[Hallicrafters] Capacitor self resonance

[Richard Knoppow]

     I got curious so I measured a 0.002uf 600V film cap 
using a Boonton 250 RX-Meter. This device is well suited to 
measuring low-Q resonators and goes up to 250mhz. The 
reactance range is limited but around resonance there is 
little reactance. The RX meter is a Schering bridge and 
measures _series_ resonance. This is unlike the Q-Meter, 
which measures parallel resonance and is best suited to 
high-Q reactances.
     I found that the self resonance of this capacitor 
appears to be around 147mhz. Below that, say at 145mhz the 
sense of the X is still capacitive, at 150mhz it becomes 
inductive. I did not try to find the exact resonance since 
that would have required many successive measurements but I 
think this is close enough to give some idea of performance.
     The cap is in a dipped package with radial leads. The 
leads are relatively short. the body of the cap is perhaps 
half an inch long and less than 1/4" wide, quite thin. I 
have other caps and will try them later. Its obvious that 
this cap would be an effective by pass cap in the 
"communication" range, certainly up to 30 mhz and probably 
up to 65mhz.
     I have film caps with values closer to 0.02 and 0.05. 
Likely they are different. I also have some ceramic caps. I 
will try these in the near future and report back on what I 
find.
     I am hors de combat as far as detecting piezoelecric 
effects by ear since my hearing has deteriorated 
significantly in recent years. I have not yet tried looking 
on a scope to see if there is any output from mechanical 
excitation but suspect there will be. If one is present the 
other will be too.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com