the basics of vhf and uhf signal propagation · the frequencies of interest to the fm and tv dxer...

The Basics of VHF and UHF SignalPropagation

The Electromagnetic Spectrum - The electromagneticspectrum is a continuum of all electromagnetic wavesarranged according to frequency and wavelength.Electromagnetic radiation is classified into typesaccording to the frequency and length of the wave. Visible light that comes from a lamp in your house orradio waves transmitted by a radio station are just two ofthe many types of electromagnetic radiation. In order ofincreasing frequency the electromagnetic spectrumconsists of radio waves, microwaves, infrared radiation,visible light, ultraviolet radiation, X-rays and gammarays.

An electromagnetic wave consists of the electric andmagnetic components. These components repeat oroscillate at right angles to each other and to thedirection of propagation, and are in phase with eachother.

VHF Signal Propagation http://www.dxfm.com/content/propagation.htm

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All electromagnetic energy, regardless of frequency orwavelength, passes through a perfect vacuum at thespeed of light (300 million meters per second) in theform of sinusoidal waves.

The Radio Spectrum - As radio and TV DXers, we are ofcourse most interested in the "radio" portion of theelectromagnetic spectrum, which exists between thefrequencies of 10 Kilohertz and 300 Gigahertz, withwavelengths from 30,000 kilometers to 1 millimeter. Asthe frequency of a signal is increased, its wavelengthbecomes shorter. For example, an electromagnetic waveat 750 KHz in the middle of the AM broadcast band has awavelength of approximately 400 meters. As weincrease the frequency to 100 MHz in the middle of theFM band, the wavelength decreases to about 3 meters.

The frequencies of interest to the FM and TV DXer aresituated within the Very High Frequency (VHF) and UltraHigh Frequency (UHF) regions of the radio spectrum. TheVHF portion of the radio spectrum is between 30 and 300Megahertz, while UHF is situated between 300 and 3,000


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Megahertz. The frequencies used for the FM band andtelevision channels 2 through 13 lie within the VHFportion of the electromagnetic spectrum, while televisionchannels 14 through 83 are within the UHF portion of thespectrum.

Signal Propagation - When we refer to signalpropagation, we are talking about the radio signalgetting from one place to another, presumably from thestation's transmitting antenna to the receiver's antenna. Signals at VHF and UHF frequencies can be propagatedby a variety of means or "modes". Depending on theparticular mode that is dominate at the time ofreception, the distances covered by VHF and UHF signalscan extend hundreds or even thousands of miles. Hereare some of the more common modes for VHF and UHFpropagation:

Ground Wave - Ground wave propagated signals aresignals that, generically speaking, travel along or closeto the Earth's surface on their path between thetransmitting and receiving antennas. Ground wavesignals are the "local" signals we receive -- the signalsthat are always present at your location, day and night,regardless of any any particular atmospheric orionospheric conditions.

The ground wave actually consists of two components,the surface wave and the space wave. The terms"surface wave", "space wave" and "ground wave" areoften used interchangeably, even though it's not exactlycorrect to do so.

The surface wave travels out from the transmittingantenna, remaining in contact with the Earth's surface. The surface wave is primarily responsible for thereception of local AM broadcast signals. The strength of


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the surface wave diminishes rapidly with distancebecause the Earth is a not a particularly good electricalconductor. Also, the attenuation of surface wave signalsincreases rapidly as the signal frequency is increased. AtFM and TV frequencies the surface wave is virtuallynonexistent. The surface wave is generally not a factorin our reception of FM and TV signals, local or otherwise.

