----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
> If the VSWR is below 4:1, RG-59 and RG-8X have very little loss below 28
MHz.
> IMO, the value of open wire line is HIGHLY over-rated. There's an
equation (and a
> graph of it) in the ARRL Handbook and Antenna Book that does a good job of
> dispelling the myth of excessive loss in lines having only moderate
mismatch. The
> real issue is matching the line to the transmitter so that the protection
circuitry
> doesn't throttle back the output stage.
>
Hi Jim,
What you say is true, but take a look at the VSWR of a
typical non-resonant antenna and you will see that VSWRs
can be significantly higher than 4:1. When you start getting
up to VSWR's of around 20:1 (1000 ohms feedpoint), there
can be a huge difference between coax and open-wire line
(just plug some typical numbers into that chart you refer to).
My 80 meter inverted-vee at 40' works great on 80 meters.
On 40, it is a total dummy load because I am feeding a
high Z antenna with a low Z feedline. The mismatch must be
horrible, so even though I am using low-loss LMR-400 coax,
it is still a very lossy system (at really gross mismatch you
have to get up to something like 1 5/8" 50 ohm hardline
before you start to compete with open wire line). Two
half-waves in phase should be a good antenna. As a matter
of fact, the antenna hears reasonably well, but I have at
probably have at least an S-unit of additional loss in the
relatively short, but grossly mismatched low-Z feedline. This
bears out in practice. I can hear everyone just fine, but they
oftentimes can't hear me.
At the local club we have an 80 meter delta loop hung
off a 60' tower. The antenna sits on top of a 400' high mesa.
It should be a pretty good antenna, but it is a dummy load on
75 meter SSB. I did some calcs, and it's probably down an
S-unit on 75 meters due to the additional mismatch loss from
the high VSWR on the feedline (the antenna is tuned for CW).
There is additional loss due to the fact that the current
maxima are so close to the ground (no radial system).
Between these two factors, there is probably at least 10dB in
system loss. One KW becomes 100W and 100W becomes
10 W. With a dipole tuned for 80 CW, you can get away with
feeding it with coax and matching in the shack with a tuner,
because the VSWR in the 75 meter phone band is not too
bad (5 or 6:1 typically). No so with a delta loop (at least the
one I modeled). The VSWR in the phone band is over 20:1
when its tuned for CW. The TLW tool that comes with the
ARRL handbook is real handy for looking at these sorts of
things (it's basically an electronic version of that chart in the
handbook). Plug some numbers in for open wire line and
hardline at very high VSWR's. I think you will find the results
to be very revealing.
Ken, N6KB is right. Resonance isn't so important in terms
of getting an antenna to radiate once the power is flowing
into the radiator. It can, however, be very relevant in terms
of getting power from the transmitter to the radiator. If our
antenna tuners were mounted right at the antenna terminals,
we could all be much less concerned about "resonance".
73 de Mike, W4EF......................................
> >Then you need a low loss antenna tuner, and you have to use it right.
>
> There's no good excuse for not having resonant antennas on Field Day, and
if the
> antenna is reasonably close to resonant, almost any tuner will be fine.
For all
> practical purposes, all operation on Field Day is confined to a pretty
narrow
> segment of most bands on a percentage bandwidth basis (other 75 phone).
How
> hard is it to hoist a dipole on some pulleys and lower it once or twice to
tweak it to
> length? IMO, the LDG auto tuner, the Elecraft auto tuners, and the Ten
Tec tuners
> are all damn good.
>
>
> Jim Brown K9YC
>
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