In message , Owen Duffy
writes
"Wayne" wrote in
:

I am using a whip antenna mounted on a metal patio cover. Results are
good on 10 and 12 meters, and contacts have been made down to 40
meters. (Matches were not obtained on 15 or 30 meters) The whip is 8.5
ft long (a longer whip is under consideration). It is fed through an
antenna tuner and about 25 feet of RG-58. I'm about to replace that
with RG-8.

Taking this to mean that the ATU is at the tx end of the 25' of RG58...

It is challenging with that topology get get good antenna system
efficiency when the monopole is less than about 17% of a wavelength.
Working that backwards, your 2.6m whip is 17% of a wavelenght on 15m
wavelength or 20MHz.

The contributions to poor efficiency are feed line loss under standing
waves, ATU loss, and if the whip is magnetic stainless, conductor losses
in the whip.

RG8 will have lower losses, but the result will be a more challenging
load for the ATU and its losses will increase, consuming some of the
benefit.

You really need to take a system perspective because there is a complex
interaction of the system components.


Any comments on how to hold down feedline losses. One suggestion is
to use two parallel lengths of RG-8 so that half the current runs
through each. Also, site geometry would allow open wire feed.

Paralelling two cables reduces Zo to one half, but the matched loss per
metre is the same. The loss under standing waves depends on the actual
load impedance which you probably don't know.


Possibilities?


A remote ATU, a longer monopole.

The OP seems to be doing what I did for many years, ie feed an endfed
monopole antenna (of undefined length) with coax, and force match it, as
required, at the shack end. [I believe it was you, Owen, who pointed out
my error in trying to use the graphs showing loss vs SWR when the coax
is electrically short (less than a wavelength.]

Although I now have a remote tuner at the antenna feedpoint, I can't say
I've noticed an outstanding improvement in performance (although, to be
honest, I really haven't really done a lot of operating since I
installed the tuner).

If you don't want use a remote tuner at the antenna feedpoint, the
impedance at the shack end of the coax will be the antenna feed
impedance, transformed by the length of the coax, and also altered by
the loss in the coax. Provided the shack-end tuner can be persuaded to
match the impedance seen looking into the coax, the system will work
tolerably well with low-loss coax.

It is obviously advantageous to use the lowest loss coax you can lay
your hands on. As, in cable TV systems, long coaxial trunk lines have
now been superseded by optical fibers, I would suggest that a cable TV
operator might be persuaded to donate some nice chunky coax (preferably
"as thick as a horse's prick", as one of my bosses once described it).
Failing that, just go for the best you can get.
--
Ian