It just so happens that I was building new suppressors for a Clipperton that
I am returning to service and the parasitic topic comes up again.
Since I have access to some very nice equipment, I can build the suppressors
and measure them very accurately. I thought I would pass along my findings.
I built several varieties using both resistance wire and bus wire with
varying turns and construction techniques. Thanks to Al Harbach who lives
right down the road for access to the nichrome wire and the resistors.
My final network has eight turns of nichrome about .25 inches in diameter
along side of a Matsushita 100 ohm 3 watt resistor. The trace reveals a
very nice characteristic with a self resonance of 1.5 Gigahertz.
Initial attempts to make the measurement were unsuccessful because I tried
hooking the network to the analyzer with clip leads and BNC adapters. The
wire and the connectors added additional effects that cluttered up the
results. I ended up using a test board that we keep on hand that has SMA
connectors properly terminated to 50 ohm transmission line. I soldered the
network in question to the board and then connected the test card to the
analyzer.
The nichrome behaved as advertised and lowered the Q of the resonance by a
factor of about 10. At these very high frequencies, your results may vary
so pay attention to the setup. Attempts to duplicate the resistance wire
with an additional series resistance did not work as the body of that device
added additional resonance albeit also at very high frequencies.
The construction technique was also critical. Wrapping the wire around the
body of the resistance added capacitance lowering the resonance frequency.
Winding spacing also changed the resonance.
Differences between the two types of wire were only evident at very high
frequencies where it lowered the Q of the self resonance and very low
frequencies where the network behaved as a pure resistor.
With the network installed in the amplifier, one could identify a loop that
included the plate capacitor, the capacitance of the plate choke, the high
voltage bypass capacitor and the suppressor that could resonate at VHF
frequencies. Because of differences in construction, this resonance
frequency may and will vary from one amplifier to another. If you happen to
get one that resonates at a harmonic of the exciter frequency or at a
frequency where there is sufficient gain from the tube, a parasitic may
ensue.
The fact that many amplifiers work fine with the stock suppressors or even
no suppressors doesn't surprise me. The variation in construction may move
the resonance to frequencies where there is insufficient gain to cause a
problem. This would make some systems more likely to fail than others.
Adding the resistance wire was painless and inexpensive. It seems like
cheap insurance to me.
Steve KT4FY
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