Date: Sat, 24 Sep 2011 13:50:16 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
Subject: Re: [Amps] RF in the Audio
I HAVE inserted the bifilar chokes between the output of a Titan 425 and
the antenna tuner and tested at 1.5kW keydown for several minutes from
1.8 MHz to 28MHz. At that point, the choke sees ONLY the differential
field, and there is VERY little heating because the field from one
conductor cancels the field from the other. Dissipation due to common
mode current is a very different matter, and is discussed at length in
the tutorial. In essence, if the choke as sufficiently choking high
impedance and the antenna is not very poorly balanced, the common mode
current, and thus the common mode dissipation, is reasonably small. If
conditions of the application (for example, impedance transformation)
place very high common mode voltage across a choke, the common mode
impedance must be much higher. In a testing situation, I have set up
very high common mode voltages and placed two chokes in series to
withstand them. DXE builds some of their impedance transforming arrays
of chokes that way.
### Brown, if ur gonna insert a choke BETWEEN the coaxial output of your titan
425 amp...and ...and the input of the tuner, why mess with a bifilar choke in
the 1st place??
[inserted at the same point]
## I'm assuming there is a 1:4 balun between the output of the tuner and the
OWL.
## If that's the case, and the choke that gets inserted between the output of
amp and
input of the tuner would best be served by either some coax wrapped around some
torroids or
a string of large ID beads slid over the RG213-U.
Now, I'm a guy who plays by the rules, and shares my work FOR those who
play by the rules, and my testing is done at that power level, at duty
cycles consistent with serious contesting. Someone who wants to run more
than 1.5kW can design and test his own solutions. :)
### A buddy of mine had SEVERE 'RF in the audio' problems....on 6m, while
running 5 kw output...esp with the yagi pointed anywhere near the house. My
suggested
'fix' was a string of large type 43 beads...right at the output of the 5 kw 6m
amp..slid over the
1/2" flexible heliax. Problem solved asap... no more RF in the audio. The
real problem was the
mickey mouse 'balun' that came with his M2 6m yagi. A few months earlier,
another friend of mine
also had RF in the audio, and also severe RF all over his house, this time..on
his 80m yagi. This was also a M2 yagi.
The 'balun' they use on the 80m yagi consisted of several wraps of 213/393
coax, inside the same box that changes
taps on the cw/ssb coil.
## It was blatantly obvious to me that 4-5 wraps of scramble wound coax ain't
gonna cut it on 80m. The short term
'fix' was again, to slide a mess of large diam type 43 beads over the heliax
output of the amp. [ well > legal limit].
Again, problem solved, no more RFI.
## Just cuz the legal limit is 1.5 kw in the usa....doesn't mean it's 1.5 kw
for the rest of the planet.
IMO.. it really doesn't matter how one obtains his erp. Once ur maxed out with
ant's either due to space
constraints, money etc, you don't have a lot of options left.
### I see very little, if any pro-audio gear these days with a pin 1 problem.
In fact, I haven't seen any pro-audio
gear, made by anybody with a pin 1 problem. You wouldn't be in business very
long building gear with pin 1 problems.
## I got around the pin 1 problem in the yaesu simply by using the chassis
instead....and installed a new RCA connector
on the rear AL portion of the chassis. From mic to all the various audio gear
is all balanced line level. At the output of all
this mess is the 20 db pad... then into a jensen balanced to unbalanced xfmr.
[with dual shields] A real short piece of
coax goes from jensen xfmr Un-balanced output to new rca input on yaesu
FT-1000 MP MK-V.
## each balanced audio jumper is made from scratch, using shielded [braid
like 58-u] balanced cable. Neutrik brand XLR
connector's are used, male at one end, female at the other end. From tip to
grnd..and also from ring to grnd.... right inside each
XLR connector, is a 500 pf @ 25 vdc disc ceramic cap.... so 4 x caps per
each jumper assy. Zero RFI...and no RF into the audio..
at any power level. I also employ a large number of type 43 [large style]
beads, slid over 393 coax, at each ant. An AS balun,
made from type 33 torroid, is installed at the input of the remote switch
box. A 2nd identical AS type 33 balun is also installed on the
SPG in the basement. Some of the 393 coax feed lines from remote switch box to
some of the yagi's is 1/4 or 3/4 wave long [ -2%]
This is one way tro kill RFI problems..and nip it in the bud.
Later.... Jim VE7RF
73, Jim Brown K9YC
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