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Author Topic: Keep your ground connections short if you want them to work  (Read 283 times)

Offline ChrisSmolinski

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Keep your ground connections short if you want them to work
« on: November 21, 2020, 1719 UTC »
Earlier today I got an email from an SWL with a KiwiSDR & AirSpyHF+. He was complaining about very high noise / RFI levels, even though he lives in a rural area much like myself (he was comparing his noise levels to what he sees when using one of my online KiwiSDR receivers).

We exchanged a few emails back and forth, so I could understand his setup. He has a 80 ft T2FD and 500 ft sky loop antenna, so comparable to some of my antennas. I asked him if the coax shields were grounded, he said yes.  OK, I asked how.

Quote
"I have a very nice ground it is a 8-ft rod into the ground and it is connected with the 12 gauge copper wire to the coax cables."
I explained that the connection from the ground rod to coax shield needs to be as short as possible. Ideally a few inches. How long is it? If it is many feet long, the inductance will be high, and it will not work as a good ground connection.

Quote
"Apx 12 feet".
Hmm... So he shortened that to 9 inches, and reported the RFI levels dropped down to as low as if no antenna was connected to the radio. Ideal background noise levels.

He was pretty happy  ;D

So the moral of the story is, if you want your ground to work, you need to keep the connection to it as short and low impedance as possible.
« Last Edit: November 22, 2020, 1642 UTC by ChrisSmolinski »
Chris Smolinski
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Offline ThaDood

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Re: Keep your ground connections short if you want them to work
« Reply #1 on: November 21, 2020, 2119 UTC »
Thus far, the best GND conductors that I've used are trashed coax cable outer braids. You've got contaminated RG-8, or some other type of coax, use them as GND conductors. Might surprise you upon how well that can work, especially 5MHz and way under. Cheaper than 6" wide copper strip conductors.
I can't decide upon what's worst, young and stupid, or old and chemically dumbed down.

Offline NJQA

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Re: Keep your ground connections short if you want them to work
« Reply #2 on: November 22, 2020, 1622 UTC »
The biggest contributor to poor ground systems is failing to understand/comprehend *why* we are grounding.   You may be grounding something for lightning protection, RFI/EMC, electrical safety, EMP, etc. - but we still call it “grounding” even though the requirements and methods may be totally different (and possibly at odds with each other).

For instance, the NEC says that an electrical safety ground is not required to use larger than #6 wire....but that would be totally inadequate for grounding a tower leg.  The electrical safety ground wire is sized to ensure sufficient current flow so that safety devices like fuses and circuit breakers will function as they should.  The tower ground wire is supposed to carry most of the lightning surge current to ground.

RF grounds are very much frequency dependent and as Chris points out you have to pay attention to how long they are.

You can’t lose track of legal requirements.  It is possible to install something that is fully code compliant (and kosher with your insurance company) but doesn’t fix your problems, and vice versa.

There is a wealth of excellent and free military, FAA, and NASA pubs on the internet that discuss this in depth...so many that it will overwhelm you.

It is no wonder so many people have problems with grounding....but the first step to success is understanding WHAT you want to accomplish and the WHY of what you are doing.


Offline RobRich

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Re: Keep your ground connections short if you want them to work
« Reply #3 on: November 23, 2020, 1614 UTC »
On a related note and probably as a last resort, a tuner can be used to create a pseudo-"tuned counterpoise" for those in difficult to near impossible RF grounding situations.

The MFJ-931 comes to mind as an off-the-shelf solution:

https://mfjenterprises.com/products/mfj-931
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