Since you have a tuner, you're in great shape. The usual problem with multi
band dipoles is trying to get a decent match on all bands, because the wires
all interact with each other. For you, though, it's probably ok if the
perfect match for 20m is at 13.9 MHz or 14.4 MHz, because the tuner will fix
it.
A couple ideas.. I have a Alpha-Delta DX-CC which is a multiband dipole with
3 elements. A 10m, a 20m, and a 40m (which serves on 15 as well, by
operating as 3/2 wavelength). It has a funky loading coil about 10 feet
from the end which does the 80m band. The 3 wires are supported about 6"
apart by plastic spacers (1/2" PVC pipe would work well, I think, especially
if you got the pink UV stabilized kind). Originally (before tuner days), I
tuned it up using a MFJ antenna analyzer, and it was a bit tedious. Start
with the 10m element, adjust, then adjust the 20 element length, then adjust
the 40 (inside the coils, then adjust the tails for 80, repeat....
Dipoles are fairly broad band (typical 2:1 bandwidth is on the order of
5-10%,) which means that the match isn't all that bad even if the length is
a bit wrong. For that matter, a dipole has approximately a 70 ohm feed point
impedance which in itself gives you a non 1:1 VSWR (although, if you bend it
a bit, as in an inverted V, it will come closer to 50 ohms).
Put some numbers to it. Say you wanted to operate at 14.2MHz, and your
antenna wound up being resonant at 13.9 MHz. That's only 300 kHz away, and
the VSWR is pretty close to 2:1, which your tuner will have no problem
matching.
You don't want to be too cavalier with the lengths, because the mismatch is
going to have some power circulating between the antenna and your tuner
through the feed line. The tuner's reactive components are probably the
dominant loss in the system, and you want to keep them to a minimum.
However, even if you wind up 5% away from the resonant frequency, it's still
only about 3:1 (into 50 ohms).
Spend your time trimming the low band elements (the band is a wider fraction
of the center frequency, (3.5-4MHz is 13% bandwidth..) However, even here,
you're still probably within 4:1 if the wire is even remotely close to the
right length.
Here's the upshot... Get out a tape measure. Cut the wires to about the
right length. Pull it up in the air, use the tuner, be happy. If the
antenna is close to the ground, contemplate making the wires a bit shorter
(you can just fold them back on themselves, or wad it up in a ball, to make
a capacity hat). When I had my multiband dipole 5 feet off the ground, I
found that I needed to shorten the wires almost a foot to get the resonance
in band.
Spend your time on getting the wires high in the air and away from other
things. That's probably more important from a loss/efficiency standpoint
than anything else.
If you need something to do while it's pouring rain, download a copy of
4nec2 (free) or multinec (nominal cost) and plug in a design and let the
optimizer find out the exact right length wires.
----- Original Message -----
From: "Tim Holmes" <W8TAH@Zoominternet.net>
To: <towertalk@contesting.com>
Sent: Thursday, October 21, 2004 12:06 PM
Subject: [TowerTalk] New Member - Need help with an antenna design
Hi Folks
I'm trying to put some work into my antenna system before winter really
gets going, and I am looking to redsign my multi-band dipole. I am
strongly considering a fan type dipole cut for the center of the General
band on 80, 40, 20, 15, 10. I use and IC706MKIIG, and have LDG AT11mp
(I think) auto tuner.
The antenna will be hung roughtly north south at about 30 feet, and will
have a 20 degree dogleg to the west in the north half.
I realize that a dipole is nowhere as good as a beam etc, but I cant put
up a beam, and I can put up a dipole, so I need some suggestions, or
links to plans etc.
Thanks for your time
Tim Holmes
Amateur Extra Operator
W8TAH
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