[AMPS] Measuring plate choke series resonance

[Jon Ogden]

>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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