I am not getting multiple minimum SWR points.`
That doesn't mean anything.
If I move the external capacitance in
between the 2 external inductors I can run 500 watts with out an arc over.
The antenna you have is ~99 feet long, although folding it back some unknown
angle electrically shortens it.
The network you hqave, and the value of components, is far out of range of
any tuner. The SWR with the components you have in the circuit you have is
probably 50:1 or more. It gets worse if you decrease either inductor from
maximum.
Assuming you have a 45 degree angle at the flat top, a 45 uH in series from
the coax to the antenna, with a 50 uH to ground at the antenna, gives about
50 ohms j0
You have a 6.7 uH in series, and a 12.7 uH to ground at the transmitter
side. The values are not only too small, they are in the system backwards.
With your antenna length, the shunt coil has to go on the antenna side.
Keeping the 100 pF external cap there and add another 100pF cap back at
the
antenna
going to ground the cap in the MFJ-989 B starts arcing around 300 watts
again.
I'd arc too, feeding a 50:1 SWR. :)
I am at a bit of a loss. My MFJ-998RT auto tuner will be back from
repairs
soon. If I
do not do something with this situation I will FRY the thing again.
I bet you will. :)
Fix the antenna, and you will not need a tuner except when you move off the
resonant frequency.
You really need to get the antenna SWR and impedance down to some reasonable
value. 50:1 SWR means you could have a little as 1 ohm at the tuner, or as
high as 2500 ohms, and anything between if it is significantly reactive.
You are working on the wrong problem, and forget everything you heard about
"hairpins" made out of inductors and Q. What you really have is an L
network, and you have it backwards for the antenna impedance you have. You
have no choice of operating Q, it will be what it is when the antenna is
matched. With an L network, like you actually have, there is no choice in
operating Q.
Your antenna is probably around 14 -j 270. You are not going to match that
with a 140 ohm inductor and an 80 ohm inductor. The numbers are not there no
matter how you wire it. Even if you put both inductors in series as a
loading coil, it isn't enough to handle the -j270. It certainly isn't even
close when you try to bump the 14 ohms up to 50.
73 Tom
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Topband Reflector
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