There is a relationship between SWR bandwidth, and Q which is valid for
"simple" circuits and for limited excursions from the center frequency.
It uses the limit of |X| = R of the input impedance, and is
Q = center frequency/SWR bandwidth at the 2.6:1 points.
For a pi-network, it is somewhat approximate, as it does not take the asymmetry
of the real part around the resonance point into account.
A discussion and a detailed derivation can be found at:
https://owenduffy.net/blog/?p=12152&; and
https://owenduffy.net/blog/?p=1736
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från : paul@paulbaldock.com
Datum : 2018-05-24 - 00:09 (UTC)
Till : amps@contesting.com
Ämne : [Amps] Relationship between Q and SWR in PI network.
Does anybody know the relationship between Q and SWR for a PI network?
For example let's say I design my amp PI output network for 7.0MHz. I
simulate the Tube Impedance with a 1K resistor. I put my MFJ259 on
the output of the PI and tweak the load and tune caps for a perfect
1:1 (50 Ohm) SWR. I now leave the caps where they were and adjust the
frequency to show the upper and low 2:1 SWR points and they are
7.2MHz and 6.8MHz. (2:1)=Fo/(Fu-Fl)=7.0/(7.2-6.8)=17.5. In practice
this is in fact about 1.5 times my design Q, but I would like to know
what theory says.
- Paul KW7Y
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