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[TowerTalk] Re: Force12

To: <>
Subject: [TowerTalk] Re: Force12
From: Mike" < (Mike)
Date: Thu, 11 Oct 2001 21:05:40 -0700
Hey Tom,

One more try -  see my comments below:

> Mike, 
> The SWR  *in the 75 ohm cable* is still only 2.14 to 1
Yes, I agree that the ratio of voltage maximum to voltage minimum 
on the 75 ohm line will still be 2.14:1, but that is not what my 50 ohm 
equipment is interested in. My 50 ohm equipment is interested in how
closely the line input impedance will match to its design impedance.
The 50 ohm VSWR meter will give an indication of that.   

> The SWR *in a 50 ohm cable*from the end of the 75 ohm cable
> to the transmitter depends of course on the load impedance 
> presented to the 50 ohm cable which of course depends on
> the transposed impedance (and therefore length) of the 75 ohm line.
> I think most readers on towertalk know to use multiples of
> 1/2 WL of  75 ohm CATV line for their 50 ohm systems
> so that the impedance presented to the 50 ohm line is the
> same as the load impedance.  That topic has been 
> discussed many times before.

Yes, I wouldn't be surprised, but it does illustrate my point.
> Even if you use an odd number of Quarter wavelengths of 75 ohm
> cable which transforms the 35 ohm load to 160 ohms, the SWR
> in the 75 ohm cable remains at 2.14.  The 3:1 SWR you mention
> only exists in the (short) 50 ohm cable from the end of the 75 ohm
> line to the transmitter.  YES, you may want to match that back to
> 50 ohms for narrow range amplifiers with limited tuning range.


> FWIW, when I was at Collins Radio in the late 60's, I was told the
> rule of thumb for antenna system design was to try to keep the
> SWR on 50 ohm cables below 3 to 1.  Anything above was matched
> at the antenna.  Anything below was matched at the transmitter.

Seems like a good rule. For convenience, I match my coax fed 80 meter dipole 
in the shack with a tuner on 160 - 10 meters. On the higher bands its a 
real dog, even with LMR-400 feedline. I haven't made measurements, but
I am sure the line VSWR is very high in most cases. One of these days, 
I will get a better antenna :) 

73 de Mike, W4EF..........

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