I think I have discovered something quite useful: How to measure the
actual Q of a plate tank circuit using an SWR meter. Here's how I did
it. Please check my approach and let me know if you think I have
blundered in any way.
First, I made a pi-tank circuit with a Q of 12, according to the
numerous Excel spreadsheets around. I used the values of L=2.72 uH
and adjusted the C1 And C2 values to resonance at 13.31 Mhz. I chose
that value of L because that's what I already had for my current
project. With that value of L and that frequency, the Q has to be 12.
All the spreadsheets agree on this. I had the tank circuit sitting on
the bench, not in the amplifier, so as to reduce external influences.
Here's my procedure:
1. I tacked a 2200 ohm resistor across C1 to simulate the load
presented by the 8877.
2. I connected the MFJ-259 SWR analyzer across C2 and set its
frequency to 13.31 Mhz.
3. I adjusted C1 and C2 for a 1:1 SWR as shown on the meter.
4. I tuned the analyzer lower in frequency until the SWR read 2:1 and
made note of the frequency, 12.844 Mhz.
5. I tuned the analyzer higher in frequency until the SWR again read
2:1, 13.867 Mhz.
6. I subtracted the low frequency from the high frequency to get the
2:1 SWR bandwidth, 1.023 Mhz.
7. I divided the center frequency (actually the numeric average of
the high and low) by the 2:1 bandwidth to get a Q of 13.06. I then
applied a correction factor (for the accuracy of the SWR analyzer) of
.9188 to get a Q of 12.
How does this sound? Any errors anywhere? I believe this should scale
up and down to any frequency within the capability of the analyzer.
This allows one to measure the actual Q as opposed to the calculated
Q of the tank circuit. This could be most useful on the higher bands
when the "minimum capacitance problem" affects the Q.
I put all this into an simple Excel spreadsheet which you can download at:
http://members.ispwest.com/dezrat1242/SWR.xls
In cell A4, change the .9188 factor to whatever your analyzer
requires. I highly recommend doing the calculations for your
particular analyzer instead of using mine, although it would probably be close.
Comments welcome.
Bill, W6WRT
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