Signal level drops when I touch radio chassis or mic

[N3GBH]

Hello all - Forgive me if this seems like a newb question, but I'm a little stumped at this point.

When I touch the chassis of my radio or my microphone (which has a metal base), my background noise level drops about by about S1 on the meter (e.g. S5 background noise drops to S4, etc). Strong, clear signals don't seem to be affected or affected as much, e.g a 5 by 9 signal isn't dropping to 5 by 8, it's mostly noticable on weak signals or background noise. I've bee working 20 meters lately, if that matters.

Here's my radio configuration: Yaesu FT-2000, Yaesu MD-200A8X mic, grounded with a 8ga solid copper run approximately 20 ft long outside of the shack to a 8 ft long copper clad 1/2" ground rod. Antenna is a portable Superantenna MP1 clamped to an ungrounded metal table with counterpoise wires dangling around the deck, approximately 16 feet off the ground.

Any ideas? By touching the chassis/mic, am I in effect simply changing the tuning of my antenna somehow, or is this a ground issue?

Greg N3GBH

[KO6WB]

Hey Greg, the reduction in noise indicates you are either altering the RF ground or the rig is acting as part of the antenna. That would usually show itself as a burning sensation on the point of contact provided you're running enough power for that to happen.
What feedline are you using?
Where is the antenna located in relation to the rig?
The ground you have at present is adequate for DC and static purposes but can be very reactive as the frequency rises. Using counterpoises at the radio is one way to effect a RF ground. You could also use a tuner to get the RF ground on your rig. This would be called an artifical ground and it can be effective.
The choice of an antenna is an interesting one. Do you have antenna restrictions? There are a number of antennas that can out perform the MP1. That wouldn't be hard to do since the MP1 didn't set the bar that high. Still it's better than no antenna.
There are numerous antenna configurations that can give you much better operation but some of them would need greater funding. A simple dipole for 20 meters is only 33 feet long or so. With it up about 16 feet off the ground it can do well. Naturally higher would be better. A ground mounted 1/4 wave vertcal is only 16.5 ft high and it would be best if you could put down radials for it. About 16 to 32 would be good and you could make them about the same length as the vertical section. The radials can be shorter and still be effective and you can either lay them on the grass or bury them about 1-2" in the ground.
These are just suggestions and if can do them I believe you will be pleased.
Hope this helps
73
Gary

[KE3WD]

Antenna is a portable Superantenna MP1 clamped to an ungrounded metal table with counterpoise wires dangling around the deck, approximately 16 feet off the ground.

Any ideas? By touching the chassis/mic, am I in effect simply changing the tuning of my antenna somehow, or is this a ground issue?

Greg N3GBH

Hi Greg,

Very likely that this compromised antenna system is also letting RF onto the outside of the coax. While that is usually a recipe for RF problems when transmitting, it can also affect receiving, in ways you have noted.

Do you have some sort of Isolation (balun) at the antenna feedpoint?

Even a simple balun made from the coax, wrapped in a coil for a few turns and of the right diameter for the frequencies of interest can isolate the coax shield (and thus the radio chassis and ground system) from the offending RF.

However, it might also be the case where the balun can reduce the already inefficient performance of the compromised antenna system, simply because the coax shield is acting as more RF capture area for you.

And, yes, by touching any ground point at the radio, mic, chassis, etc. -- you are very likely changing antenna tuning somewhat. Since it REDUCES NOISE and not signal, it is also very likely that the coax shield and your grounding system are picking up some local noise from something.

A Balanced Antenna such as a dipole, along with a balun at feedpoint, would likely make hamming a lot more interesting for you, if you can erect same. If not, at least try the balun at feedpoint to the compromised antenna system.


73

[WX7G]

Attach a 16.5' wire to the GND post on the radio.

[N3GBH]

The choice of antenna is primarily due to extremely tight restrictions due to home owners association. I live in a townhome community, and we're not permitted to erect any external antennas of any type, I'm skirting around the restriction by using a "portable" temporary antenna. I may be able to set up a inverted V from the corners of my deck to the bedroom level above -- it would be almost invisible unless someone came right up on it, and even then - only another Ham would recognize exectly what it is.

My noise level in the middle of the night is pretty low on 20M, and I got a 5 by 9 clear transmission report from a contact in Slovenia a few nights ago. I'm pretty happy with my substandard, non-ideal antenna so far. I'll give the balun a try, maybe more counterpoise tuned for 20m and see what happens.

Thanks for the comments!

BTW feedline is a 75 foot piece of RG-8X coax. I could probably have gotten by with a 50 foot piece, but 25 was definitely not long enough.

Greg N3GBH

[KJ3N]

... grounded with a 8ga solid copper run approximately 20 ft long outside of the shack to a 8 ft long copper clad 1/2" ground rod.

I don't care if it is 8ga, that's not a ground. Lose it.

[KZ4Y]

greg,

Sounds like you have a loose ground connection somewhere. I, too, have HOA restrictions. However, I do have a 240 ft loop on the roof along with a 20M antenna and, soon, a 10M antenna up there. My compromise is hiding the coax behind the downspouts and gutters. So you are not alone and keep thinking outside the box, as it were.

Ken
KZ4Y