Phil Levin wrote:
>SWR at best is 4:1. Am using MFJ analyzer on input, relay
>closed w/ match pack and antenna for load on output. (Which has better
>than 2:1 match). I know I need a Q of 2, how in the world do I get this
>match. Please steer me in the right direction. Have removed SWR bridge
>coupling in amp and ran good RG8/x for all coax runs. Help, I'm abt to
>go POSTAL.*
Phil,
You are making a fundamental mistake in your measuring that is easy to
do. You are trying to drive your tubes into conduction with an MFJ SWR
analyser.
The fact is that the input impedance of the tube depends on the level of
drive. In fact, the input impedance varies over the drive cycle. In
reality you end up looking at the "average" SWR for matching purposes.
Given all that, the MFJ analyzer does not have enough of any kind of
signal to drive your tubes into conduction. An SB-200 is not a class A
amplifier. Therefore, the tubes are in cut-off unless a certain level of
RF is applied. This is usually on the order of Watts and not micro-watts
that the analyzer outputs. Once you start driving the tubes into
conduction, then you can measure your SWR with an SWR meter. But again,
the impedance varies with drive level, so you really need to measure the
input network performance at your nominal drive.
Here is what I suggest: Build the networks as suggested by theory, other
designs, etc. Plug in your transceiver, SWR meter, etc. and fire up at
full power. Does it work? If so great. Otherwise, adjust your caps,
coils, etc and try again and keep going till it does.
Another thing you can do if you really want to use your MFJ is to
simulate the impedance of the tubes. Leave the tubes in the sockets
(unpowered of course) and connect a resistance equal to the driving
impedance of the tubes from the input pins to ground. This will simulate
the tubes under conduction conditions and yet you can use your MFJ. ALL
THIS MUST BE DONE WITH AC POWER OFF!!!!
The bottom line is that the MFJ is a small signal device and you can't
use a small signal device to measure large signal impedances.
Hope this helps.
73,
Jon
KE9NA
--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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