[R-390] Mechanical filter tests?

[DJED1 at aol.com]

Thanks for all the suggestion on filter testing, both on and off the  
reflector.  Here's what I finally did, which seems to make sense in light  of all the 
comments:
 
I connected the URM-25 high level output through a 100K resistor to the  
input of the filter, and tacked a 100 pf air variable across the input.   
Similarly, I put a variable cap across the output, along with a 100K load.   I used my 
HP 400EL AC voltmeter, which has an input resistance of 10M, shunted  by 25 
PF.  To decouple the meter more, I used a scope probe at 10X to  connect the 
meter to the setup.
 
To obtain a reference level, I connected the meter to the input of the  
filter, then adjusted the trimmer cap for a peak.  I then transferred the  meter to 
the output, and peaked the output cap.  Finally, while measuring  the output, 
I repeaked the input to compensate for the effect of the meter in  setting 
the reference.
 
I measured about 9 dB of insertion loss on my spare filter, reasonable  
ripple in the passband, and a sharp falloff outside the passbands.  I  couldn't 
measure levels of filter rejection because I was too close to the lower  limit of 
the meter. I'm pretty happy with this setup- the filter resonated  as 
expected, the loss is reasonable and the filter shape looks OK.  My only  problem is 
one responder said the loss should be about 25 dB.  I'll have to  try and 
tracl down the original filter specs.
 
Anyone have any further criticisms/suggestions?  Now I have to decide  if I 
want to go to the trouble of replacing the filter, given that I never use  it.  
Or should I put the spare in the "someday" drawer with my  spare 3TF7s.
 
Ed  WB2LHI