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Artifical Ground




 
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WA2SQQ
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« on: February 03, 2020, 01:41:01 PM »

So, during this weekend’s AM Rally I had an interesting conversation about these devices called “artificial grounds”. They are basically tuning the counterpoise of end fed antennas. So, I asked, “OK so how has it improved your signal or reception?” (no response).  “Will this device provide the same electrical protection that a “genuine ground” provides”? (no response) I tried to explain that in situations where a good ground was not available they can reduce the stray RF in  the shack, but they should not be considered as being better than a genuine ground. The scary part was that this station had a very good “genuine” ground and saw this device as an upgrade.
What am I missing, it seems that theses artificial grounds are the latest RF Voodoo.
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W1ITT
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« Reply #1 on: February 03, 2020, 09:31:51 PM »

Ground is not always RF ground and, at any given frequency, ground is not always where we think it is or where we want it to be.   A few years ago I built a swell multiband speech processor for SSB use.  It worked well and sounded good.  But on 15 meters, RF got into it.  After messing around with a second wide  copper ground strap from the station window to my outdoor ground system, which was extensive, I had to try something else.  I cut a quarter wave of hookup wire, around 11 feet long, crimped a lug on it to connect to the back of my speech processor and ran the wire up the wall and around the ceiling moulding.  That cured my RF problem on 15 meters and no ill effects were observed on other bands.  I had created an RF ground in mid air.
Those store bought artificial ground boxes are certainly more versatile, and they do have their place in some circumstances.  I suppose if I had copper flashing on my radio room floor and walls I wouldn't have had to worry.  And if RF didn't have a wavelength we'd all be at ground all across the bands.  I don't need any such thing in my current configuration but it's another trick in the toolbox of hams who build stations with less than commercial budgets, in less than optimum situations.  Build a real ground first, as best as you can, then do the artificial trick if necessary.  And if it's not really necessary, the only thing it will hurt is your bank account.
73 de Norm W1ITT
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KK4YY
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« Reply #2 on: February 06, 2020, 12:41:40 PM »

Feeding an end-fed antenna is like trying to push on a rope. You need something to push against to feed one. All too often, what's being pushed against is the outside of the coax cable transmission line. That puts RF on the outside of the cable which leads right back into the shack.

Proponents of end-fed antennas will usually loose their enthusiasm for them as soon they buy an amp for their 100W transceivers. The rude awaking of a bite on the lip from the microphone is their first clue. "Ouch! RF in the shack? I never noticed that before."

This is what I think is happening...
The end-fed wire that we see as the antenna is only half an antenna. The box we call an "artificial ground" is a tunable other half. If that box is in the shack, you're tuning to resonance the outside of your coax, to be that other half. A tuned transmission line of a sort.

For my money, the solution to an end-fed antenna is a balanced antenna. Short of that, an adequate ground system has to be used. This could be a ground plane for a vertical "end-fed" antenna like the roof of your car is with a 2M mag-mount on it. Or a gazillion radials buried under the lawn and an inverted L for 160M.

Given certain circumstances, sometimes one can only manage an end-fed zepp antenna. An artificial ground might help there, but still, you're leading a hard life.
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WA2SQQ
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« Reply #3 on: February 06, 2020, 03:04:04 PM »

So the way I do it is to feed the end fed with either a 4:1 or 9:1 Un-Un. Preceding the UNUn I have a big a*s choke. I have a ~ a 40ft counterpoise hanging off the unun, about 2-3 feet above the ground. 1200W and no RF problem in the shack.

I was told that with a counterpoise the choke wasn't required. I still use one.
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Patrick J. / KD5OEI
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« Reply #4 on: February 06, 2020, 09:46:10 PM »

Grounds and grounds!

No tuned ground or articifcial ground unit was needed, but a workaround was done.

When I used a GRC-106 (400W HF with built in antenna matcher designed for whip or long wire) with an end fed antenna, it liked to be seriously hot..  Inside the shack there was not a good RF ground, even though the metal building is grounded with a couple of rods. Building sits on a 6 inch thick slab copiously filled with rebar. I didn't want to try boring through the floor and hammering a ground rod in.

A fix was to leave the ground of that transceiver connected to the station ground (the previous attempt) but also attach a length of cable shield about 25 FT long and strew it around on the floor. Apparently this coupled it to the rebar enough to 'fix' the problem.

