>What is the usual method of measuring the series resonance of a plate
>choke?
Well, what I have seen done and suggested is to short the ends of the
choke together and measure the resonance with a dip meter. This is how I
measure mine.
>
>Probably a silly question, but could there be more than one series resonant
>frequency?
Yes, there is actually. The graph of a choke's impedance versus
frequency actually looks similar to what the graph of a bouncing ball
would look like. It starts out low (as the ball is thrown upward from
floor level), peaks, comes back down, hits the floor, bounces up again,
reaches a second but smaller peak, comes back down again, etc. Each
succeeding peak is smaller and smaller until everythings all damped out.
The impedance of a choke looks this way as well. For a given inductance
at some particular "low" frequency, the choke looks like a very low
impedance. Then as the frequency increases, the impedance rises.
However, eventually the parasitic capacitance of the windings, etc, begin
to take their toll and the impedance begins to drop off. I believe the
peak is what you could call the "parallel resonance point." Finally when
the iductive and capacative reactances are equal (and in an equivalent
circuit in "series") we have our series resonance point of minimum
impedance. However, as frequency continues to increase, we go beyond the
series resonance point and the impedance increases until we hit another
parallel resonance point and then as we continue to increase we hit
another series resonance, etc....
>From my experience, albeit limited, I have found that the trick is in
building a choke that has a large enough inductance so that at the low
end of your frequency range you have a large impedance, but yet at the
high end, you don't have that first resonance right in or near your
desired band of operation. Some people build chokes with a first
resonance around 11 or 12 MHz. The second resonance is then typically
around 25 MHz or so. Before the WARC bands, you could pick a good spot
like that to put your choke resonance. However, if you want to include
the warc bands it's not so easy.
Hope I helped.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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