[Amps] Q-value of a plate tank

[Mikael Larsmark]

Hi everybody

I have been wondering a bit about how to actually measure the Q of the 
plate tank of an amplifier. Of course I have been looking around a bit 
and I have two different things that kind of contradicts each other, at 
least from what I understand. Or I have missed something.

First, there is the method of using for example an MFJ 259B, which is 
explained here,
http://web.archiveorange.com/archive/v/0RLC1crR63v6fj2qn5uL

By this method they use the 2:1 VSWR bandwidth to determine the 
bandwidth of the filter and thereby calculate the Q-value. This is OK if 
it is actually the "antenna bandwidth" that does determine the Q-value 
in a Pi-tank.

Another method is to use the same as for example when measuring the 
Q-value of a BPF, where we use the -3dB point as a measure of the 
bandwidth (half power).

But for me 2:1 bandwidth and -3dB point is not the same, or have I 
missed something?

Mike, SM2WMV (SJ2W)
http://www.sj2w.se/contest/