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.
<< Hello K7GCO,
I am in the throws of putting up a 132' dipole fed with 450 ohm line. I am
to try to bring the line into the shack through a 9:1 balun made of 10 turns
RG8 on a bleach bottle (about 6" in diameter) and a pig tail to the shack
the back of the Drake MN2700 tuner. I will have about 200' of twin lead in
run. Have you ever tried the coax method outlined above? Any help you can
tnx es 73 de n8csp k
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