Reception of local FM and TV signals relies almostentirely upon the space wave component of the groundwave. The space wave signal path is the so-called "line-of-sight" path between the transmit and receiveantennas. The curvature of the Earth is the primarylimiting factor for the maximum distance a space wavepropagated signal can travel. The space wave will traveloutward from the transmitting antenna until it reachesthe horizon. Beyond that point, the space wave isblocked by the Earth itself, and reception is no longerpossible for a receiver located on the surface of the Earth(as most are). It's important to note, however, that theoptical horizon (the horizon you can see) and the "radiohorizon" are not quite the same. In reality, the spacewave does not quite travel in a straight line as it movesaway from the transmitting antenna. Instead, the signaltravels in a slightly downward curved path that keeps itnearer to the Earth's surface, thus extending its path alittle further than the optical horizon.

If you are into math, the approximate distance (in miles)to the radio horizon can be calculated by multiplying thesquare root of the antenna height (in feet) by 1.415times. For example, the theoretical distance to the radiohorizon for an antenna 1,000 feet above the ground isjust under 45 miles.


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The distance, D1, to the radio horizon for thetransmitter is 1.415 times the square root of h1(feet). The theoretical maximum line-of-sightdistance between two elevated points,presumably the transmitter (h1) and the receiver(h2), is the sum of the two distances to the radiohorizon (D1 + D2).

All this, of course, assumes that the Earth is a perfectlysmooth sphere, and that no signal disturbances orenhancements occur along the path between thetransmit and receive points. As we know, the Earth isnot a perfect sphere, and the space through which theradio signal travels is not perfect either. FM and TVsignals can be either attenuated or enhanced by variouspath imperfections. Hills, buildings, trees and otherphysical obstacles along the signal path often reduce FMand TV reception distance. On the other hand, a varietyof atmospheric and ionospheric conditions serve toenhance our FM and TV reception distance. We willconcentrate on the enhancements that make it possiblefor VHF and UHF signals to travel hundreds and eventhousands of miles.

Refraction, Refraction, Refraction - Refraction isdefined as "...a change in direction of a wave as itcrosses the boundary that separates one medium fromanother." While this may sound a little imposing, it'sreally a simple principle of physics -- one that weprobably observe daily. Refraction, in one form or


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another, is the primary mechanism that enableslong-distance FM and TV reception.

Refraction explains theapparent "bending" of anobject when it is partlyimmersed in water andviewed from above thesurface.

As stated earlier, a radio wave travels through a perfectvacuum at the speed of light (300 million meters persecond). However, when the medium through which thewave travels is not a perfect vacuum, the wave's travelis slowed. For example, an electromagnetic wave travelsslower through air or water than it does through aperfect vacuum.

Refraction comes into play when a wave enters a newmedium at an angle of less than 90°. As the wave entersthe new medium, a change in the wave's speed occurs


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sooner on one side of the wave than on the other. Thiscauses the wave's direction of travel to be bent.

Under normal conditions, a signal that is not blocked orobstructed simply travels in a straight line out intospace, never to return to Earth again. However, variousatmospheric conditions often cause the normal path ofFM and TV signals to be bent downward, returning thesignal to the surface of the Earth, sometimes a greatdistance from its point of origin.

Tropospheric Enhancements - Within the broadclassification of tropospheric enhancement, there areseveral different and distinct propagation modes thatmake it possible for FM and TV signals to travel fargreater distances than the normal radio line-of-sighthorizon.

Tropospheric Scatter - Tropospheric scatter is the mostcommon form of tropospheric enhancement. Tropo-scatter is always present to some degree just abouteverywhere. Tropospheric scatter at FM and TVfrequencies is caused when the paths of radio signals arealtered by slight changes in the refractive index in thelower atmosphere caused by air turbulence, and smallchanges in temperature, humidity and barometricpressure. The signal is scattered in random fashion. Thetiny portion of the transmitted signal that is scatteredforward and downward from what is called the "commonscattering volume" is responsible for signal paths longerthan the normal line-of-sight horizon.


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Geometry of Tropo-Scatter Signal Propagation

The height of the scattering volume that is common toboth the transmitting and receiving stations determinesthe maximum tropo-scatter path distance. Above about6 miles refraction in the troposphere becomesinsufficient to return any signal to Earth.

