So what inductance should the shunt feed RF Choke be?
Take a 'classic' choke - the old National R-175, used at the kW DC input in AM
days. It's 145 microhenries, so with something around 2kV rms across it, and
its reactance of about 3.2kohm at 3.5MHz, there's 625mA of RF in the choke.
Plus the DC, of course. Assume that the choke Q is 100 (probably being generous
here) and we have 12.5 watts dissipation in the choke.
I have a 4 'pie' choke, reputedly used in the NCL 2000. That's 950 microhenry,
and issipates a couple of watts. But the problem is the series resonance - in
the way I have it mounted, that just happens to be at 21.5MHz.
Some transmitters I've seen used under 100 microhenries, but had no series
resonances in their operating range.
So what's the best to use?
The lower the value, the better the chance that there won't be a nasty series
resonance, but there's more tuning capacity needed, and more current through
the bypass cap at the HV end. Incidentally, I seem to remember that Rich has
made some comments about the value of the bypass capacitor, which obviously
has to handle the current. It seems that you need an extra choke between the
bypass cap and the HV supply if you're really going to keep the RF out of the
supply, and thus the mains. I know Alpha in some of their amps switch the
choke, which is one approach. But especially when you're covering 160 as well
as 10, it's obviously not straightforward as to what choke to use. I've tried
the tack of a solenoid choke (possibly ex ART13 - solenoid, about 1/2 inch
diameter, about 4 or 5 inches long, 95 microhenry) in series with the NCL 2000
choke. Series resonance for the pair is 24MHz.
Question: how far away from the operating frequency is acceptable for a series
resonance? The resonances seem to have quite high Q.
Measurement method is to remove the tubes, use a scope probe as that gives
about the same capacity as the tubes, connect a load resistor of the calculated
laod value between plate and ground and feed a signal generator in (via a
return loss bridge) at the pi output. Tune the pi for max volts across the load
resistor consistent with max return loss. Vary the sig gen frequency, and you
can see frequencies where there's very sharp dip in plate volts - down pretty
well to nothing. Which I feel is a delicate test for the series resonance.
It does seem, prima facie, that a lot of the chokes in use are rather low in
inductance for ideal operation.
73
Peter G3RZP
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