Tim,
I can speak from experience on this issue. I have a 4 element Steppir and 2
element Cushcraft XM240 (40m) beam at 70 and 85 feet, respectively, on a 70
foot tower that is about 50 feet from my shack. While running "barefoot,"
I had no issues with RF in the shack (I use a single point ground and the
towers, antennats and station are well grounded generally, to each other and
to the house ground as well). However, when I added a 1500 watt amplifier,
I got tremendous RF into the audio on SSB, but only when the beams were
pointed generally at the shack. Away from the shack, no problem. Anywhere
in the general direction of the shack, and my audio was unintelligible,
something that I could tell while monitoring my own audio and, of course,
others told me as well.
What did I do to solve the problem?
I started adding ferrite beads to the incoming audio lines. My Heil boomset
mic/headset goes to a Microham u2R SO2R controller and, from there, into the
rig's and computer's mic input and audio (headphone) input. I started
adding type 43 ferrite beads that I had bought years ago on eBay on all
incoming mic or earphone cables (I put beads on the earphone lines because I
was using a boomset mic/headset combo and because both were going into the
u2R). Where feasible, I looped the wire through the bead, as opposed to
just running the wire straight through the bead. I experienced immediate
and dramatic reduction of the symptoms, but not complete resolution. I then
ordered about $50 worth of Fair Rite ferrite beads from Mouser Electronics.
Although type 43 ferrite material is what is usually touted in the journals
and offered by other sellers, such as DX Engineering, I ordered type 31,
which is more specific to the 30mhz and lower frequency range. I was
careful to inventory what wire diameters would need to be beaded, so that I
ordered exactly the right number of beads needed to fit over the wires, or
looped wires, that I wanted to protect. Looped wires through a bead are
much better than a bead over a straight through wire. I put beads on the
coax between the rig and the amp. I put beads on the coax from the amp. I
put beads on all available locations in the incoming mic input line and the
incoming earphone input line, both into the u2R and from the u2R into the
computer and into the rig. I put beads on the incoming power line into the
rig, on the theory that the RFI might be entering via the power line.
I had no chance to check the effectiveness of these changes until last
weekend's contest, the ARRL DX SSB contest.
Bottom line: ZERO RFI into the transmitter.
Conclusion: my problem was, as I had suspected, caused by RF, due to power
level and proximity, getting into the rig, either through the power line or
through the incoming mic input line or earphone input line or both. Had I
had more time and patience, I would have explored each of these
possibilities one at a time until the problem was solved. I didn't have the
time nor the patience. I just choked every line I could think of. It
worked, so I'm happy. If I have a bunch of ferrite beads on lines that
aren't actually needed, who cares?
I would strongly suggest the use of type 31 material, versus the more
commonly recommended type 43.
Hope this is helpful.
73, Doug W7ZZ
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