Hello,
In the early nineties, I installed a fairly equilateral, 75 meter delta loop
utilizing the paralleled and taped coaxial feedlines described. The loop
was fed with 2 each RG-62 lines (93 ohm coax) which had the braids soldered
together at both the antenna feedpoint and station end. Therefore, the
resulting coaxial balanced feedline had an impedance of roughly 186 ohms
(double the characteristic coax impedance). The station or source end was
terminated to a 1:1 current balun under our ham station building with a
short run (10 ft) of RG-213 to the shack. Oh, the loop was mounted on three
telephone poles at heights of 80, 50 and 40 feet, and presented a near
perfect match near 3850 KHz, with correspondingly great matches on some
portion of the 40, 20, 17, 15, and 10 meter bands.
This antenna system was located on Keesler AFB, MS (surrounded by Biloxi,
MS) as a part of the well-funded military club station, K5TYP. Needless to
say, it was a KILLER antenna on 75 and 40 meters, and played equally as well
as the TA-33 tri-bander at 40 feet on 20 and 15 meters most of the time. I
remember running a kilowatt or more to it with a TS-830S / Henry 2K Classic
X on 75 and 40 meters and being 25 to 35 over S9 to most everywhere in the
country. In fact, some of my friends at the time in the northern Midwest
(3913 KHz) thought I was God when I came on from the Deep South. Although
it was a concern, the high power level didn't seem to affect the coax lines
or balun that I could see. I figure it was due to the excellent match we
had (keeping the voltages down). One proof to its effectiveness would be
the 1991 Sweepstakes Phone Contest Delta Division 1st place finish, new
Delta Division record and "clean sweep" that year. This skyhook loop
antenna was used for most of the contest on 75, 40, 20 and 15 meters, with
occasional switches to the TA-33 at 40 ft.
I believe this antenna and feedline arrangement were as good of a matched
system as I have ever seen. The height of the loop, along with the fact
that it was "in the clear" of its surroundings, should have made it present
a feedpoint impedance in the neighborhood of 150 to 200 ohms. It did, and
easily and completely matched the described feedline system on multiple
bands.
I fondly remember operating this skyhook, and look forward to building a
similar antenna again. I hope yours coaxial balanced line feeders work as
well as this example.
73.
Steven J. Gehring, KZ9G
Bothell, WA
E-mail: kz9g@arrl.net
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Michael Tope
Sent: Tuesday, July 29, 2003 6:35 PM
To: Chuck Counselman; Towertalk@contesting.com; Jim Lux
Subject: Re: [TowerTalk] Balanced Line using Coax ?
Don't feel bad, Jim. I went to bed last night thinking that it was 1/2 the
loss, but when my head hit the pillow I started to imagine two separate
generators (one driving each coax with 1/2 the total power) which made me
start to think that the loss was the same in either case. Fortunately, when
I came back downstairs I realized that I hadn't hit send yet :):)
Anyway, I still think its the same, but perhaps are more rigorous analysis
will show otherwise. I'll leave that to someone else as this will all be
academic if I don't get my tower application started :):)
73 de Mike, W4EF............................................
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "Chuck Counselman" <ccc@space.mit.edu>; <Towertalk@contesting.com>
Sent: Tuesday, July 29, 2003 10:54 AM
Subject: Re: [TowerTalk] Balanced Line using Coax ?
> Now I am really confused.. enough of the facile quick stuff.. time for
> the rigorous analysis.. stay tuned, but more facile analysis below
>
> At 12:55 PM 7/29/2003 -0400, Chuck Counselman wrote:
> >At 9:34 AM -0700 7/29/03, Jim Lux wrote:
> >>...Looking at loss as a dB/foot for the coax, you divide the power
> >>into two pieces of coax, so the absolute loss (in watts) will be
> >>half in each piece of coax, but you've got two coaxes, so the total
> >>loss is exactly the same....
> >
> >
> >No. To deliver the _same_power_ as a single coaxial line, two
> >coaxial lines operating in "push-pull" deliver half the current at
> >twice the
voltage.
>
> But half the power flows through each line, no? and, half the power
> isn't half the current.
>
>
> > At HF, virtually all of the loss is ohmic, and the power dissipated
> > per unit length of conductor is equal to I^2*R', where I is the
> > current in the conductor, I^2 is I squared, and R' is the resistance
> > per unit
length
> > of conductor.
>
> Yup..
>
>
> > With half the current, the power dissipated per unit length of
conductor
> > is quartered;
>
> Ok.. but, is the current halved, or 0.707'd... P/Z0 = I^2... current
> would go as the square root of the power.
>
>
> >the length of conductor is doubled; so the total power dissipated in
> >the pair of push-pull coaxial lines is one-half that dissipated in a
> >single coaxial line carrying the _same_power_.
>
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
> "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
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