[Amps] RF in the Audio

[Jim Thomson]

Date: Sun, 25 Sep 2011 12:14:16 -0400
From: "Carl" <km1h at jeremy.mv.com>
Subject: Re: [Amps] RF in the Audio

The last place I would insert a balun is at any point in a OWL fed antenna.

Do the twists as have been the norm since the 30's and live with whatever 
unbalance remains.

###  They put XXX turns of twist in balanced audio cable + telco cables for a reason, same deal. 





Since OWL theses days assumes the use of a tuner then spend the bucks and 
buy/build a truly balanced one especially if running an amp. Compromises 
with QRP and barefoot can get away with a barely functional T200-2 iron 
powder 4:1 balun that comes with the low end tuners.

## The 2 x roller coils ganged together +  a single vac cap looks like it 
may be a winner.  Balanced L network.   Since the 2 x caps of the L network are
in series, they can now be replaced with just one cap...and  1/2 the value.  It would
be easy to switch the single  cap from the input to the output side.   I think Palstar still
makes the balanced L.   Rauch mentions it on his site.   Measures has had it on his site
since day1.  Dunno if measures cooked it up originally or not.   





I always use a LPF between rigs and amps and amps and coax feeds.

##  Ok, what is the purpose of a LPF [with say a 33 mhz cut off] between
xcvr + amp ??   The xcvr already has LPF on each ham band..with typ a >60 db
cut off just above each ham band in question.    Most GG amps use a  PI net
for a tuned input, which is essentially another LPF.    I can understand 43/31 beads
but not the LPF between xcvr + amp.    Using a LPF  on the output of the big amp makes sense.

## Instead of a LPF, perhaps a 200w bandpass filter, one per band, between  xcvr + amp
would do more good. 


 A 12 large 
bead sleeve balun of 43 mix is at the input of each LPF and appears more 
than sufficient to keep RF inside the coax on any band. More beads are at 
every antenna feed point. All these conform to or are close to the 1000 Ohm 
impedance rule that has been a sort of ham standard for decades. Im also 
aware of the 1966 CIA document mentioned by K9YC on his site as I was 
Tempest cleared at the highest level at Sanders Associates 1969-78, a major 
DoD supplier and deep into stealth technology even back in the 60's. The CIA 
and other 3 letter agencies were regular visitors. I didnt remember the 5000 
Ohm recommendation however which showed some serious thinking that far back.

## The 3000-6000 ohm recommendation is a good one.  Just beware that to pull
that off, ur gonna  need a lot of type 31 material.  Where the problem starts is
when folks have a 1500 ohm Z  with their type 31 material..then wonder why it blows
up in their face.   Sure  type 31 will work.. but it's all resistive.  That's  wonderful..but
it's still  I squared x R.   IE: 2 x chokes, each is 1000 ohms..and one is type 43  and the
other is type 31..... the type 31  will go kablamo every time.   To get the type 31 to work
right,  it's Z  HAS to be sky high...so the  current ends up really low....low enough  so
the type 31 material doesn't heat up. 


Remember also that the initial use of sleeve balun beads was due to TVI and 
when a dozen 1" beads could tame any tribander or trap vertical it was 
considered good enough. We didnt have PC's, switchers, digital everything in 
the house, etc, back then.

###  good point. 

While K9YC's site has a lot of very good information there is also a bit of 
disagreement with what others have published and I dont see that changing 
much in my lifetime. He hasnt bothered to reply to my request for a test of 
a balun feeding OWL fed dipole covering 160-10M and at 1500W. Lets try this 
with a 4:1 and 9:1 as those are the common ones in use as well as 
deliberately varying feed line lengths to present worse case scenarios on 
different bands.

### The OWL  length will have to be tweaked precisely to cover a bunch of bands.
A buddy  smoked and blew up every  4:1 balun made..including palstar, and also the 
amidon 4:1 unit.  [ both were rated for 5/10 kw].   The only commercially made balun
that didn't blow up was the 4:1  made by AS.[ 5/10/20 kw]    Previous to all of this, he had tried various
chokes on the INPUT side of the hb T  type tuner.   They all overheated and blew up, including
the 5 kw palstar, made with bifilar wire on a stack of torroid's.   The torroids cracked. 
The  hb choke, consisting of a huge double stack of 8 x large diam
type 43 cores and teflon coax also overheated.  Put too many turns on torroids, and you will get parasitic arcing between adjacent turns, AS warned us of that effect. 
 My conclusion is/was... the HB T type tuner, when used in
'balanced mode', with OWL is NOT a balanced tuner, not even close... which is most of the 
problem.   The arrl fubar trick of using a choke on the input side of a T tuner is a dud, dead loss. 

##  Now if a balanced L network was used, with ganged roller coils and a single, say 500 pf @ 15 kv
vac cap..... + a heavy duty choke on the input side, IMO..that should work good.    Putting a few twists in the
OWL would help with maintaining better balance.




One test I rarely see mentioned is to test your coax first.
Leave in place and terminate the far end in 50 or 75 Ohms and then tune the 
bands recording any crud frequencies. Then add a bead balun and reterminate. 
Record any differences in signal levels. If you have a quiet receiver then 
any pickup with the antenna is likely in "antenna mode" as I like to call 
it. Many get confused with all the technical terms used. Any additional crud 
picked up in "interference mode" will be small and easily eliminated at the 
shack end with another bunch of beads and hopefully a decent RF ground.

###  Ok, this is a new one on me.   This is a damn good idea.   Are you saying
to terminate the coax at top of tower with a 50 ohm plug ?  I would have thought
that would result in zero  local noise pickup. Any small, shielded dummy load should work.

##   What about terminating the OWL  with a  300/450/600 ohm  non inductive resistor ?
Then listen for local noise on each band.   Of course the ant tuner would have to be tweaked
on each band to 'match' the  dummy load. 

later... Jim   VE7RF




 This 
is no different than the Beverage coax procedure as has been in ON4UN's Low 
Band DXing and various web pages for awhile.

The expense of DXE or other overpriced Beverage "boxes" is a waste of money 
unless you are incapable of following the well documented alternatives that 
can be tailored to individual requirements. A one size fits all box can have 
a wide range of performance in the real world.

Carl
KM1H