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Author Topic: RF Ground 6 meters???  (Read 2629 times)
KA9TZU
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Posts: 52




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« on: March 29, 2011, 05:20:15 AM »

We live on the river. I moved my shack from the ground floor that had a standpipe just out the window that went down to our deep well ground a ground to the second floor with 0 access to anything remotely usable as a ground as far as I can tell. The well pump is more then 30' away down stairs. There is a cold air return and a heater vent that go to on outside furnace/A/C unit and the electrical ground on the outlets. All my radios, amps and equipment ground to one copper bar behind the equipment table. I ran my Dentron MLA 2500 at full power on the last 160 and 10 meter phone contest with very little issues.

I just purchased a King Conversion 1 kilowatt 6 meter amplifier directly for m Lou. Now the problem start on 6 meters. With the amp at even 250 watts, I broadcast through anything with a speaker. Lou assures me its a ground issue as does Dale at Par who's antenna I use. Using 468/f, the longest ground wire I can have is around 9' and I am not sure if that includes the 4' bar copper bar that everything is grounded to. The MFJ-931 Artificial RF Ground is not an option for two reason. First is only works to 30 MHz and second it's only rated to 300 watts. So my big question is, Now What???
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KD8GKR
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« Reply #1 on: March 29, 2011, 06:07:29 AM »

Isn't Par Omniangles only rated for 200 watts? The Stressed moxon is rated for 1000 watts. Have you tried unhooking the ground and see if the problem still exists. Does it do it when transmitting into a dummy load.
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K5LXP
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« Reply #2 on: March 29, 2011, 06:56:02 AM »

I don't think you understand what purpose a ground is serving here, and frankly I don't think your advisors do either.

If you're RFI'ing anything, it's from fundamental overload.  If your antenna is close enough and/or the affected equipment susceptible, no amount of grounding anything is going to fix it.  If you have RF on your chassis and safety ground, you need to address that, whether it's from common mode energy from the antenna, radiating feedline, or something not right with the amplifier. 


Mark K5LXP
Albuquerque, NM

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WB6BYU
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Posts: 13341




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« Reply #3 on: March 29, 2011, 10:17:32 AM »

Quote from: KA9TZU
..With the amp at even 250 watts, I broadcast through anything with a speaker. Lou assures me its a ground issue as does Dale at Par who's antenna I use.

No, an RF ground won't help.  The problem is that the speaker leads are close to resonance
act as an antenna, picking up RF and turning it into audio.

Where is the RF radiating?  If it is from the antenna (as it should be), then an RF ground won't
help at all, since in the best case all the RF would STILL be radiated from the antenna.

If not, then the RF is either being radiated from the coax or the power cable.  An RF ground
can change the currents on the coax if there isn't a balun at the feedpoint, but it is just
as likely to make things WORSE as to make them BETTER.  If the problem is RF on the power
cables, a ferrite choke and some bypass capacitors is a better choice, as the RF ground may
not make any difference to that, either.


Quote
Using 468/f, the longest ground wire I can have is around 9' and I am not sure if that includes the 4' bar copper bar that everything is grounded to. The MFJ-931 Artificial RF Ground is not an option for two reason. First is only works to 30 MHz and second it's only rated to 300 watts. So my big question is, Now What???

468 isn't the proper number to use here:  a half wave ground wire will actually provide a
reasonable RF ground, but the center of it will act like an antenna in the process.  A quarter
wave wire (which should be 246/f, since the 468 includes a correction for end effect where
the insulators are attached) will invert the impedance, so putting a good ground at the far
end gives a poor ground at the rig and vice versa.  While a quarter wave open wire will act
like a ground radial and provide a low impedance at the rig, it will still radiate like an antenna
if there is sufficient current flowing on it.  So really you want something shorter than, say,
1/10 wavelength, made of flat copper sheet rather than wire.  That is rarely practical.

The MFJ "Artificial Ground" only works reliably on wires less than perhaps 3/8 wavelength
or so if they are open at the far end, and all it can do is make them look like 1/4 wave
wires - they will still radiate like an antenna.  If the ground wire presents a high impedance
at the shack end, no amount of reactance cancellation will help.  You could build your own
6m version of the circuit, but it won't work any better than a 54" wire connected to the
ground lug on the back of the radio.


Now what?  Make sure you have a balun on the coax, and that there isn't any common mode
current.  Wrap the power cord several times though a couple of snap-on cores.  If that
doesn't fix the problem, either add RF bypassing or chokes to the offending speakers or
replace the devices with ones that have proper filtering.
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KM3F
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Posts: 509




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« Reply #4 on: March 29, 2011, 04:44:00 PM »

There is no real RF ground.
Proper Antenna installations are complete unto themselves.
Example a dipole out in space.
You just greatly increased power on 6 meters making the field radiation off the antenna more intense in the area.
Grounding does not usually clear this condition.
Bypassing, shielding moveing the equipment having the issue or  moving the antenna is the best solution.
One item it be sure of is making the match at the antenna less than 3 to 1 VSWR.
As insurance to keep outer shield return current from getting back to the radio area is by making a coax coil of 3 ro 4 turns on a 4" diameter just after the antenna feed point.
Good luck.
« Last Edit: March 29, 2011, 05:03:06 PM by KM3F » Logged
K5LXP
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« Reply #5 on: March 30, 2011, 11:27:09 AM »

> One item it be sure of is making the match at the antenna less than 3 to 1 VSWR.

Why would that matter?


Mark K5LXP
Albuquerque, NM
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WY3X
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Posts: 768




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« Reply #6 on: March 31, 2011, 01:53:43 PM »

One thing I didn't see anyone mention is distance. How far is the antenna from the devices you are interfering with? If you are using a short pole to the peak of your roof, and just a few feet above that sits your antenna, it's TOO CLOSE TO YOUR HOUSE! Not only will you increase the chances of interfering with other devices, at the kilowatt power level on 6M you also stand a good chance of exceeding the RF safety rule!

You need to get your antenna 50 or 75 feet away from your house and get it up over 50 feet. Yes, this probably will require a tower. If you're going to run a kilowatt amp and play with the big boys, the expense of a proper tower should not be an issue. If you want a cheap way to "test" this to see if it helps, look for a tall tree about the right distance from your house and figure out a way to hang your antenna from a rope at that distance. (Make sure the rope doesn't come in contact with the radiating surface of your antenna.) I'd be willing to bet this will go a long way toward curing your issue. The reason this helps is that distance will lower the field strength of your signal inside your house, and lowering the field strength will most likely cure a lot of your problems.

73, -WY3X
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