On 6/20/2012 1:16 PM, Herb Schoenbohm wrote:
> Jim, How about 7 turns of coax then an independent ground rod and on
> the other side another 7 turns on type 31
Two chokes with a rod in between is a great move, but you need at least
14 turns to get the choke resonance down to 160M, and more turns is better.
>
> Seven turns of RG6 on a 2.5 inch type 31 is about all I can get on a
> core.....or what about stacking cores? would that be any better?
>
Yes, it's OK to stack cores, but you still need a lot of turns.
Inductance increases linearly with the number of cores, but increases as
the square of the turns, capacitance increases linearly as turns but
only slightly with number of cores, so turns move the resonance more
than cores.
Since it's an RX antenna, use smaller coax for the choke(s). RG58 or
RG59 or even mini-coax. A small break in the Zo doesn't matter. Or wind
one of the bifilar chokes (really a parallel wire line) shown in my RFI
tutorial.16 turns of a pair of #12 THHN will give you a 5Kohm choke from
160M up to 15M with a Zo of about 100 ohms. Tape the wires together,
then wind them on the core(s) and put SO239s on each end. Do the same
thing with #12 or 14 enameled wire and you'll get about the same 5Kohms,
but not as broadband, and a Zo of about 50 ohms.
73, Jim K9YC
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