Use of ferrites:
Choking of the RF by ferrites is more than simply putting a few turns of the
computer cable or power cable around a clamp on core, or around a toroid.
You need to use a simple coil of wire connected to an RF indicator, like a
1n34 diode connected to a 50 microamp meter, to slid along the various cables
to find the hot spots of RF. A coil 10 turns no. 20 1 inch in diam. would
work, as would a smaller diameter coil. You are not worrying about a resonant
coil, but just something sufficient to show deflection on the meter. Can be on
a cardboard tube or anything handy.
When you identify hot spots, try putting the ferrite there, and evaluate the
RF attenuation. You might have to slide the ferrite along the cable to get
the best effect. Tape it, or use nylon ties to keep it at the best spot.
Now, if you do not find any obvious RF cable radiation, another thing to try
and good anyway for most ham installations, is to not rely on an 8 foot or any
length ground rod for your RF Ground! These are principally for DC grounding
for lightning protection. From your ground bus for the rigs, amplifier and
accessories, run quarter wave insulated counterpoise wires, as straight as
possible out from the shack. But, if the only thing you can do is route them
inside, around the baseboards, do that. Since you only have the problem on
two bands, cut the wire for the lower band, and see if that is effective on
the higher as well. IF not, add a second for the higher band. Cut them by the
standard formula 234 over F. Where F is in M Hz. Check the resonance, by
cutting them 5 % long, and bring them to the center of the operating frequency
band by folding back excess, or cutting measuring with a grip dip meter, or
antenna analyzer. The idea is to make RF on the problem bands see the
counterpoise as a preferred conductor, along with the antenna, in place of
power leads, computer leads, etc. Tape up the ends which will operate at high
RF voltage.
The fact the problem increases when you plug in the computer lead, says you
have RF in the shack. Is the computer case grounded to AC ground? Might
experiment with tying it to the shack RF ground bus also; however many
interface cables for RS 232,for example, take care of chassis ground on a pin.
In these RF feedback situations, you have to look at the whole installation,
and it is right to question how the AC runs to the amplifier. Is the lead in
the attic for example, or down on the slab or floor level? How long is the
feedline to the antenna(s), and are the antennas resonant for the bands in
question? A resonant antenna is easier to feed RF. Does any antenna matching
unit have sufficient range of adjustment to operate at the optimum points on
the Tee tuner? See the W4RNL (L. B. Cebik), web site for discussion of
matching units, and antennas, and I believe he has listed the way to tune a
Tee tuner for optimum low loss results. The output side capacitor should find
a match at a lower capacity for lowest loss, I think was the suggestion, if
there are multiple match points.
Other concerns are length of mic cable, bypassing of any active (amplified)
mike, etc. It is always good practice to put RF bypasses right where all
leads of a transmitter exit the chassis, or in todays designs, use a ferrite
bead on the wire. Remember there are beads for various frequency ranges, and
pick the right material for the bands in question, in this case.
Hope all the above is useful to suggesting more things to try. The ground bus
in a station should be a wide flat copper bar, in preference to a braid, by
the way. You want lowest inductance, which the wide and flat brings, and just
in case of massive surges, braid can actually blow apart, leaving you
groundless.
73, Stuart K5KVH
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