>> Likewise, a HIGH-PASS filter cutting off just
>> below channel 7 (174 MHz) will be the ideal situation. This would also
>> reject two meter ham signals. I don't know if such filters are readily
>> available in the marketplace.
I would also use one or more common mode chokes near the preamp to prevent
fundamental overload by VHF and HF transmitters that could excite the
downlead as a long wire antenna. Use the guidelines in my Cookbook for the
transmit frequencies in use.
http://audiosystemsgroup.com/RFI-Ham.pdf
At 6M, you want 2 turns for most cores; at 2M and above you want a string
of beads. See either my data or Fair-Rite data, and aim for at least 1K -
2K ohms with the resonance near the transmit frequency. Note that Fair-Rite
is now publishing data for 1, 2, and 3 turns for many of their suppression
beads. To find it, go to the pdf catalog, select the part number to get the
data sheet for that part, then click on the data at the bottom of the page
to see the multi-turn data.
For 440 MHz you'll want #61 cores. For 6M, 2M, and 220 MHz, #43 or #31.
Note also that #43 is cheaper than #31, and #43 is equivalent above 5 MHz
and slightly better above 20 MHz. The advantage of #31 cores shows up below
5 MHz.
73,
Jim K9YC
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