Tropo-scatter enables the reception of signals from out toabout 500 miles, depending primarily on the the powerof the transmitting station and the quality of thereceiving equipment being used. Maximum tropo-scatterpath distances of 200 to 300 are more typical on aday-to-day basis. Tropo scattered signals arecharacteristically weak, "fluttery" signals that oftensuffer from random fading.

Tropospheric Refraction - The "standard" atmosphereis defined as air at sea level having a temperature of 59°F, a barometric pressure of 29.92 inches of mercury, and a density of0.002378 slugs per cubic foot. As we increase altitude within ourstandard atmosphere, the temperature, pressure and density decrease atfixed rates. The U.S. version of the standard atmosphere table


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for altitudes up to 10,000 feet looks like this...

TrueAltitude

abovemean sea

level

BarometricPressureinches ofmercury

TemperatureFahrenheit

Densityslugs percubic foot

0 29.92 59.0 0.0023781,000 28.86 55.4 0.0023092,000 27.82 51.9 0.0022423,000 26.82 48.3 0.0021764,000 25.84 44.7 0.0021125,000 24.89 41.2 0.0020496,000 23.98 37.6 0.0019887,000 23.09 34.0 0.0019288,000 22.22 30.5 0.0018699,000 21.38 26.9 0.001812

10,000 20.57 23.3 0.001756

As stated previously, when a wave enters a new mediumat an angle of less than 90°, the change in speed occurssooner on one side of the wave than on the other,causing the wave to bend. Close to Earth, the mediumthrough which radio waves travel is air. The air throughwhich the radio wave travels is an ever changingmedium due to changes in temperature, barometricpressure and density. The "refractive index" of air in astandard atmosphere is sufficient to bend a radio signalever so slightly downward, accounting for the fact that"line-of-sight" signals travel just a little further than theoptical horizon.


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An illustration of why refracted signals go furtherthan the optical line-of-sight.

What happens when the atmosphere does not follow thestandard model? Let's say everything is "normal" untilwe get to 4,000 feet, where we encounter a rise intemperature instead of the normal decrease -- acondition known as a temperature inversion. The suddendiscontinuity in the medium through which our radiowave travels will have a higher refractive index than ourstandard atmosphere model. In other words, our radiosignal will be bent at a sharper angle where it encountersthe discontinuity. If the bending is in a downwarddirection (back toward the surface of the Earth), thenormal range of the radio signal will be extended.

Various weather conditions increase the refractive indexof the atmosphere, thus extending signal propagationdistances. Stable signals with good signal strength from500+ miles away are not uncommon when the refractiveindex of the atmosphere is fairly high.

Tropospheric Ducting - This is where things startgetting interesting for the FM and TV DXer. Strongtemperature inversions with very well definedboundaries sometimes form from as high as severalthousand feet above the surface of the Earth. If the


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inversion is strong enough, a signal crossing theboundary into the inversion will be bent sufficiently toreturn it to Earth. The inversion boundary layer and thesurface of the Earth form the upper and lower walls of a"duct" that acts much like an open-ended wave guide. Signals "trapped" in the duct follow the curvature of theEarth, sometimes for hundreds or even thousands ofmiles. In the tropics and over large bodies of water,strong inversions that cover large geographic areas arequite common, and stable ducts can remain in tact fordays on end. This form of ducting is responsible for fairlyreliable propagation between California and Hawaii atVHF and higher frequencies.

An illustration of tropospheric ducting.

While somewhat less common, ducts sometimes formbetween atmospheric boundary layers at much higheraltitudes, with the upper boundary having an altitude ashigh as 10,000 feet or more. The upper refractive layerbends signals downward, while the lower refractiveboundary bends the signals upward, forming a signal-trapping duct that acts much like a wave guide. Whenthis condition exists, FM band signals can travelthousands of miles. Indeed, there is no theoretical limitto the distance a signal can travel via troposphericducting. With respect to the receiving station,


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tropospherically ducted signals will usually come from ageographically selective area. A distant station may beheard in favor of a closer station on the same frequency. Sometimes conditions are such that multiple ducts form,bringing in distant stations from many different areas atthe same time.

