[TowerTalk] 75ohm cable

[Jim Thomson]

Date: Mon, 19 Jul 2010 18:26:27 -0700
From: "Rick Karlquist" <richard at karlquist.com>
Subject: Re: [TowerTalk] 75ohm cable


Gary Schafer wrote:
> You can have up to a 2:1 mismatch (SWR) at the transmitter using 75 ohm
> line
> if the antenna is a flat 50 ohms. If your 75 ohm line happens to be a
> multiple of a quarter wave length that will transform the 50 ohm load to
> 100
> ohms seen at the transmitter end.
>
> 73
> Gary  K4FMX

Actually, the transmitter mismatch will be 2.25:1 (112.5 ohms).
Additionally, if the "50 ohm" antenna is actually 33.3 ohms,
it would be a 1.5:1 VSWR with 50 ohm coax but 169 ohms
with 75 ohm coax.  Your 50 ohm  transmitter (or tuner) would
effectively be working into a 3.4:1 VSWR.  This may or
may not be OK.

Rick N6RK

## agreed.  And if the "50 ohm"  ant is actually less than 33 ohms,
it gets even worse. 

##  Run YC-156 power into high swr's = asking for trbl..even with the mating vac caps tuner. 
Everything from arced connector's to melted hot spots on the coax.  If the broadband 
Array solutions  50:75 xfmr's are used at each end, then you won't have any
problems.  With 9 x HF bands, and say just one remote switch box used... on the
end of 75 ohm coax... your chances of  the 75 ohm coax being a multiple of a quarter
wave long are very good.  I operate at the  upper/lower band edges [and in between],
so the chances of a multiple of a qtr wavelength is pretty good.  

## In Rick's  example above, the swr at the feed point is really 1.5:1    but the TX  'see's
3.4:1   .   Who needs that?   

Later... Jim  VE7RF