[Towertalk] 160 Mtr Phased Verticals -Problems and Trivulations
Guy Olinger, K2AV
k2av@contesting.com
Sun, 10 Nov 2002 15:49:32 -0500
A collection of short answers...
There is almost no feedline around that will be lossy enough healthy
to "broaden" the apparent SWR of an antenna on 1.8 MHZ, aside from
damp string or saltwater in a hose. But you can do it with a skimpy
radial system on a short vertical system.
The radial system you have down will beat the silly pants off a pair
of raised radials. You did right.
The reason why your SWR is that narrow with that short of an antenna
over an ideal radial system is because it is supposed to be. SWR
*NARROWS* as you do things that raise the efficiency of the antenna
system, all else held constant. LOSS broadens it. Like the ground
losses you would have with only two raised radials.
Two resonant antennas that close WILL couple each other and change the
match. If you model the combo you'd dig up the why of the match you
get. But modeling linear loading is a bear.
Using the configuration you expect to use the most, center it's tuning
where you like to operate and use your shack tuner to go up and down
the band.
Then go work out the band and enjoy your antenna (and radial) system.
73, Guy.
----- Original Message -----
From: "Rick" <richardw@mho.com>
To: <towertalk@contesting.com>
Sent: Sunday, November 10, 2002 2:12 PM
Subject: [Towertalk] 160 Mtr Phased Verticals -Problems and
Trivulations
> I have two Force 12 160VX antennas. For those not familiar, they
are 60 ft
> tall (46% of full size) and use two sets of linear loading wires
that attach
> at about the center of the antenna. The manual recommends the use
of two
> tuned elevated radials, 16 ft above the ground. The claimed
bandwidth for
> 2 to 1 SWR is 42 kHz..
>
> My antennas are spaced 1/2 wavelength apart; and for reasons still
unknown
> to me (maybe some subliminal message I received from the ham gods
while
> asleep, combined with advice from "experts" on this reflector), I
decided
> to use ground radials rather than two tuned elevated ones. The
more I
> read on this subject, the less I seem to know and the more confused
I
> become; it is worse than trying to figure out what a politician is
telling
> you.
>
> Anyway, after walking the better part of 10 miles I laid out
slightly
> more than 100 ground radials under each one. The radials are
spaced
> evenly and most are 130 ft or so long (there are a few that I
could only
> get about 100 ft - to the W thru NW and then a couple to the N on
one of
> them).
>
> Each antenna is fed with RG-213 phasing line (3/4 wavelength) for
in-phase
> use, and I can switch in a 1/2 wave delay line (also RG-213) to one
antenna;
> allowing the pattern to change from broadside to endfire.
>
> I tuned each vertical using about 30 watts of power at 1.830 KHz,
and a
> SWR meter right at the base of each vertical. By manipulating the
linear
> loading and the hairpin matching coil, a 1 to 1 match was achieved.
>
> I have two questions/problems that I hope someone might be able to
help me
> with.
>
> (1) Problem: The 2 to 1 SWR bandwidth is only about 21 KHz;
exactly
> the same on both antennas. This is the same when checked at the
antenna
> or in the shack; I don't seem to have any "added bandwidth" due
to SWR
> smoothing with the long coax run to the shack. Additionally, I
erected
> one and got it working before I even started on the second one.
It's
> bandwidth was 21 KHz and stayed the same when I installed the second
one -
> installing the second one did not seem to de-tune or otherwise
effect the
> original one.
>
> Question(s): Does tuned elevated radials increase the
bandwidth of an
> antenna as compared to standard old ground radial? If not, does
anybody
> have any idea why the bandwidth is narrower than other hams seem to
get with
> the antennas? Should I go to the trouble of walking another
520 ft
> (plus or minus) and install a couple tuned elevated radials? If
I do
> that, do you use them in tandem or disconnect the ground ones?
>
> (2) Since I am not using any L network at the "T" where I split
the
> signal to the verticals, I obviously lose the 1 to 1 match when
feeding
> the antennas together. The effect of this (I believe which is
due to
> coax length and SWR) is to "shift" the apparent resonant freq of the
array
> at the radio to 1.846 KHz with a min SWR of 1.4 to 1 when feeding in
phase;
> and to 1.840 KHz with a min SWR of 1.2 to 1 when feeding 180 degrees
out of
> phase. I am not a "double E", but I believe these readings would
vary
> considerable, depending on just where you happen to insert the SWR
bridge.
>
> Question: Is it best to "just live with it" and use the antenna
tuner in
> the shack to make the amp "happy"; or would it be prudent to make a
couple
> of L networks and switch them in out at the "T". I am not sure
how
> effective this will be since the bandwidth is so narrow anyway, and
I will
> still need to use a ant tuner some of the time.
>
> Hope someone out their has some answers, opinions, or ideas for
another
> hobby.
>
> Thanks
>
> Dick K8ZTT
> Franktown, CO
>
>
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