An illustration of a high-altitude troposphericduct.

An interesting characteristic of this form of ducting isthat both the transmitting and receiving antennas mustbe inside of the duct to gain the maximum signalenhancement. A receiving antenna located outside ofthe duct will hear little or no signal from a transmittingantenna located inside the duct. For this type of duct tobe useful to us, the signal must get in and exit the ductsomewhere along the signal path. This can occur if theends of a duct are open at each end, or through "holes"that form along the bottom layer of the duct.

A basic principal of radio is that the wavelength of asignal gets shorter as the frequency of the signal isincreased. Because of this, the size of the troposphericduct determines the lowest signal frequency that it cansuccessfully propagate. This is knows as the LowestUsable Frequency or LUF of the duct. A physically small


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duct, a duct with its upper and lower boundaries closetogether, will propagate only higher frequency signalswith very short wavelengths. As the distance betweenthe boundaries of the duct increases, the signalfrequency the duct will propagate decreases. In otherwords, a larger duct will accommodate a lower frequencysignal having a physically longer wavelength. It'spossible for a duct to form that only supports signalpropagation at UHF television frequencies, while noteffectively passing anything in the VHF television or FMbands.

Ducted signals from 900 - 1,000 miles are fairly common,but it's more common for ducted signals to travel 500 -800 miles. Ducted signals are typically quite strong,sometimes so strong that they can cause interference tolocal signals on the same frequency.

Weather Suitable for a Duct - Tropospheric ductingmost often occurs because of a dramatic increase intemperature at higher altitudes. If the temperatureinversion layer has a lower humidity than the air belowor above it, the refractive index of the layer will beenhanced further. There are several common weatherconditions that often bring about strong temperatureinversions.

While not usually the cause of strong ducting, radiationinversions can bring about pronounced signalenhancement, extending the DX range up to a fewhundred miles. This is probably the the most commonand widespread form of inversion a DXer is likely toencounter on a regular basis.

A radiation inversion forms over land after sunset. TheEarth cools by radiating heat into space. This is aprogressive process where the radiation of surface heat


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upwards causes further cooling at the Earth's surface ascooler air moves in to replace the upward moving warmair. At higher altitudes the air tends to cool more slowly,thus setting up the inversion. This process oftencontinues all the way through the night until dawn,sometimes producing inversion layers at 1,000 to 2,000feet above the ground.

Radiation inversions are most common during thesummer months on clear, calm nights. The effect isdiminished by blowing winds, cloud cover and wetground. Radiation inversions are often more pronouncedin dry climates, in valleys and over large expanses offlat, open ground.

Another meteorological process called "subsidence"often produces strong ducting conditions and excellentDX. Subsidence is the process of sinking air thatbecomes compressed and heated as it descends. Thisprocess often causes strong temperature inversions toform at altitudes ranging from 1,000 feet to as high as10,000 feet. Subsidence is commonly produced by large,slow-moving high-pressure zones (anticyclones). Thesealmost stationary high-pressure zones often form overthe eastern half of the United States during the latesummer and early fall months. They usually move out ofCanada, traveling toward the southeast. As thehigh-pressure zone stalls over the Midwest, stronginversions form, bringing outstanding 1,000+ mile DXthat can last for days at a time. This condition is mostcommon in the Southeastern states and lower Midwest. It also shows up from time to time in the upper Midwestand East Coast states. It rarely shows up in the Westernstates.

The following weather maps are from September 5 and 6of 2001. They provide a real-world illustration of


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tropospheric ducting associated with a slow movinghigh-pressure zone.

