grounds - archive.nsarc.caarchive.nsarc.ca/hf/hf_min112708.pdf" conductor resistance very small...

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NSARC HF Operators 127 Nov08

HF OPERATORS

GROUNDSby

John WhiteVA7JW

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WHAT GROUND?

! EARTH GROUND

! POWER GROUND

! SAFETY GROUND

! CIRCUIT GROUND

! RF GROUND

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EARTH

! Earth is whatever the ground is." soil, rock, gravel, sand, water, clay, concrete, muck …

! Not an insulator, just medium to high resistance

! Think of each small volume of earth as a high value resistor

! All in parallel – more resistor, less overall resistance

! Total resistance can be small as volume increases

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EARTH “RESISTANCE”

• Ground resistance is measured as Resistivity• Values are given in ohm-meters

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EARTH “CONDUCTANCE”

! ARRL Tables ! Conductance in Siemens S = 1/R in S/m

! Other than salt water, earth is not good for antennas! S = 5, R = 0.2 ohm-m 10X better than anything else

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EARTH GROUND

! Rods, Wires, Plates commonly used" Conductor resistance very small

! Resistivity of the earth is usually negligible because there so much of it available to carry current

• The limiting factor in electrical grounding systems is how well the grounding electrodes contact the earth, which is known as the soil / ground rod interface

" 1 ohm excellent" 10 ohms good" 100 ohms typical" 1000 ohms not good

! http://www.newarkinone.thinkhost.com/brands/promos/Earth_Ground_Resistance.pdf

Equipment

Earth

Rod

Wire

10 -100 ohms

0.01 ohms

0.01 ohms

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POWER CIRCUITS

! AC Line - Equipment Power Supply – Transmitter

! Power ground is earthed

TRANSMITTERCIRCUITS

POWERSUPPLY

ServiceEntrance Power

Transformer

WallSocket

LineCord

Line (black)Neutral (white)Ground (green)

14 kV

Power LineTransformer

120VAC

0VAC

black

bare

white

EARTH GROUND

CIRCUIT GROUND

RF

TRANSMITTER EQUIPMENT

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AC POWER CIRCUIT

! Hydro Transformer " primary side 14kV" return side earthed

! Secondary Side" 120VAC residential" neutral earthed

! Residential Ground" bare copper wire " earthed" third prong of plug" connects chassis

Chassis connection

ServiceEntrance

Tr

WallSocket

LineCord


14 kV

Power LineTransformer

120VAC

0VAC

black

bare

white

EARTH GROUND

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SAFETY GROUND

! Chassis connection is Safety Ground

! GREEN WIRE in cords and ground systems

! Protects against insulation failures

! Prevents YOU from becoming part of the BC Hydro power line circuit from Line (HOT) to Earth

! Do NOT disable this connection

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SIGNAL CIRCUITS

! Power transformer isolates direct connection to Hydro! Secondary to Power Supply

! AC to DC Operating voltages! All circuits have a common conductor! Circuit Ground

" low DC resistances" low AC impedances" common for all AC signals – audio , RF, control etc

TRANSMITTERCIRCUITS

POWERSUPPLYPower

Transformer

LineCord


CIRCUIT GROUND

RF


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CIRCUIT GROUND

! RF output of all modern transceivers is via SO-239

! Note RF connection to CHASSIS

! Output RF is developed between circuit / signal ground

! Has meaning INSIDE transceiver

! What about outside ?

! Is there an RF ground outside ??

TRANSMITTERCIRCUITS

CIRCUIT GROUND

RF

SO-239

CHASSIS

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GROUND SUMMARY

! Earth / Power Ground systems" equipment safety to prevent electrocution " operates at 60 cycles" long leads " engineered for low resistance

! Circuit / Signal Ground (inside equipment)" common signal path for all circuits" resides inside equipment" operates at kHz / MHz / GHz" short leads" engineered for low impedances

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QUESTION (Dave VE7DKR)

Q: In a radio station setup, what sort of a connection is made between the ground rod, or plate, and the soil/ earth. Is it:a) resistiveb) capacitivec) inductived) or a combination

1. All components are resistive – wire, rods, earth2. Voltage and current are in phase3. Therefore it is a resistor to earth, and earth is a big resistive medium of

tremendously varying value

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DOES RF GROUND EXIST?

! Here’s K9YC, Jim Brown’s opinion on the matterhttp://audiosystemsgroup.com/RFI-Ham.pdf

Ground Wiring: Some hams like to think of the earth as if it were somehow a "sink" into which all noise can be poured, never to bother us again. Indeed, you'll find lots of bad advice to solve RFIproblems with "a better ground." In fact, nothing could be further from the truth. An earth connection is rarely part of a solution to RF or noise problems.

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EARTH GROUNDS for RF?

! NO

! Earth Grounds are resistive and lossy

! OK at 60 cycles, but not suitable for RF

! RF means frequencies at MHz and GHz

! Grounding wire lengths comparable to RF wavelengths

! Wires connecting equipments and power systems to grounding systems present complex impedances, not low resistances

! Impedances are indeterminate

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But I NEED AN RF GROUND

! Problems with RFI ?

