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[TowerTalk] 75 ohm feedline

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Subject: [TowerTalk] 75 ohm feedline
From: (Greg Clark)
Date: Sun, 06 Jul 1997 16:36:37 -0700
Pete Smith wrote:
> At 12:23 PM 7/4/97 -0400, Bob Locher wrote:
> >I don't understand the reluctance to use 50 to 75 ohm impedance
> >transfomers. I built two of the ones featured in all recent ARRL
> >Handbooks - took me about an evening, and total cost was about $30
> >including the boxes. They work GREAT. When I put them into my system, I
> >had one in backwards because of a stupid mistake, and the resulting SWR
> >was 2:1. I figured out what was wrong and turned the transformer around
> >and all was beautific.
> >
> >I run 1500 watts to them and they stay cool. I can't imagine why anyone
> >would bother to do it any other way!
> Raises an interesting question, but one beyond my quantitative abilities.
> Assuming the autotransformers are built well - adequate wire size, proper
> core, big-enough box, etc. - what is the expected loss in such a 50:75
> transformer?  If I had that number, i think I could answer your question,
> because according to TL (thanks, Dave), on 20m. my 230-odd feet of 75-ohm
> hardline has an added loss of only 0.05 dB, when terminated on the antenna
> end by a matched 50-ohm antenna or transmission line.  I suspect that's
> less than 2 very good toroid transformers could deliver.
> 73, Pete Smith N4ZR

It's been interesting following this thread for the past few days.  In
Pete's reply he almost said what I'm about to:

Why bother with the transformers at all??

I have many runs of 75ohm hardline running to 2 towers.  They vary in
length from 120ft to 300ft.  I feed monobanders, tribanders, wire and
shunt-feed a tower with it.  I don't use transformers.  Did I try
them??...yep.  Did I see any type of performance  So, they
were done away with.  One less thing to go wrong in the middle of a

As Pete stated....the loss in having a 75ohm feedline going to a 50ohm
antenna is minimal.  It would be interesting to see if the transformers
in-line would cause more harm (be it a small amount) than good.


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