Mike,
You were a bit vague on a few points, making it difficult to 2nd guess the
problem.
1. When you say your openwire fed dipole (OW-FD) dipole "won't perform", what
do you mean by "perform"? Do you mean actual on the air performance, or do you
mean you can't manage to match it?
On the air, an OCFD will work no different than the OW-FD. Assuming approx.
same height and length their performance (in real life) will be nearly
identical. As to which one has less loss, down to 1/10 of a dB, that will
depend on the cores and design of the balun on the OCFD and the type of
matchbox used on the OWFD.
I have always been able to find a way to match an OW-FD regardless of length of
feedline (to an extent). If it won't match on 40m, it only takes about 12 ft.
of feedline to move it far enough to match. Twice that on 80m. I don't know
how you came up with the 100' number. Reading on the wrong web site, I guess.
I used that kind of antenna for 45 years and moved about 25 times during those
years. I used all sorts of lengths of the dipole lengths and of the feedline.
You need to describe your matchbox to us.
I have had good success running the Ten-Tec model 238 and a coax Ugly balun.
Of course you must often use an external capacitor on the back for 160 and/or
80m.
Never had to use the ext. cap for 40m.
With all the vintage equipment you have, I assume you are trying to match with
a Johnson Viking Matchbox. Regardless of how the old timers rant and rave
about this box, it did have a somewhat limited matching range. I can show you
how to improve that quite a bit.
See:
http://www.dj0ip.de/antenna-matchboxes/symmetrical-matchboxes/j-viking-upgrade/
(if the link breaks here, paste the link into a browser and remove any spaces).
2. It's tough to get 80/40/30 to work together in an OCFD. Probably the best
design to get these three bands is to use the one W8JI proposes (or my similar
one) with the feedpoint located about 1/5 from one end. You Must cut it for
the very bottom of the band on 80m, then use a matchbox for the upper portion
of the 80/75 and possibly also on 30m. But these can be a simple Transmatch.
You don't need something expensive.
On the other hand, if I only wanted 80 and 40, I would build a fan dipole. I
have had good luck just hanging the 40m radiator under the 80m radiator,
separated by about 18". Use thin PVC tubes as spacers.
3. Before you go spending money on a balun for and OCFD, you should do a bit of
due diligence. Saying you have a 1:1 current balun is about as informative as
telling us you have a car. Which exact toroid is inside? Is it wound with
bifilar windings or coax? If bifilar, are the wires Teflon insulated or just
bare enameled copper? I won't use a balun at any power level (except QRP) that
has enamel insulated wire. And if it is wrapped with coax, is it
Teflon-insulated coax? In any case, how many windings are on the core? ALL OF
THIS MATTERS. If you don't know why, find out before wasting you money buying
possibly the wrong bits.
How much power do you run? Is it only SSB and CW or do you run high power in
RTTY?
If high power in RTTY, I would avoid the OCFD.
The addition of 1:1 balun in the shack always helps a little, but that won't
solve the problem if you don't get it right at the feedpoint.
If you run with the W8JI version of the OCFD, there are two methods I have
built that work and reduce the CMC to a point that you have no problems.
1. Use a dual-core 4:1 Guanella wound on two FT-240-43 toroids; each
transmission line uses its own dedicated core. In addition you will need a
Maxwell choke attached to the balun. The Wireman 8231 will work for that task.
For the Guanella, wind 12 turns of #16 or even #14 Teflon-Insulated wire,
bifilar onto each core. This one will normally work up to about 700 or 800w if
your antenna is hanging in the clear and not running over a rooftop - no parked
18-wheelers nearby, and not running parallel to a metal fence. Standard stuff.
2. Alternative: build a hybrid balun consisting of a 4:1 Ruthroff (voltage)
balun and a 1:1 Guanella (current) balun. Use an FT-240-61 for the Ruthroff.
This one really only works as a transformer here. It does not do much to
impede CMC. For the condx pointed out above, wind 16 turns of RG-142 onto an
FT-240-43. Series it with the Ruthroff (the Ruthroff goes on the antenna
side). For higher power, you'll need to stack 2 of these cores together and
wind the coax around them as if they were one thick core. I'm going to peg
this one at max 1200w. I've never owned an amp that put out more than that.
Problems occur when the antenna is hung near other objects which can cause the
CMC to increase. In that case balun option #2 is preferable. If the problem
is really bad, the only recourse is to start doubling the number of cores. In
fact if the environment is such and you don't need so many bands, scrap the
idea of OCFD and stick with a straight dipole.
I always place a Maxwell outside, just before the coax comes into the house.
In addition I have another Maxwell directly at the amplifier.
If you do incur CMC, replace the Maxwell's with Guanella's.
As Jim suggests, for longer runs of coax, I break it up with RF chokes.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Mike Bryce
Sent: Tuesday, September 22, 2015 7:57 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] It's getting cold. Perfect antenna weather for a OCF
dipole
The antenna I’m planning on installing, the off center feed dipole, will be
replacing the 132’ on a side center fed zepp feed with 600 ohm open wire that
up there now.
After all the work I put into this antenna, it simply won’t perform at my QTH.
Either I’d have to move the shack out into the middle of the back yard, or
somehow wrapped an extra 100’ of feed line someplace. I’m not about to move the
shack and even MY neighbors would raise a bitch if I strung up an extra 100 +
feet of open wire so the feed point impedance would make me a happy camper.
Today, the UPS guy dropped off the HYTOWER 160 meter inverted L upgrade. so,
that’s what I’ll be using on 160 meters this winter. I’ll let everyone know how
it works.
So, for 80, 40 and perhaps 30, I’ll try the OCFD. 20 and up, I have the beam.
If it won’t work on 30, that’s cool, too, as I installed a bazooka two weeks
ago for that band.
I didn’t get the chance to call the torroid guys out in W6 land, but will
tomorrow. I’ll see what they have in stock for the CMC baluns.
I have a 1:1 current balun in the shack from the center zepp experiment, I’m
thinking of putting it in a new box with a SO-239 connector instead of the
screw terminals.
I read someplace that to counter common mode currents, the addition of a 1:1
balun on the shack side of the coax helps along with one on the antenna end.
I’ll let everyone know
Mike, WB8VGE
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