>
>> Stray capacitance at the middle of the choke is the problem at the
>> first resonant frequency, so removing turns there and making an air
>> gap will shift the first series resonant frequency higher.
>
>The choke sold by RF Parts Co. and Ameritron has been a "God-send"
>to me in the last few amps I have built. I tried several other things over
>the years; using two chokes and switching one in and out, (good, but
>expensive) mounting two chokes at right angles, (crispy-critter time)
>and using B&W 800's and 802's, even though the L was too low for
>adequate operation on 160 meters.
>
>Let's look at the RF parts/Ameritron choke. It is around 210 uh on a
>1" x 5 1/2" ceramic form. The windings are split into three sections;
>5/8" 7/8" and 1 1/2". The distance between windings is 5/8" and
>1 1/4." The 1 1/4" gap in the winding is in the center of the form.
>The first resonance is marked on each choke, which is parked
>between 11.7 and 12 mhz.
>
>I think the reason that the "gapped choke" described in the ARRL
>handbook does not change much from a standard winding is that
>the gaps were too close to their adjacent windingsto make much
>difference, as Rich has pointed out and I have confirmed by building
>one. I saw no advantage to the ARRL choke at all.
My guess is that a gap would work if it was about 10-inches. Mounting
chokes at right angles seems to work better and save space. A switched
vacuum relay to short the larger choke as needed would undoubtedly work
best. This is how commercial. general-coverage amplifiers deal with
hv-rfc resonances. .
>
later, Phil
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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