In a message dated 16.11.99 07:33:13 Pacific Standard Time, K7GCO writes:
Using a 450 ohm balun connected to a 450 ohm open wire line seldom works
as does the use of a 4:1 balun. You would have to just luck out with the
right length of open wire line. It assumes the Z at whatever length the open
wire line is--is 450 or 200 ohms resistive which it frequently isn't. With
the typical SWR's the Z's change on the length from high to low every 1/4 WL.
It will be great if your antenna load is 450 or 200 ohms and the feedline a
1/2 WL multiple. You can get any R value of 3-5000 ohms down to 50 or less
and it's usually reactive. I use a "magic length" of 1/2 WL multiples on all
bands. That assures the least reactance if not zero on certain frequencies.
It's handy if you know how to GD open wire line for a particular resonant
"magic" length--which is easy to do.
Fortunately the Johnson MB will tune out a fair amount of reactance but
its best to have the least you can. I'd find a 1 KW MB or the 275W model,
take it all apart and inspect everything for arcs. Remake all connections.
I use them without the case. They use the same tap on one side for 15&10M
which unbalances the tuner some on 10M. It will work on 30, 18 & 12M also.
Make provisions for using other than just 2 turns in the link if needed. Use
RF ammeters in the output or shunted light bulbs. If you can get 1:1 SWR,
maximum transfer of energy occurs--most of the time. Did you see the bit I
had on the MB in TT and the dual differential modification or reversal for
certain odd loads. I sometimes use a variable Xc in the link but it's not
needed most of the time. I use 100 ohm balanced coax into it a lot (2 RG-8's
connected in series). Add another SO-239. I have made my own mono-MB with
commercial plug in 75,150, 500 or 1KW coils or made my own for 20-2M with
silver plated 1/4" tubing.
k7gco
<< Hello K7GCO,
I am in the throws of putting up a 132' dipole fed with 450 ohm line. I am
going
to try to bring the line into the shack through a 9:1 balun made of 10 turns
of
RG8 on a bleach bottle (about 6" in diameter) and a pig tail to the shack
and to
the back of the Drake MN2700 tuner. I will have about 200' of twin lead in
the
run. Have you ever tried the coax method outlined above? Any help you can
be is
appreciated.
tnx es 73 de n8csp k
<< goodnews@mail.cmedic.net >>
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