This also works with the KW sized AM transmitter. Although it drives a balanced line to an  dipole there was still some RF getting in to the audio circuits, fixed by the strewn cable.

No tuning of the 'ground' is required.
I also wonder if a storebought box would burn up under AM QRO conditions. The "3KW" MFJ tuner has.

Moot point now since the dipole is up, a balanced antenna. Still, a 25 FT length of coax braid cast out on the floor removed the remaining RF so that the mike or audio processor never has feedback. I could put carpet down to hide this but there is so much crap around that no one even would notice it.

The mike has a 20FT cable on it so I can stand in front of the transmitter with it when wanted, without RF problems.

YMMV.
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KK4YY
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« Reply #5 on: February 06, 2020, 09:51:56 PM »

So the way I do it is to feed the end fed with either a 4:1 or 9:1 Un-Un. Preceding the UNUn I have a big a*s choke. I have a ~ a 40ft counterpoise hanging off the unun, about 2-3 feet above the ground. 1200W and no RF problem in the shack.

I was told that with a counterpoise the choke wasn't required. I still use one.
I think your 'counterpoise' is a very good idea. That becomes the other half of the antenna. You've essentially created a dipole!

The UNUN and the choke make the feedline look like a high impedance and reduces RF from traveling on the outside of the coax. Well done.
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Steve - K4HX
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« Reply #6 on: February 13, 2020, 02:37:30 PM »

Artificial grounds or tuned counterpoises are not the latest voodoo. They've been around since the beginning of amateur radio. The more modern tuners for a wire counterpoise are mostly to help with RF in the shack problems. I used one with from the second floor of a townhouse and an end-fed wire many years ago. It worked FB.
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n4joy
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« Reply #7 on: February 13, 2020, 03:01:26 PM »

Artificial grounds or tuned counterpoises are not the latest voodoo. They've been around since the beginning of amateur radio. The more modern tuners for a wire counterpoise are mostly to help with RF in the shack problems. I used one with from the second floor of a townhouse and an end-fed wire many years ago. It worked FB.

I used the MFJ artifical ground as well and it worked fine--it has been around for a while.  Quoting K5KVH "Artificial ground tuners are series resonant circuits with a current sensing metering circuit so that you can tune for maximum current in the series circuit, an indication of optimum cancellation of the reactive ground lead, (resonance)." 

It is referenced in the April 1988 QST.

Interesting article about a home made artificial ground: https://www.qsl.net/on7dy/Documentation/Artificial%20Ground-GD4EIP.pdf
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Steve - K4HX
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« Reply #8 on: February 13, 2020, 03:36:27 PM »

Just to be clear, tuned counterpoises were used back in the spark days.
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R. Fry SWL
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« Reply #9 on: February 14, 2020, 08:14:39 AM »

... Quoting K5KVH "Artificial ground tuners are series resonant circuits with a current sensing metering circuit so that you can tune for maximum current in the series circuit, an indication of optimum cancellation of the reactive ground lead, (resonance)." ...

For consideration... below is a NEC4.2 analysis showing the performance effects of using an "artificial ground" to improve the radiation efficiency and reduce the amount of r-f in the shack from a transmit antenna system.

This NEC analysis shows that the use of a tuned/resonant, z-matched conducting path for the artificial ground conductors in this scenario does not produce "optimum cancellation" of radiation from that conducting path, but increases it.





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AG5UM
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« Reply #10 on: February 14, 2020, 10:06:08 AM »

My storebought MFJ 1982HP halfwave Endfed has a matching transformer and choke built into it right out of the box.

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AG5UM
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« Reply #11 on: February 14, 2020, 10:30:28 AM »

The Heathkit DX-100 manual has some interesting info. on grounding.
"an 8 foot wire from transmitter to ground would be a direct short at 160 meters but becomes
one-quarter wave length at 10 meters and therefore places the chasis at high RF potential."
"if the transmitter becomes hot at one particular frequency, the addition of a ground wire cut to one-half wavelength
at this frequency may clear up the difficulty." Etc. etc. etc.....1/4 wave radials, random lengths etc.
Much practical info. on page 58.
"Some symptoms of inadequate grounding are manifested in the form of :
1. Inability to load the antenna properly.
2. High RF potentials on the chassis or case of the transmitter.
3. undue voltage strain on output circuit components.
4. Final plate current reading affected by touching the panel or case."
There s much more info. in the manual.
AG5UM

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