On September 5th, 2001, a very slow movinghigh-pressure zone was pushing out of Canada towardthe southeast. As the high was centered over NorthernMichigan, we observed excellent tropospheric DXconditions here in Lexington, Kentucky. Strong FM andTV signals out of Minnesota, North and South Dakota,Iowa, and Illinois were plentiful throughout the day. Signals from 800-900 miles were common.


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On September 6, 2001, a full 24 hours later, the sluggishhigh pressure zone had moved only as far as westernNew York. Here in Lexington, our DX zone had expandedeast. The 800-900 mile TV and FM signals from theMidwest were still present, but strong signals from theNortheast as far away as central Ontario were also addedto the mix. This excellent DX "opening" lasted almost afull 48 hours.

In the northern hemisphere, the strongest signals andlongest signal paths will usually be observed to the southof the high-pressure center. In the southern hemisphere,the reverse is true -- the best signal paths will be to thenorth of the high-pressure center. Subsidence ducting isoften intensified during the evening and early morninghours when the effects of radiation inversions are addedto the mix.

Well positioned warm and cold fronts sometimes bring


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about ducting and enhanced DX conditions.

A warm front is the surface boundary between a mass ofwarm air flowing over an area of cooler, relativelystationary air. Enhanced DX conditions will often beobserved out to approximately 100 miles ahead of theadvancing front. The best paths will be along a lineparallel to the frontal boundary.

Likewise, cold fronts can also produce some nice DXconditions. A cold front is the surface boundary betweena mass of cooler air that pushes itself under a mass morestationary warm air. This forces the warm air up andbehind the advancing front. The ducts produced by apassing cold front are often unstable. The best signalpaths will be behind and along a line that's parallel to theadvancing front.

On November 9, 2001, this cold front and well-positionedhigh-pressure zone (over southeast Kansas) produced a


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full day of outstanding tropospheric FM and TV DX herein Lexington. We were solidly open to Arkansas,Alabama, Louisiana and Mississippi. Path distances wereupwards of 600 miles with very strong signals.

On December 7, 2001, this well positioned cold frontproduced excellent DX paths into eastern Tennessee. This very geographically selective opening didn'tproduce very long signal paths, but even low powerstations were heard with very strong signals.

Since the tropospheric enhancements we've covered sofar are all weather related, you can see why it'simportant for the DXer to pay attention to day-to-dayweather conditions.

Sporadic E - This is probably the most interesting andexciting forms of signal enhancement for the FM and TVDXer. Highly ionized patches or "clouds" occasionallyform in the E region of the ionosphere at altitudes


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between approximately 50 and 70 miles. We call thesesporadic E clouds. Sporadic E clouds are usually fairlysmall in size, but larger clouds or multiple clouds oftenform during substantial openings. These clouds often,but not always, travel from their point of origin to thenorth and northwest at speeds up to several hundredmiles per hour.

It's interesting to note that after almost 70 years of studythe true cause for sporadic E is still unknown. There aremany different theories as to how and why sporadic Eclouds form.

It was once believed that the formation of sporadic Eclouds was directly related to the eleven year solar(sunspot) cycle. You'll still see that theory expressed insome text books even though overwhelming evidencesuggests that this belief is wrong. There seems to be nocorrelation between the ionization level or formation ofsporadic E clouds and the eleven year sunspot cycle - atleast not in the mid latitudes away from the geomagneticequator and poles. It was noted all the way back in the1930s and 1940s that the formation and intensity ofmid-latitude sporadic E clouds does not substantiallyvary over the course of the eleven year solar cycle.

There is evidence to suggest that the primary cause ofsporadic E cloud formation is wind shear, a purelyweather-related phenomenon. Intense high altitudewinds, traveling in opposite directions at differentaltitudes, produce wind shear. It is believed that thesewind shears, in the presence of Earth's geomagneticfield, cause ions to be collected and compressed into athin, ion-rich layers, approximately one-half to one milein thickness. The area of these patches can vary from afew square miles to hundreds or even thousands ofsquare miles.


