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Author Topic: Problems On 40m Due To Length Of Ground Connection?  (Read 1978 times)
AK4SK
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Posts: 205




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« on: September 13, 2017, 09:49:38 AM »

I have various problems on 40 meters, and only on 40m, when I transmit. I get slight RF burns touching the mic connector, issues with equipment connected to my PC such as the radio going into TX and staying keyed, USB ports disconnecting, and certain household electronics malfunctioning. The issues aren't always severe but occasionally they are.

My shack is on the second floor. I have a 2" flat copper strap that runs about 12' from my entrance panel to a ground rod directly below the panel. Inside I have a single point ground for all of my equipment. I have a choke on the inside of my entrance panel and a 4:1 balun on my OFC dipole and a 1:1 balun on my 80-40-20 fan dipole.

I think the problems are due to the length of the ground connection but I don't understand why. Is 12' a high impedance length for 40m? I think 12' is about 1/12th of a wavelength. I have the issues on both antennas. I believe that a 1/4 wavelength ground run would be bad, I think it would have a high impedance and radiate just like an antenna. But I wouldn't think 1/12th would not be an issue.

Any help understanding what is going on here is appreciated.

73 Chris AK4SK
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WB4SPT
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« Reply #1 on: September 13, 2017, 10:17:26 AM »

Hi Chris;   Describe the chokes.   Grounding is not the best method for dealing with CM currents.   Think how to reduce a current.   Would providing a lower Z path for a current increase it or decrease it? 

Better choking is usually the better solution; although I will be the first to admit with these things not all fixes are universal.  It did work for me and others. 
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AK4SK
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« Reply #2 on: September 13, 2017, 10:30:39 AM »

Hi Chris;   Describe the chokes.   Grounding is not the best method for dealing with CM currents.   Think how to reduce a current.   Would providing a lower Z path for a current increase it or decrease it? 

Better choking is usually the better solution; although I will be the first to admit with these things not all fixes are universal.  It did work for me and others. 

The inside chokes are two large type 31 (I think) ferrites with a length of coax coiled through them a dozen times or more. I forget the exact construction but I can figure it out later.

Why would I just be having a problem on 40m?

73 Chris
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KH6AQ
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« Reply #3 on: September 13, 2017, 10:34:33 AM »

Which antenna does the problem occur with?
How far from the house is the antenna?
Have you tried disconnecting the ground strap?

A 12' x 2" strap at 7 MHz has an inductive reactance of roughly 120 ohms. The resistive ground connection (ground rod) adds to this.
« Last Edit: September 13, 2017, 10:48:16 AM by KH6AQ » Logged
AK4SK
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Posts: 205




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« Reply #4 on: September 13, 2017, 11:21:45 AM »

The problems occurs on both antennas.

I'll have to try disconnecting the ground.

The antennas are right on the house basically. I have a 35' mast for their feed point and the mast is partially support by the house. The mast is about 10' down the wall from the shack so the antenna wires are pretty close.

Chris
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KH6AQ
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« Reply #5 on: September 13, 2017, 02:42:05 PM »

I'm wondering if the antennas are coupling RF into the house AC wiring and that that is the cause of the high RF potential on the equipment.

A fix I've had success with several times is to connect a 1/4 wavelength wire to the transceiver ground post. The wire is insulated (it's carrying RF current) and is routed around the room or out the window. The wire places a lower RF impedance at the transceiver chassis and can reduce the RF potential.

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AK4SK
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Posts: 205




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« Reply #6 on: September 14, 2017, 09:55:56 AM »

I'm wondering if the antennas are coupling RF into the house AC wiring and that that is the cause of the high RF potential on the equipment.

A fix I've had success with several times is to connect a 1/4 wavelength wire to the transceiver ground post. The wire is insulated (it's carrying RF current) and is routed around the room or out the window. The wire places a lower RF impedance at the transceiver chassis and can reduce the RF potential.



Thanks. I would think if that was the case it would happen on other bands other than 40m but there very well could be something going on there I don't understand (I obviously don't understand the problem). Some of the electronics that are bothered by the issue are kids toys that are battery powered. There is one that will start playing music.

W8JI has an interesting article here for grounding a 2nd floor shack: https://www.w8ji.com/second_floor_grounding.htm It is much more elaborate than yours but similar in concept I think. Whenever I get around to replacing the carpet in my shack I'll probably do that.

I may give what you suggest as try.

73 Chris AK4SK
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KB8FE
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« Reply #7 on: September 15, 2017, 04:29:24 PM »

I wonder what the length of your coax feed line is for each of the two antennas.

If they are close to a quarter wave for 40 meters (33 feet) then you might try to extend those to about 52 feet each and see if the problem persists.  The problem might only be for 40 meters because that is the only band for which your feed lines are about a quarter wave in length.

It just seems to me that your feed lines might be creating a quarter wave leg for 40 meters and your rig and grounding might be acting as the other half of the half-wave antenna.  This would place the maximum radiation at the rig.  This is just a thought as I only have what I have read in the posts to consider.

It also sounds like your mast is about a quarter wave long for 40 meters.

Just something to consider.
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KD8IIC
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« Reply #8 on: September 15, 2017, 11:56:27 PM »

  Add five or six feet of coax at the rig and see what you find. Quick and easy possible cure.
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KB8FE
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« Reply #9 on: September 16, 2017, 02:42:10 AM »

Also, stating the brand name and model of the 4:1 balun and the 1:1 balun might be helpful to the readers.

There are voltage baluns and there are current baluns and each has its proper application.
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