimprovedtheprospectiveboard.

I am planning to have a small supply of printed circuit boards available forsalebyFebruary2020,whenthisarticleisscheduledtobepublishedinthe‘PacketStatusRegister’.Howtoordertheboardwillbepostedonmywebsite.Ifdemandexceedstheinitialsupplyofboards,Iwillattempttosourcemore.

Acknowledgements

AnearlierarticleofminedescribingtheexperimentalnatureofthisprojectwaspublishedintheMay-June2018editionoftheRadioAmateursofCanadamagazine,TCA(TheCanadianAmateur).IwishtoacknowledgetheassistanceIreceivedfromtheTCAeditorandreviewersoftheearlierarticle.Manyoftheirsuggestionshelpedmyapproachtotheprojectandwritingaboutit.SincethenIhaverefinedtheautotunertomakeagooddeviceevenbetter.WiththepresentarticleIwaspleasedtohavetheadviceoftheTAPRreviewerBruceRaymond,ND8I.Anyandallproblemsassociatedwiththeautotuner,however,areentirelymyfault.Thereremainstheneedtodemonstratetheautotuner’scapabilities

with a wide range of small magnetic loop antennas used by other



hams.Also,theautotuningapproachdescribedinthisarticleshouldbeapplicableforotherkindsofantennas..Therearealwaysopportunitiesforexperimenting.

About the Author

Andrew Cornwall resides in Nova Scotia, Canada where he has Basic withHonoursamateurradiocertification.Hewasfirstcertifiedin1992andretainsthecallsignVE1COR.HespendsconsiderabletimeintheUnitedStatesandhashadaGenerallicensesince2008withcallsignKB1RSE.Heisretiredfromcareersineconomicsanalysisandininformationtechnologysystemgroupmanagement.AndrewCornwall,VE1CORandKB1RSEEnfield,NovaScotiaCanadaEmail: [email protected]: www.acornwall.ca

Footnotes

1.Thewebsitewww.hoa-usa.comclaimsthatthereare351,000suchhomeownersassociationsintheUnitedStates.CommonlyhomeownersassociationsbanfullsizeHFantennas.2.UnitedNationsDepartmentofEconomicandSocialAffairs,2018

RevisionofWorldUrbanizationProspects.3.Ihavenotusedasmallmagneticloopantennaonabalcony,but

others have, for instance: www.qsl.net/kp4md/balconyloop.htm4.Theautotunercircuitdesignallowsforvariablecapacitorsthathave

an180degreeturninglimit.Thisrequirestheadditionofaturnlimitsensorandamodifiedmicrocontrollerprogram.

5.Theadjustingmechanismofavacuumvariablecapacitorisnotsuitableformyautotunermechanism.6.Someofthepeakinessofthesmalltuningrangeisduetothevery

highSWRoutsideofthevicinityofantennaresonance.Thisresultsintransmission line loss and also causes the transmitter’s foldback function toreducepoweroutput.7.Thereisthepossibilityofmeasuringtheoutputsignaloftheantenna

otherthanasamplingantenna,whichwouldnotbesubjecttoextraneoustransmissions.Theautotunercircuitandlogicwouldnotbealtered.8.Measurementsofsignalstrengtharerelativeandrangefrom10

toabout1000(anythingbelow10ismeaninglessandnotreported).Avalueof1000correspondstonearly5voltsfromtheoptocouplertothemicrocontroller.9.Thedatasignalfromtheautotunertoacomputercanbegarbled

whenafairlyhightuningsignal(sometimesaslowas5watts)isbeingtransmitted and the small magnetic loop antenna is close to the computer, ortheradiotransmissionlineisintheproximityofthedatacable.Thesolution is to move the antenna, separate the transmission and data cables,and/orlowerthepowerofthetuningsignal.10Runningsequentialtrialsisnotassimpleasitmightseem.Even

withthetuningsignalreducedto5watts,froma100wattcapabletransceiver, holding the key down repeatedly for about a minute heats up thetransmitternotably.Thiseventuallyreducestuningpowerbyasmallamount.Whileautotuningisaccurateforthetrail,theresultsarenotcomparabletoothertrails.Thesolutionforconsistencyistoallowtimeforheattodissipatebetweentrials.

###

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PSR#144Winter2020,ISSN:1052–3626PublishedbyTAPRPhone(972)413–8277E-mail [email protected] www.tapr.orgFacebookwww.facebook.com/TAPRDigitalHamTwitterwww.twitter.com/taprdigitalTAPROfficeHours:MondaytoFriday,9amto5pm EasternTime

TAPR Officers President: Steve Bible, N7HPR, [email protected] VicePresident:ScottyCowling,WA2DFI,2018,[email protected] Secretary:StanaHorzepa,WA1LOU,[email protected] Treasurer:TomHolmes,N8ZM,[email protected]

TAPR Board of DirectorsBoardMember,CallSign,TermExpires,e-mailaddress John Ackermann, N8UR, 2022, [email protected] Steve Bible, N7HPR, 2020, [email protected] GeorgeByrkit,K9TRV,2021,[email protected] ScottyCowling,WA2DFI,2021,[email protected] StanaHorzepa,WA1LOU,2020,[email protected] JohnKoster,W9DDD,2021,[email protected] Dave Larsen, KV0S, 2022, [email protected] Bruce Raymond, ND8I, 2022, [email protected] DarrylSmith,VK2TDS,2020,[email protected] TAPRisanot–for–profitscientificresearchanddevelopmentcorporation[Section501(c)(3)oftheUStaxcode].ContributionsaredeductibletotheextentallowedbyUStaxlaws.TAPRischarteredintheStateofArizonaforthepurposeofdesigninganddeveloping new systems for digital radio communication in the Amateur Radio Service, andfordisseminatinginformationrequired,during,andobtainedfromsuchresearch.

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tapr psr #144 winter 2020 president’s corner · for direction and swr, activity is infrequent for...

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