Vic, K2VCO, asked about measuring the self resonance(s) in plate chokes.
Others mentioned doing the measurement in-situ. I also believe that it must
be done in the actual chassis or a model box which has similar wall
spacings, dimensions. The capacitance from the open end will certainly
detune it, and (if the box is ferrous (steel), the inductance at the closed
end near ground may be altered as well.
Using a GDO is fine to see resonances, but harder to discern series or
parallel resonances. But it is certainly do'able. If you ground one end of
the RFC (assume that is the DC power supply side, suitably bypassed with a
GOOD capacitor) and leave the other end open, then the lowest frequency
resonance is the one of concern, where the parallel resonance of the
self-capacity and the inductance resonate. We all should be aware of the
nuances of coupling too closely (pulling the frequency), or tuning past the
dip (high Q) too fast, and the proximity effects of our hands holding the
Using a vector impedance meter, one can qualitatively compare RFC's. The
Hewlett Packard 4815A is a favorite unit, now available in surplus for a
few hundred dollars. I got mine from an ad in Nuts and Volts magazine a few
years ago, without a probe. Someone did some scrounging for me, and found a
The probe is quite expensive, having an active sampling circuit to get the
complex voltage and current at the tip, to make the impedance calculation.
These go for $500-1000, and are no longer supported by HP.
The tiny tip is very fragile, so most of the times, we make a substitute
tip from a small screw, and connect to a copper strip to the coil under
test. Ground on the probe must be near the chassis end of the RFC. These
instruments will measure from an ohm up to about 100K ohms, along with the
Using this instrument, one can measure and plot the various parallel and
series resonances of a choke, and even compare the Q, up to 110 MHz. The
later generation vector Z meter, the Hewlett Packard 4393A, has a different
probe than the 4815A, and has GPIB bus output as well as digital readouts
of the impedance. I wrote a general program to automatically run this unit,
and take data in a range of frequencies, and plot it on an HP85 or similar
ancient controller. We are still using it, even today, to design our RFC's
without resonance problems in the frequencies of interest.
It appears that HP has recently discontinued the 4193A as well. The
replacement instruments are more general purpose inpedance/gain/phase
I mention all this because the 4815A instruments are becoming more
widespread in the surplus market, and a prudent ham would gain an excellent
tool by acquiring one, instead of spending the money on a better grid
dipper. I would hope that amplifier manufacturers use similar tools to
build their RFC's.
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