[Amps] Relationship between Q and SWR in PI network.

[Paul Baldock]

I found an online calculator 
http://chemandy.com/calculators/return-loss-and-mismatch-calculator.htm

A resistive load of 300 Ohm in a 50 Ohm System represents about a 3dB 
loss over a 50 Ohm load. A resistive load of 300 Ohms is 6:1 SWR. I 
also tried a load of  150+150J, which gave a similar 3dB loss and SWR 6:1.

With a known test circuit, my MFJ259 shows about 4:1 at the 3dB points.

Close enough I think.

- Paul



At 01:15 PM 5/25/2018, Fuqua, Bill L wrote:

>    It is not an easy comparison.
>
>A pi network is modeled by 2 back to back L-networks. Each having a Q.
>
>So the bandwidth of these are somewhat smaller than a single LC network.
>
>   Also, 2:1 points in SWR are not the same as the -3 dB points.  -3 
> dB points are
>
>where only 1/2 of the power passes thru the filter. 2:1 SWR 
>represents about 89% power
>
>passed thru the system.
>
>    I went thru this exercise many years ago and but it has been to 
> long since I had done the
>
>math.
>
>
>73
>
>Bill wa4lav
>
>
>
>
>
>----------
>From: Amps <amps-bounces at contesting.com> on behalf of Paul Baldock 
><paul at paulbaldock.com>
>Sent: Wednesday, May 23, 2018 6:09 PM
>To: amps at contesting.com
>Subject: [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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