[TowerTalk] Using 75 Ohm Hardline

[Tom Rauch]

> But, regarding the mismatch between 50 and 75 ohms, if on
all bands my antenna were a 1:1 (at 50 ohms) I would not
have worried about the immediate jump to 1.5:1 or the extra
loss created by such.  My antenna at best is around 1.5:1
and near the band edges gets higher than that.  My concern
was the added SWR to this when using the 75 ohm cable.

With a perfect 50 ohm antenna the SWR normalized to 75 ohms
on the 75 ohm line is 1.5 : 1, but that isn't what the
50-ohm system in the shack sees!

The actual SWR at the radio caused by using 75 ohm cable in
a 50 ohm system isn't "1.5 : 1" , it is somewhere between
1:1 and 2.25 : 1 in the shack. This assumes the antenna is
50 j 0.

Since you have an SWR of 1.5:1 at the antenna (normalized to
50 ohms) the antenna impedance can be as low as 33.3 ohms.
If it is 33.3 ohms SWR back in the shack can be as high as
3.38 : 1 (169 ohms) with some cable lengths or as low as 1.5
: 1 if you use an exact multiple of 1/2 wl.

We have a complex system when we mix impedances throughout
the system, and I didn't even consider a reactive load in
the case where you had 1.5:1 SWR in the 50 ohm system.

We often fail to mention another thing that happens. As the
line length gets longer and the SWR on the 75 ohm line
higher (normalized to 75 ohms) , SWR bandwidth becomes
narrower.

None of this means the 75 ohm cable won't work OK. It just
means things are not as simple as they first seem to be when
we look only at SWR and not impedances in the system. Keep
that in mind.

73 Tom