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Along the same line is the theory that sporadic E cloudsare formed in the vicinity of thunderstorms by theintense electrical activity associated with the storm. There is often (but not always) a correlation betweenthunderstorm activity and the formation of sporadic Eclouds, enough to make this theory very tantalizing. However, strong thunderstorms often form along frontalboundaries, and intense wind sheer is usually foundalong the same frontal boundaries that producethunderstorms. Likewise, strong sporadic E activity oftenappears when there is no apparent thunderstorm activityalong or near the propagation path.

Yet another emerging theory suggests that sporadic Eclouds are formed by concentrations of meteoric debris. Again, there seems to be a strong correlation betweenmeteor shower activity and the number and intensity ofsporadic E clouds.

The point is, nobody has presented a definitiveexplanation for how and why sporadic E clouds form.There are many excellent papers on the subject. Justenter "sporadic E" into your favorite search engine, andstart reading. It's entirely possible (perhaps even likely)that sporadic E clouds are formed as the result of acombination of factors, perhaps involving wind shear,cosmic debris and thunderstorm activity.

The amount by which the path of a radio signal isrefracted by sporadic E clouds depends on the intensityof ionization and the frequency of the signal. For a givenlevel of ionization, the signal refraction angle willdecrease as the frequency is increased. Above a certaincritical frequency, refraction of the signal will beinsufficient to return it to the surface of the Earth. Thiscritical frequency is known as the Maximum UsableFrequency or MUF.


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Sporadic E is very common on the low VHF TV channelsduring the summer months. From time to time, theintensity of Sporadic E cloud ionization increases to thepoint where the MUF rises into and sometimes above FMband frequencies (88 to 108 MHz). It is common for theMUF to rise up to and then stop at a particular frequencywithin the FM band. Distant signals will be heard belowthe MUF, while only local or tropospherically enhancedsignals will be heard above the MUF. It has beenobserved over the years that the signal strength ofreceived sporadic E signals will be greatest just belowthe Maximum Usable Frequency. Also, since the bendingangle (angle of refraction) decreases as signal frequencyis increased for a given ionization level, we can surmisethat the most distant receptions will occur as weapproach the MUF. In other words, an Es cloud willsupport longer signals paths at 100 MHz than it will at 50MHz.

The above illustration shows an actual Es cloudconfiguration and the associated skip zone that occurredduring the summer of 2001. Three different DXers are


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represented by the numbers 1, 2 and 3. The Es cloudwas over Eastern Kansas and Western Missouri. Theyellow band is the DX zone. Using the same sporadic Ecloud, a DXer in one part of the country will hear oneassortment of stations while a DXer in a different part ofthe country will hear a completely different set ofstations. If the DXers were to plot lines between theirrespective locations and the stations they were eachhearing (as I did here), the approximate location of thesporadic E cloud will be above where the lines intersect. You will note that both the DXer and the stations beingreceived are in the yellow shaded DX zone. DXersoutside the yellow shaded zone did not benefit from thisparticular Es cloud.

DXer #1 (me), located in Lexington, KY, heard stations inNew Mexico and Colorado. DXer #2, in West Texas heardstations in Wisconsin and Michigan. DXer #3, located inWestern South Dakota heard stations in Mississippi andAlabama.

Es signal paths are usually bi-directional. In other words,if the DXer in Kentucky is hearing FM stations inColorado, a DXer in Colorado will be able to hear stationsin Kentucky.

Since Es clouds often move with respect to the receivingstation, the DXer will often hear a changing selection ofdistant signals.


