I guess this choke has quite a wide following. Well, it deserves it. As far
as I can see, it was the first commercial product that was broadband enough
for ham service.
I just happened to stumble across an ad by National in an old Handbook (1956)
in which they were "announcing" the new and improved R-175A. The primary
claim for the updated model was that is was now resonance free up to 50MHZ,
good news for the 6M crowd. I looked carefully at the picture in the ad and
it looks like the choke that I have. Moreover, my best guess about the
pedigree of my particular choke leads me to believe it was purchased new in
the 60's, so let's assume it is the -A model.
I will list all of the resonances I found, and for the series resonances
(which are the only dangerous ones) I also give the actual value of R at
resonance. This is of some minimal use for evaluating its effect in a plate
circuit. The parallel resonances have values of R (parallel circuit model)
that are too high for me to measure easily (>25 to 50K), so I didn't bother.
Parallel resonance: 6.5, 18.2, 35.8
Series Resonance: 16.7 (70ohms), 23.55 (270) , 30.6 (1500) ,35.5 (600)
That 30.6 peak is a higher-order resonance but it does have a phase null. I
checked it several times and it always showed up right on frequency, so it is
for real. The only one of these that has any real danger is that 23.55
series-res. lurking just below the 12M band. If anybody thinks it is
important, I will analyze that resonance in more detail and see if it needs
attention. In a QRP rig it would probably go unnoticed, but in any amp with a
real power supply, it could become a shack warmer or worse.
73
Eric von Valtier K8LV
P.S. The choke also reads 2200 ohms at 2.0MHZ (175uH) and the DC resistance
is 4.5 ohms, using #30 wire. Ni-Cr wire of that gauge is 5ohms/ft and this
choke has 10-20 feet of wire (my estimate). The real problem is that at the
time that choke was designed, Kryptonite hadn't been discovered yet. If wound
from Kryptonite, the resistance would have been even lower, and the ability
of Kryptonite to eat all parasitics would have made it the ideal component in
every way.
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