[TowerTalk] RG11 Cable

[Dan Zimmerman N3OX]

On Tue, Aug 25, 2009 at 5:10 PM, DF3KV <df3kv at t-online.de> wrote:

> But only if the line length is a quarter wave electrically or an odd
> multiple


Yeah, for the "worst case" 2.25:1 SWR with an actual ohm load.

But it's a warning about the casual use of 75 ohm cable in 50 ohm systems
based on an experience I had when I didn't even know the cable I had was 50
ohms.  I took my trusty MFJ-259B out to my brand new 40m groundplane
antenna.  I adjusted the length until I had the best SWR across the band,
which was quite good.  Then I hooked up my coax and went inside only to find
that the SWR in the shack was nowhere  close to under 2:1 over the whole
band, when it was nowhere worse than 1.5:1 over the band out at the antenna.

Turned out that about half the coax run from the antenna switch to the
vertical was RG-11.

The point is there are lots of antennas that are absolutely acceptable loads
with respect to the SWR they present to a 50 ohm system, like a 40m
groundplane vertical across the whole band, but you run them through SOME 75
ohm coax and back to 50 ohms, and if you
don't  realize and understand the transformative properties, you tune up
your antenna, and get blindsided by 3:1 SWR where you were thinking there
would be no mismatch issues to speak of.

Specifically on a 1 inch diameter 40m ground mounted vertical in EZNEC
resonant around 7.030, you need to be within about 15% of a half wavelength
line to keep the *50 ohm* SWR at the shack end under 2:1 across the band,
and if you get closer to a quarter wavelength, it's never under 2.5:1,
despite the fact that your analyzer out at the antenna is under 2:1 across
the whole band and under 1.5:1 across the whole thing.

A multiband vertical that was a bit short on 40m should be even worse.

Don't get me wrong.  I agree with everyone else that there is no efficiency
penalty to just tossing the cable in there and letting 'er rip with no
matching.
But there are situations in which adding some 75 ohm cable in a system that
showed good SWR on 50 ohms could get you into the "foldback zone" on a lot
of equipment across the whole band.

If we're thinking about tube amps or tribanders that show 55+j0 midband,
there's no problem.  You don't have to give it a moment's thought.  If
instead you're installing some sort of short-ish vertical over a good radial
system way out in a big field (hence the need for hardline) and a
Tokyo Hi-Power HL1.5KFX with a 1.8:1 SWR foldback and no high power tuner,
you've got a problem for many/most  hardline lengths, and it becomes worth
it to match at least up to 75 ohms somehow.

Dan