Questions about my use of 50 vs 75 ohm cables:
Robert Shohet wrote:
>
> Ok, now I am really curious....
> > >
> > > Are you saying that you run 75 ohm coax from
all the antennas to the
> antenna
> > > switches and then from the antenna switches,
run 50 ohm to the
> wattmeter?
> >
> > between the switches on the towers and the outside
wall of the shack all
> the
> > lines are 75 ohm. from the
switches to the antennas is 50 ohm, and from
> just
> > outside the shack wall all the way to the radio is
50 ohm.
> >
>
> Let me ask a naive question.... Let's say that the
antenna really is at 50
> ohms with 1:1 swr. The you
add the 75 ohm piece to bring it inside the
> shack. SWR is now (at least) 1.5:1 not
including the effects of any
> reactance since the 75 ohm
piece is not a multiple of a 1/2 wavelength. Now
> you go back to 50 ohm. Doesn't that multiply the mismatch by at
least
> another 1.5:1? Or does that somehow reduce the mismatch
by 1.5 :1?
But
> then since the piece inside
the shack is not a multiple of 1.5:1 and
> reactance is a factor of some
unknown quantity, doesn't this create a total
> "witches brew" of the swr
and impedance at the radio and amp?
Please help
> me understand this.
_ 50 ohm
75 ohm
50 ohm |||
|_|------------================================--------------|||
50 ohm antenna
|||
tx
ok, from right to left...
on the 50 ohm line attached to the
antenna the swr is 1:1
on the 75 ohm line the swr is 1.5:1
at the left end of the 75 ohm line
the impedance can be anywhere from 50 to
112.5 ohms due to the impedance
transformation along the line.
every 1/2 wave
the 50 ohms will repeat, and half
way in between at the odd multiples of 1/4
wave you get 112.5 ohms.
so on the 50 ohm line to the
transmitter you can have anything from 1:1 to
2:1(approx) swr.
on a monoband
system there are 2 easy fixes if you get an impedance you don't
like... i
always leave a bit extra hardline anyway so that can
be trimmed to
bring it closer to the 1/2 wave
multiple. or
in one case (160m where trimming
1/4 wave wasn't very practical) i
made the line from the hardline to the tx out
of 75 ohm rg11 which made the
impedance much better.
all my systems are made like this
and the only ones i have had to play with were
160m and 15m a bit, i guess i was just lucky that the others were close enough
to 1/2 wave not to bother
with. you
could of course measure out the hardline in
advance with an analyzer. there are also
other methods like one that puts
coaxial transformers at each end,
or the 50-75 ohm un-un... for hf though both
of these seem like overkill and add
complexity with extra connectors and other
things to break or cause more loss
than the mismatch does.
btw, on mine i
always stop the hardline just outside the shack and
bring rg8/213
into the shack. its just too
hard to bend that catv stuff around without
kinking it.
>
> > >
> > i never measure swr on the 75 ohm cables, only one the 50 ohm ones inside
> the
> > shack, or on the 50 ohm pieces going to the
antenna when i am tuning them
> on the
> > ground. i
have never had a problem with this.
> >
>
> Now does this also mean that with my hardine stubs (75 ohm fused disc CATV),
> when I am making 1/4 sections
of coax (to connect the stubs to each other)
> that I should have used 75 ohm coax (RG11 say) instead of 50 ohm
coax
> (RG213)?
no, because the 1/4 wave stubs are
meant to look like an open or short at the
end where they connect to the 50
ohm feedline the actual impedance of the stub
cable doesn't matter. so you want to
keep the section of coax between them the
same as the rest of the cable that
the goes to the transmitter.