The AM Forum

Gotta question...

While doing some testing with a FS meter in the shack,
Note the condx:
75m, 375 watts output, to CF Zepp antenna, ladder line, Johnson
ATU, etc....
External speech amp, 32V3 driving HB Class C / Class B combo...

 I found to my surprise very little RF floating in the shack. FS meter on MAX
sensitivity, very little around ATU, and Not much more RF found near the
transmitter....but
when moved the FS meter near the 500 ohm AF cable between the speech  amp and modulator, and the PTT control cable (both shielded) my FS meter was maxed out!
I never installed RF line filters on the power cords of the high power deck,
The modulated sine wave looks fine on a scope....
any thoughts?
73 steve
8tow

Hi Steve;

Is that 500 ohm AF cable between the speech amp and moduator a balanced line? Maybe some common mode ferrite chokes on that cable might help?

What type of ground system are you using in your shack? common point star ground type setup?

Unless your antenna is a great distance from your shack, you are in the near field. All bets are off as to what sorts of field-strength readings you will get. The increased reading could be as a result of "extra" RF on your audio lines, or something else. As long as it's not showing up on your transmitted signal, I wouldn't worry about it.

Could be coming FROM the xmitter, not being picked up from the air.  Everything bypassed OK FINE?

A few aditional notes:
Antenna is 65 feet above the shack...almost directly above...
but there is almost NO RF present with the FS meter place
on top of the Johnson ATU.

The 500 ohm line is 2 wire...between the 500ohm output
of the speech amp and the input of a 500 ohm to grid  driver
trany....I can't remember if the cable has one leg grounded or not...
but I am certain it is shielded and thus the shield is ground...

As far as the signal..well, there is no RF visable on the scope riding on
my audio, nor has anyone ever said anything about it...

Perhaps just some ferite beads would solve this? hummmmm
73 steve

woops... I should have read more carefully. I didn't realize you did not have RF on your audio.

Since the Antenna is directly overhead, it's gonna be very difficult if not impossible to sniff RF hot spots from you equipment with a FS meter. You would need some sort of near field probe...

right Huzman?

current probe would be easier.

Steve the shield of the audio cable could be coupling RF to the FS meter antenna when you put the FS meter antenna close.  This does not mean the wires in the shielded cable have RF on them.  As you said, you don't see evidence of RF on the transmitted signal. 

If it is not causing any problems, you might curing a problem that does not exist.

Possibily the best idea it exactly that...if it aint broke, dont fix it!

I might say:
This issue became next to address in the shack after resolving several other little
technical problems which have been lingering for years...
Among those was the fact that my 75m center fed Zepp "HAD" one leg touching
a tree...when it was wet outside, the SWR was all over, and naturally, I had a
bit more RF in the shack...That problem was resolved.
Next, I started evaluating the level of RF floating around the house...I hate having
a antenna above the house...but no choice...With a 900 watt CW signal into the antenna, there is PLENTY of RF , everywhere ....even with the antenna at 65FT!!!

So...evaluating my feedline, grounds, and such led me to the control cable and the audio cable .....That is the whole story guys....

To be honest, after 25 years of using open wire...I might actually just go to
good coax, a BIG balun, and single band dipole...I just don't use the
ZEPP on any other band these days...so not much reason to use the open wire...
thanks for the thoughts and ideas! vri 73 steve 8tow

Steve if you go to coax I suggest you just get the slip on beads from Palomar.  Forget the money for a balun becasue many, if not all will present a reactance above about 20 megacycles  no matter how they are constructed.

Beside that, they are cheaper than a big balun.  Of course you can roll your own but that would be a lot of coax up there to support.

Roger on the balun vs beads... On my 160m/40m dual band antenna,
I use the "bead" approach...works fb....I have dumped a kw into it w/o
a problem....and sure cost much less!

Last eve I installed snap on ferite beads on the AF and Control lines
to the big rig....I now have so little RF in the shack it is hard to detect with the
FS meter!!!
I doubt there was any "REAL" problem, but atleast I now have one less
technical conundrum to keep me awake at night!
vri 73steve
8tow