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Various geometries of Sporadic E SignalPropagation

This illustration shows three sporadic E clouds. Cloud #1is more intensively ionized, and is thus capable ofrefracting signals at a sharper angle, producing a shorterskip distance for a given frequency. Signals beingrefracted by Cloud #2 are returned to Earth at a lesserangle, thus producing longer skip distances. With clouds#2 and #3 in alignment along the signal path,"double-hop" skip can occur. With this cloud alignmentsignals from both the "transmitter" and the "Single-HopZone" would be heard at the receiver location.

A sporadic E cloud producing short to medium distanceskip at lower frequencies (TV channel 2, for example) islikely to produce longer skip at FM frequencies if the MUFis high enough.

The maximum distance for a single-hop sporadic Epropagated signal is approximately 1,500 miles. However, if multiple, sufficiently ionized patches exist ina line along a particular signal path, it's possible for agiven signal to reflect off the surface of the Earth afterthe first hop and get refracted back to Earth by a secondsporadic E cloud. This can extend the range of E-layerpropagated signals out to 3,000 miles and beyond.


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Statistically speaking, the "average" skip distance forsporadic E propagated FM DX seems to be between 950and 1,050 miles. During 2001 I received 205 stations viasporadic E (a decent statistical sample). The averagedistance of these receptions was 997 miles.

Single-hop E-layer propagated signals are often as strongas local signals. Indeed, I have witnessed more than onesituation where a local station was completely "covered"by a distant one, only a few miles from the local station'stransmitter location (that's when they call the engineerto see if he can "fix" the problem). Since the surface ofthe Earth is not a very good signal reflector, multi-hopE-layer propagated signals will usually be weaker thansingle-hop signals, and are often covered by signalscoming from stations in the single hop zone. If themid-point of a double-hope happens to be on water (suchas the ocean), the signals will be stronger and the therewill likely be no interference from mid-point stations(unless someone happens to be operating an FM or TVbroadcast station aboard a ship!).

Sometimes we hear stations via Sporadic E that don'tseem to fit the normal model in terms of path distance. It's not uncommon to receive signals beyond the rangeof what would be considered "normal" for a single hop,but at less than the range expected for "normal" doublehop.

Many theories have been advanced to explain thisphenomenon, including paths along multiple, tiltedsporadic E clouds. Here's an illustration of how thismight work.


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Geometry of "Tilted" Es Cloud-to-Cloud SignalPropagation

Based on both Earth and satellite based ionosondereadings and readings from rockets sent through Esclouds, it is known that tilted Es clouds do form. As such,this theory does provide at least one plausibleexplanation for longer than "normal" Es signalpropagation paths.

I think there is another, simpler means by which Essignals are propagated longer than "normal" distancesout to almost double-hop distances. This would alsoaccount for receptions where signals from double-hopdistances are received without the usual interferencefrom stations in the single hop zone.

Geometry of "Non-Tilted" Es Cloud-to-Cloud Signal


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Propagation

In this illustration, neither Es cloud is sufficiently ionizedto return a single-hop signal to Earth. However, with thetwo "weak" clouds working together, the refractionangles of each Es cloud are essentially added. Thiswould have the effect of raising the apparent MaximumUsable Frequency and ultimately returning the signal toEarth at a greater than "normal" Es distance. This is asomewhat more simple (thus more likely) Es cloudconfiguration than that of the "tilted" cloud theory. Itwould account for variable path distances which fallbetween that of "normal" single- and double-hopsporadic E path distances.

In theory, a similar configuration could exist with three ormore Es clouds, producing much longer signal paths. However, as the complexity of the signal path geometryincreases, the likelihood of such configurations formingand becoming usable diminishes.

Other Es cloud configurations are certainly possible. Picture, for example, a larger sporadic E cloud, which isnot uniformly ionized...

Possible Path Geometry of a Large Es Cloud withNon-uniform Ionization


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A signal entering the "weaker" side of the large Es clouddoes not return to Earth. Instead, it is propagated to apart of the Es cloud that is more intensely ionized. Ionization in this region of the Es cloud is sufficient toreturn the signal to Earth. The effect of such aconfiguration is not fundamentally different than thetilted cloud or cloud-to-cloud examples presentedabove. It is another possible means by which Es signalscan be propagated longer than "normal" single-hopdistances.

