> 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.
Here are some proven facts about the RF Parts/Ameritron choke.
1. It works in a wider variety of layouts than other chokes.
2. It works from 160-10 meters and runs cold on all bands @ 4 KW.
The argument can now start on the hows and whys, but I use this
choke exclusively now on all my projects. It saves a lot of time,
money, and chassis space. It does the job on 9 ham bands and
runs cold while doing it. Just what I needed!
(((73)))
Phil, K5PC
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