Posted by
Don Wilhelm-3 on
Jan 22, 2007; 6:49pm
URL: http://elecraft.85.s1.nabble.com/Rf-in-the-shack-distorting-audio-tp443216p443225.html
Bob,
You counterpoises - are they 1/4 wave long at each of those bands? Are the
far ends isolated? Are the far ends seaprated for the various bands? The
answer to each of these questions should be yes.
It is difficult to achieve a good RF Ground with a long run to a ground rod.
At frequencies where the length of wire to the ground rod is 1/4 wavelength,
there will be a high impedance to RF at those frequencies - a 16 foot run to
the ground rod would actually create a high impedance at 14 MHz in your
shack. In that case, the counterpoise wires are a better solution. You may
want to try connecting the counterpoise wires to the coax side of your
balun.
You could try inductors in series with the AF line in the K2 to get rid of
the distorted audio, but the real solution is to get the RF out of the
shack.
I wonder why you are using a 9:1 balun? You may be better off using a 1:1
balun instead. Yes, I know that 50 times 9 is 450, but that is only the
characteristic impedance of your feedline - the feedpoint impedance will be
quite different than 450 ohms, and may vary from very low to very high
depending on the frequency. It just may be that your 9:1 balun is creating
more RF in the shack. If you have no 1:1 balun available, try removing the
balun and connect the feeders to the center conductor and shield of your
coax - and connect your counterpoises to the shield at that point too - you
may find your RF in the shack will drop dramatically.
73,
Don W3FPR
> -----Original Message-----
>
> To make a short story long, I don't have a very good ground
> system, so there's a high level of RF in the shack - the shack is on the
> 2nd floor, about 18 ft above the ground rod. My primary antenna is a 100
> ft multiband horizontal dipole feed with ladder line into a 9:1 balun,
> which is connected to my KAT100 via 6' of RG8. While the antenna works
> great on all bands (the KAT100 has no trouble tuning it), there's a
> fairly high SWR on the feedline on the higher bands. I have two
> different length wires connected the ground rod to the KAT100, and
> adding counterpoises for 20, 17 & 15 meters hasn't change the high level
> of RF in the shack on these bands.
>
> A week or two ago, I put up a second antenna, a vertical
> parallel dipole for 20, 17 & 15 meters, fed with RG8 - this antenna has
> very low SWR on these bands. When using this antenna, I have no problems
> with the computer speakers screeching, so there's apparently not much RF
> in the shack. This weekend, during the NA QSO party, when using the
> ladder line fed dipole on the higher bands, several times, I was told
> that my audio was distorted, and switching to the vertical coax fed
> dipole immediately cured the problem.
>
> So, the question is, what can be done to fix the distorted audio
> when there a high level of RF in the shack (besides getting a better
> ground system - I plan to put in one or two more ground rods, but I
> won't be able to do that until after the ground defrosts in the spring).
> The mic I'm using is a RadioShack Electret replacement element in an old
> mobile mic case - the case is plastic and not shielded, but the cable is
> shielded, and I have grounded the mike jack in my K2.
>
> All suggestions about how I can eliminate the distortion will be
> greatly welcomed, as well as suggestions about how I can reduce the
> high level of RF in the shack besides adding more ground rods.
>
> Thanks
>
> Bob W1SRB
>
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