Date: Wed, 23 May 2018 15:09:52 -0700
From: Paul Baldock <paul@paulbaldock.com>
To: amps@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
## I believe it was myself that discovered that trick with moving the
MFJ-259B
up and down in freq to find the 2:1 swr points....after tweaking the tune +
load
for 1:1 swr initially. I just did it for a laugh, on 160M, to see how wide
the 2:1 swr points
would end up at. Then tried the same stunt on 80m, 40, 30, 20, 17 , 15m.
My amp does
not work on either 12 or 10m. I have no interests in those bands.
## I used GM3SEKs spread sheet to find the values for the coil, and tune and
load caps,
for a given plate load Z and design freq.
## I used a Q = 10 using the spreadsheet, which amounts to an input Q of 8
....and
an output Q of 2. Total = 10.
## On my 3x3 amp, the 2:1 swr points were 67 khz wide on 160m, and 134
khz wide on 80m,
and 275 khz wide on 40M. 565 khz wide on 20M. That was also using a total Q
of 10.
And a plate load Z of 1960 ohms.
## On our mutual friends amp, the 2:1 swr points were FAR different on each
band vs my amp.... his used a
different tube and an even lower plate load Z. I believe his were all
narrower, but I may be wrong, his might have been
a lot wider. But in any event his 2:1 points kept doubling as he doubled the
freq.
His was designed around a lower plate load Z than mine..about 70% of mine.
## I just chaulked it up to some anomoly..like using different plate load Zs.
What I did not try.... was to design for a different plate load Z,
but same Q, then re-tap the tank coil for each band... then re-run the 2:1
swr tests.
You might want to try that, just as a temp experiment, to see if the 2:1 swr
points
narrow or widen, when designing around a different plate load Z.
## What I did note was that my 15M 2:1 swr bandwidth was way outa
wack,,was way too narrow.
It should have been around 800 khz, but was way less than that. That was
cause I had to use a higher Q
on 15M, since the vac tune cap only had 10 pf of min C..and the tube C alone
was 33 pf. So 43 pf,
with tune cap fully un-meshed..... forcing me to use less coil than
optimum..resulting in a higher loaded Q.
The eventual fix for that was to insert aprx .66 uh between plate block cap
and C1 tune cap. That forms
the step down L network..which reduced the plate load Z way down on 15M, then
a total Q of 8 could be
employed... which rises to a Q of 12, when 15M tap re-used on 17M.
Then the 15M 2:1 SWR bw improved, and was where it should be. Since I have
no bandswitch,
the 20M tubing coil was tacked onto end of roller. The combo 17 + 15M tap
was done with a
modified HV contactor.
### Since I use the roller coil on 160-30m.... then a tapped tubing coil
for 20-17-15M... I should have
temp changed the roller setting on the lower bands.... designing for a higher
and also lower plate load Z... but
designing around the same loaded Q, and re-run the 2:1 swr tests. The idea
of course is to see if there is
any correlation between 2:1 SWR BW and plate load Z.
## IF also changing the loaded Q in the design, then the 2:1 swr points
WILL change of course, with higher
loaded Q designs having narrower 2:1 swr points.... and vice versa.
## You have a 400 khz BW for your 2:1 swr points on 40M....for a 1 k ohm
plate load Z. Mine is
only 275 khz wide on 40M, but I used a total Q of 10. If I use a Q of 12, my
2:1 swr points would be even narrower.
## Using your formulae, my loaded Q = 25.45 on each band... which is
nonsense of course. It would be on
fire with that high a loaded Q. I believe the formulae is invalid in this
case.
## Typ the current through the coil is aprx the loaded Q X the plate
current in amps. l temp inserted an
RF ammeter in series with the 50 ohm end of the tank coil..and the measured
current was dead on. That was
done on 160, 80 m only. I also tried inserting the RF ammeter in series with
the tuned input coil..and
also got identical results. Q X 2A... 200 watts into a 50 ohm load..... =
RF ammeter current.
## I would suggest not losing sleep over it, and use the calculated coil
value.. and calculated tune and load
cap values...and call it a day.
Jim VE7RF
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