[TenTec] It's getting cold. Perfect antenna weather for a OCF dipole

[Rick - DJ0IP / NJ0IP]

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 at 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