! RF in the shack - RF tickles and nips off station equipment

! Noise, Xtalk, Coupling etc ?

! Addition of a “ground” may change performance but doesn’t address the problem

! Changes to system impedances due to wavelength effects of ground wire simply move voltage and current nodes around

! Need to look elsewhere for root of the problem ….

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HERTZIAN ANTENNA

! Very FIRST antenna

! Heinrich Hertz – 1880

! Demonstrated ability to transmitand receive electromagnetic waves

! Balanced system

! No “grounds”

! The early Dipole

BALANCED DIPOLE

BALANCEDTRANMISSION

LINE

TRANSMITER

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MARCONI ANTENNA

! Earliest form of commercial antenna" Marconi Spark transmitters" operation well below 1 MHz" 1 wavelength >> 300 meters

! Hertzian dipoles not practical

! Operates against Earth Ground" earth is the “other half” of the antenna circuit

! Earth is not a good ground " salt water is unique and excellent" wire radials

VERTICALANTENNA

EARTH GROUND

TRANSMITER

Courtesy 1918 Wireless Telegraphers handbook

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SHIELDING

! RF is generated inside a metal box (the rig) ! RF is contained inside the cabinet

" conductive enclosure prevents radiation outside the box! Only (engineered) way out is through the Ant Jack > Coax! Coax contains RF between center conductor and inside

surface of braid

! Blue “fill” indicates area ofRF signal and is containedby the shielding effect of the cabinet and coax

! RF currents flow on SURFACE of conductors, not THROUGH


TRANSMITTERCIRCUITS

POWERSUPPLYPower

Transformer

CIRCUIT GROUND

COAX

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DIPOLE ANTENNA

! Balanced (dipole) antenna looks like a 50 ohm resistor! R is the radiation resistance! Circuit is complete – no ground required

! Power Safety ground is connected from chassis to earth but plays no part in the RF circuit, coax or antenna system

BALA

NC

ED A

NTE

NN

A


TRANSMITTERCIRCUITS

POWERSUPPLYPower

Transformer

CIRCUIT GROUND

COAX

Equipment Safety Ground

RADIATIONRESISTANCE50 ohms

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BAL ANTENNA with ATU

! Balanced Antennas with Antenna Tuner Unit ! Balanced line contains EM field (non radiative) ! No ground system required


TRANSMITTERCIRCUITS

POWERSUPPLYPower

Transformer

CIRCUIT GROUND

COAX


BALA

NC

ED A

NTE

NN

A


BALUNATU

ANTENNA TUNER

BALANCEDLINE

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VERTICAL ANTENNA

! Radiation resistance of vertical antenna ~ 37 ohms ! Ground resistance appears in series with radiation R

" 8 ground rods / radial system maybe ~ 20 - 40 ohms wet" 1 ground rod > 100 ohms

! Antenna efficiency suffers / ground currents flow on coaxTRANSMITTER EQUIPMENT

TRANSMITTERCIRCUITS

POWERSUPPLYPower

Transformer

CIRCUIT GROUND

COAX


GROUNDRESISTANCE100 ohms

VERTICAL ANTENNA


RF current flowon outside of coax &

equipmentsbetween grounds Use ferrite on coax

to impede braid currents

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LONG WIRE ANTENNAS

! Use with an antenna tuner! ATU has to be connected to station ground (whatever that is)

! Ground wire is acting as a counterpoise is main component! Ground may be a minor component! RF currents flowing in shack can be a major issue

ANTENNA TUNER

Random Long Wire

Ground Wire

RADIATIONRESISTANCEUNKNOWNBALUNATU

ANTENNA TUNER

Station Ground

Jum

per

GROUND RESISTANCE100 ohms

XREACTIVECOMPONENT

Ground currents

COAX

COAX

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LONG WIRE ISSUES

! Excerpts from Practical Wire Antennas, John Heys, G3BDQ

! Advises counterpoise wires are absolutely necessary! Short counterpoises can be effective Google AA5TB

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RF GROUND FOUND !

! A, second line “.. Lack of an RF ground”

! It is not Earth

! It is a COUNTERPOISE

! RF Ground is a misused description for the missinghalf of vertical and long-wireantennas

Courtesy of ARRL QST Jan 06

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Summary

! The Station Ground is a Safety Ground to Earth

! Circuit Grounds exist inside equipment

! RF Ground means COUNTERPOISE

! Some housekeeping rules to keep RF out of shack" Use counterpoise for “one sided” antennas" Ensure all signal leads are well shielded to control egress of RF from

Transceiver" Ensure all power leads are well filtered" Ensure all control leads, if not shielded, are bypassed" Employ Ferrites and Baluns on feed lines

grounds - archive.nsarc.caarchive.nsarc.ca/hf/hf_min112708.pdf" conductor resistance very small...

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