> >Hi,
> >Anybody have any experience with the FL-2100Z showing a High VSWR on 18
> >Megs and 24 megs,??any fast track to the cause?? .....snip......
Probably the network Q ids too high to cover those bands when
the input is adjusted for 15 and 10 meters. There is so much feedback
in PA's using triodes like 572's on the higher bands, and the
driving impedance of the tubes is so high, it is tough to make a
really broad banded input.
> In order for a g-g amplifier tuned input to produce a good match to a
> solid state exciter, the Q needs to be at least 2. . In other words,
> the reactance of C1 should be no more than about 25 ohms. At 1.8MHz,
> this works out to be roughly 3600pF.
The more Q, the narrower the bandwidth and loss in a network.
Q only needs to be some value more than the square root of the
impedance ratios (approximately). If the tubes are 100 ohms
impedance, and the source is 50, the Q at the high Z end needs to be
more than sqrt 100/50, so a Q of any value more than 1.5 will work
fine and give you a 1:1 SWR. That may be where the 2 came from.
(By the way, calculating the value at one end, like we do with tanks,
does not give the exact value of Q. It is only a close
approximation.)
If the tube impedance is 500 ohms, Q needs to be just over 3.3 .
If the tube impedance is 5000 ohms, Q needs to be sqrt 5000/50, or
10.
To be conservative, most handbooks use the rough formula of
the square root of the impedance ratio needed plus one for Q.
The matching network actually acts like a transmission line. In
order to match the two impedances the surge impedance of the
pi network must be lower than the geometric mean of the two
impedances. The MINIMUM impedance of the network winds up being the
same as you would calculate using a 1/4 wl line section for
matching. In that case network phase shift is 90 degrees, it looks
just like a "lumped transmission line".
If the network does not have enough Q, the system will not
match. This was the problem in many Dentron tuned inputs on the
lower bands. The Pi's on the inputs don't act like Pi's at all,
because the network impedance (Q) is too low..
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|