[Elecraft] Antenna Theory

[Don Wilhelm]

Mike,

The discussion points all seem to apply to use with non-resonant antennas -
generalizing will not necessarily lead to valid conclusions. The REAL answer
is "It all depends..."

There is nothing wrong with coax at all.  Its benefits are that it can be
placed almost anywhere with no ill effects - throw it on the ground, coil it
on the floor, etc. and it will still do its job.  Some coax types do have
higher matched line loss than parallel line, but compare the loss of 3/4
inch hardline with 300 ohm tubular twinlead - that coax is very good
especially when used with low SWR.  Belden 9913 and RG-8 foam dielectric are
not far behind, but OTOH RG-174 rates quite badly on the loss per 100 feet
scale.
Keep in mind that the length is a big factor too.  Very short runs of most
any kind of coax will be low loss because of the short length.

The rub comes if one is using a transmission line at high SWR conditions -
such as when feeding a non-resonant antenna.  Good balanced line will have
lower total loss than coax, but more care must be taken to keep the parallel
line spaced away from other conducting materials, only use long gentle bends
when changing direction, keeping it supported along its length so it doesn't
change coupling with other objects in the wind, and other difficulties.

So the answer is YES (well at least 'maybe'), if one is willing to install
the parallel line correctly, it will provide a lower loss transmission line
than coax.  Incorrectly installed, parallel line may introduce even more
loss than coax.

73,
Don W3FPR

----- Original Message ----- 

> So I'm going to ask a really elementary question about HF antenna
feedlines:
>
> It would appear to me from all I've read/heard that overall a balanced
> feedline is generally much less 'problematic' and in fact often preferable
> to unbalanced coax.
>
> Since there seems to be no clear advantage to using coax (other than the
> fact virtually every piece of equipment made has a SO-259 on the back) why
> then is coax THE most prevalent feedline in actual use?
>