[TowerTalk] 75 ohm hardline/twinlead

DavidHarper@tx.slr.com DavidHarper@tx.slr.com
Wed, 29 Oct 1997 11:08:02 -0600

Hi Bill.
I imagine you will get responses from people who know more than I, but
here are my thoughts (mostly just stuff I have heard/read over the

> I have 2 questions:
> I recently obtained some 75 ohm hardline from my cable company.  In
> talking to the gentleman who gave it to me (and is also a ham with a
> lot of antenna experience), 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
> will?
I have heard that at HF it makes no difference, and also the type of
splices and/or homemade connectors you might use also make no difference
at HF (as long as they are sufficiently protected against weather).  But
I also know that the purists like to use impedance transformers and
"proper" connectors.

> 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.  Comments please!
I think that ladderline could be used with good results, but there are
some possible drawbacks.  First, ladderline might be desired for really
long runs due to the low loss.  With your 60' run, any loss reduction
would be negligible compared to decent coax, especially since you are
using an antenna that probably presents a pretty good match to coax.
Ladderline has to be routed carefully to keep it from coming too close
to the tower or other conductors (getting it into the shack can also
take some thought), and I've heard about needing to have a half-twist
every few feet to keep the ladderline from radiating.  Also, rain/snow
affects the impedance.  Dipoles fed with ladderline need an antenna
tuner in the shack.  You don't want to have to adjust a tuner every time
you change bands with your beam, do you?  I'm not sure if this would be
needed or not if you use a balun at both the beginning and end of your
feedline run.
When I build a quad "some day" (I have had this cool aluminum spider
mount for years!) I may try to feed it with ladderline to avoid having
to switch in impedance tranforming stubs for each band.  We'll see.

> Regards,   Bill - WX8S
> wbh3@chrysler.com
Dave WD5N  /  VP5EA

FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search