In the northern hemisphere, seasonal sporadic E seasonpeaks occur during the months of May, June and July. Anadditional minor peak often occurs in late Decemberaround Christmas time when the Sporadic E season is atits summertime peak in the southern hemisphere. Onetheory to explain this phenomenon is that intensesporadic E clouds formed in the vicinity of the equatormanage to hold their configuration as they drift towardthe north and northwest, thus producing our shortDecember sporadic E DX season. The best time of dayfor sporadic E seems to be mid morning and midafternoon. However, sporadic E DX can happen atanytime, day or night, and can pop up any time of theyear. Sporadic E DX usually lasts from a few minutes toa few hours. However, I've seen it last several full daysand nights, causing a great lack of sleep!

Aurora Effect - During periods of high solar andgeomagnetic activity, aurora or "northern lights" may bepresent. FM signals can be returned to Earth from theauroral curtain. However, the constantly varyingintensity of the aurora and its highly variable reflectivitygive auroral propagated signals a fluttery quality. Theflutter will usually be in the range of 100 Hz to 2,000 Hz,


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producing a "buzz" in the received signal. In somecases, this effect can be so strong, normal voice or musicmodulation ends up becoming distorted to the point ofunintelligibility. In the northern hemisphere, auroralpropagated signals will generally come from the north,regardless of the true direction of the transmittingstation.

Look for aurorally-propagatedsignals following major solaractivity, or the announcement thatvisible northern lights will be seennear your area. Obviously, thefurther south you live, the lesslikely you are to hear auroralenhanced radio signals. The bestplace for auroral effect in theUnited States is in Alaska or theNortheastern states. It'll be rarebelow latitude 32 in the Southeastand latitude 38 to 40 in the Westand Southwest.

The theoretical maximum distance for Auroral enhancedsignals is about 1,300 miles. 200 to 800 miles is typical. High quality, very sensitive, receiving equipment isrequired for this DX mode.

Meteor Scatter - This interesting form of enhancementresults from signals bouncing off of the intensely ionizedtrails of meteors entering and "burning up" in the Eregion of the ionosphere. The strength and duration ofmeteor scatter signals decreases with increasingfrequency. Thus, the effect is much more pronounced atthe lower FM band frequencies than at the upper end ofthe band. Meteor scatter can be heard anywhere,


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anytime of the day or night. However, bursts are moreplentiful around dawn, and during known major meteorshowers.

The radio signal returns to Earth after bouncing off themeteor trail.

Meteor scatter is characterized by a sudden, short burstof a distant, strong signal. The length of the burstdepends upon the length and intensity of the ionizedmeteor trail. They can be as short as a fraction of asecond, producing a short "ping" at the receiver. Largermeteors, or meteors entering atmosphere at a glancingangle, have been known to produce signal bursts lastingup to several minutes. If you try, you'll hear manymeteor scatter signals. However, you have to be lucky


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to actually catch a station identification at exactly thesame instant the meteor burst occurs. High qualityequipment helps, but meteor bursts can even be heardon the "average" car radio if you know what to listenfor..

Summary - There are other, more esoteric signalpropagation modes that are often at work to enhancelong-distance reception of FM signals. Signals bounce offof airplanes and even formations of birds. With the rightequipment, it's even theoretically possible to recoverbroadcast signals bounced off the surface of the moon! However, the propagation modes outlined above are themore common ones you are likely to encounter while FMDXing.

Note: All the propagation mode illustrations shown here,are obviously not drawn to scale. The signal "bending"angles shown are very exaggerated. In reality, they areusually fairly slight angles.


30 of 30 15-07-2015 20:36

the basics of vhf and uhf signal propagation · the frequencies of interest to the fm and tv dxer...

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