[Elecraft] Rf in the shack distorting audio - the results

[Solosko, Robert B (Bob)]

 Hello All,

	Several weeks ago, I posted the attached e-mail describing my
problems with RF in the shack distorting the audio when transmitting. I
received many responses with good suggestions about how the cure the
problem -- thank you to all of you who responded, and especially thank
you to Don, the "Friendly, Patient Radioman", with whom I subsequently
had an e-mail dialog about the design of by balun and the materials
used.

	The results: problem cured!

	The first thing I did was move the counterpoises from the ground
on the KAT100 to the ground side of the coax where it connects to the
balun. That had the most effect, significantly reducing the level of RF
in the shack, as measured by my field strength meter, and mostly
eliminated the RF getting into the computer speakers. 

	The next thing was to use the 4:1 taps on my voltage balun
instead of the 9:1 taps - that reduced the RF level a little more, at
the expense of the KAT100 not being able to get down to a 1:1 SWR on
15m; the lowest SWR achieved was about 2:1 (I was using the 9:1 taps
because the KAT100 could get down to essentially 1:1 on all bands).

	Next, AA1SB gave me a few toroid cores of unknown type that were
just large enough to fit over a PN-259 connector- I slipped them over
the coax where it connects to the balun. That made a slight difference
in the RF level on 10m, but had no other affect on any other bands.

	The final step was to replace my 9:1/4:1 voltage balun, made
using a T300A-1 core, with a 4:1/1:1 current balun made using a pair of
FT-240-43 baluns. Don, W3FPR, had suggested that the FT material in a
current balun would be much more effective at eliminating the RF
problem. I finally got this current balun made and installed this
weekend, and just as Don predicted, the RF in the shack is essentially
completely eliminated. Also, the KAT100 now can achieve a 1:1 SWR on
15m. However, you don't get something for nothing - with this balun, I
lost the upper 40 kHz of 160m... The KAT100 doesn't get the SWR down
below 9:1 and just gives up. But, below that, there is about a 10kHz
range over which the minimum SWR that the KAT100 can achieve goes from
1:1 to 9:1. This is not quite surprising since my antenna is a 100'
shortened multi-band dipole which by design has a very narrow bandwidth
on 160M. Furthermore, the inductance of the 4:1 current balun made with
the FT material is about 50 times greater than the 9:1 voltage balun I
made. On 160m the balun impedance was likely less that the impedance at
the balun end or the ladder line (the rule of thumb is that the balun
impedance should be at least 4 times the impedance at the balun
input)... So, it's likely that the KAT100 was really tuning to the balun
impedance and not the antenna impedance. However, with the new 4:1
current balun, the balun impedance shouldn't be much of a factor and the
KAT100 is probably seeing more of the actual characteristics of the
antenna... And, now that I think of it, all of the few QSOs that I've
had on 160m have been below the upper 50kHz or so.

	So, thank you all again - I'm very satisfied with the results.

Bob W1SRB

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Solosko, Robert B
(Bob)
Sent: Monday, January 22, 2007 12:57 PM
To: elecraft at mailman.qth.net
Subject: [Elecraft] Rf in the shack distorting audio

Hello All,

	Several month ago, after I finished my KPA100, I found that when
transmitting SSB on the higher bands (20, 17 & 15), the computer
speakers across the room, when turned on, would screech - adjusting the
speaker volume had no effect (but turning off the speakers quieted
them).  During a QSO a few weeks ago, it was commented that it sounded
like I was getting RF into my audio. 

	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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