Help! RF on 40 meters

[W1UJR]

Hey fellows, I need some help with getting rid of a pesky RF problem.

The 30K-1 works jsut FB on 75, but has RF getting into the audio on 40 meters.

For some background, here is the set up
1) 30K-1 which has a balanced output. To make things easy to handle in the shack, I've driven one side of the output to the chasis, then used coax to feed the Johnson Matchbox.

2) Johnson Matchbox, coax in, balanced feedline out. I monitor the output balance with a pair of neon bulbs, output appears to be balanced and should cancel out feedline radiation.

3) Microphone, D-104, non-amplified unit.

On 40 meters I have RF getting back into the audio. I use a 1/4 phone jack on the mic cable.
The ground braid is connected to the mic body, and the two inner leads are used for T/R switching and audio feed.

I have not RF getting into the audio if the head is removed, but once installed it starts the squeal.
What's the first step here?

73 Bruce W1UJR

[K1JJ]

Bruce,

If normal shielding and grounding doesn't do it, then use low pass R/C filters in each of your audio interconnect leads between audio boxes. These boxes usually have low 1K input and output impedances.

What always works for me is to use a 500 ohm to 1K resistor in series with each audio cable with a .001 to ground at each input and output audio plug. ie, go in with the resistor and then the .001 to ground.

On 10 meters, I find this to be important when running QRO.

If you use 1/4" plugs, you can fit a 1/4 watt resistor and tiny disc .001 inside the plug.  Depending on the type of microphone, (if low impedance, not D-104 xtal type) you can put one on it's output too.

*** This is a last resort method, since if you exhaust your shielding and grounding and moving things around possibilities, it will most likely be cured. This assumes you have shielded audio boxes to begin with and the RF in the shack is not an extreme case, like an end fed wire, etc. 

There really is no downside to this R/C method other than losing some audio gain (and maybe some unwanted stray RF) throughout the system, if the low pass cut off freq is above the 100Khz area. Just crank up the audio throughout the affected areas to compensate. 

As good measure, sweep the rig to insure the extreme audio highs are not affected.

Good luck, OM!

T

[WA1GFZ]

The RF on the mic cable shield (audio return) is getting on the center conductor. The head is completing the loop.  You could change to a different mike connector that does not have audio return on the shield or maybe  You could put an over braid shield over the cable and terminate it to ground on one end only. a little filter inside the plug will also help but you may have to filter the PTT also.
You are most likely in an electric field with a high Z mic. A PITA to fix. Filters with caps to ground could kill D104 high end.

[Jim KF2SY]

Another approach that may be easier in the long run.
See Cebik's section on "Series-Connected Coaxial Cables"
(a shielded paralell transmission line for the shack portion.)
I assume you are using a 80M dipole on 40.
If so, high impedace, difficult for tuner.


http://www.cebik.com/trans/spcoax.html

[WA1GFZ]

If your shack is close to the antenna changing the feed line won't do anything.
Running a single ended high Z mike with a return sharing the shield is tuff when there is a lot of rf floating around.

[flintstone mop]

Hi Bruce
It sounds like you will have to find a way to shorten the "antenna/Mic cable and or convert the mic to balanced audio. I had a similar problem with my system on 40M getting into a broadacast type audio console and it luckily turned out to be re-seating the XLR connectors  on the outpoot side of the board.
You can try wrapping around 10 turns of mic cable into a ferrite core. RF is nasty, but essential in our hobby.............hi
Fred