[TowerTalk] 75 ohm hardline/twinlead

wbh3@chrysler.com wbh3@chrysler.com
Thu, 30 Oct 1997 17:53:30 -0500

        kc5ajx @ hotmail.com
        10/29/97 07:58 PM
To: William B Haselmire/SCI/Chrysler
Subject: Re: [TowerTalk] 75 ohm hardline/twinlead

Bill I'm no expert at this and my comments are from what I've learned by
reading different sources not from persoanal experice.

 I asked him about a means of matching the
>75 ohm hardline to the 50 ohm input  for my Icom 765.  His comment was
>basically not to even bother or worry about it.....just hook it up and
>go for it.  While he certainly seems to know what he is talking about,
>I am still curious as to whether this will work as easily as he says it

 If you divide 75 ohms into 50 ohms you will get a diference of 1.5
which will be the SWR of the hardline into a match antenna. This is
within the tollerances of even solid state rigs therefore don't worry
about the mismatch just spend the time turning the antenna.
  If you use a 75 to 50 ohm transformer to match the hardline to a 50
ohm load then you have to make sure that the transformer can take any
power that you decide to run at the time or at a later date. The
transformer would be the weakest part of the feed system and thus the
most likly to fail.

>second question:  I have also heard (and wondered why not)  that
>instead of using coax to feed    my beam, that I should just try  400
>ohm ladder line (I assume with a proper  balun of some kind) to feed
>the beam and I should get real good results with minimum loss.  Since I
>am only going to have a short  run of about 60 feet to the beam, why
>not use the ladder line instead of the more expensive coax?  It seems
>that I have only heard of a few stations that use this method, everyone
>else going with the standard coax.
  Ladderline is used mostly in antennas that are not matched ie do not
present a 50ohm load to the rig and are usally much highert than 50 ohms
ie loop antennas and end feed long wires.
When using a beam the impedance is close to 50 ohms thus coax is the
perfered feed line. Also ladderline is affected by metal objects close
by so you can't run it down a tower or metal mast without some kind of
standoff, ladderline is also affected by ice and water ie rain. also
ladderline is usally conected to a tuner then to the rig so as to
present a 50ohm load to then rig. Remember even though it is called an
antenna turner it doesn't tune the antenna and the mismatch on the
antenna side of the turner can cause coax to breakdown but ladderline
can handle it. (that last statement is not really right but I can't put
it in the right words). Basicly ladderline can handle the sometimes
large mismatch where coax can't.
 I hope I made it clear what I'm trying to say, I have a little trouble
with that sometimes. Also if I'm mistaken in my statements then someone
will correct me and I will learn something also


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