To: <topband@contesting.com>
>Date: Mon, 16 Dec 96 20:37 MST
>From: Rod Greene <w7zrc@micron.net>
>Cc: topband@contesting.com
>Sender: owner-topband@akorn.net
>Precedence: bulk
>
>Eric,
>
>I would appreciate a description of how you built this one.
>What do you mean by "modified"?
>
> Thanks and 73, Rod
>
>At 01:40 PM 12/16/96 -0700, Eric Gustafson wrote:
>------------------snip-------------------------
>>Anyhow, the following year, we built a modified delta loop that matches 50
>>ohms directly and operates at a much lower system Q. It works better as a
>>TX antenna than the "J", requires only one small support (98 ft at apex),
>>and is broadband enough that the tuner is not necessary. Since the high
>>votage ponts are 86 feet apart and not at a wire terminus, even Larry can't
>>burn it up.
>----- Rod Greene, w7zrc@micron.net, <>< -----
>
Rod,
What I mean by "modified" is that the geometry has been
changed from the standard equilateral triangle delta loop so
that the antenna matches 50 ohms directly instead of
approximately 200 ohms. This modification also results in
being able to hang the radiator from towers which are too
vertically challenged to hang the normal loop from.
Here is a text description of the 50 ohm Delta Loop. I hope
that it is good enough to be useful for you. I will
describe it as line segments between points located on an
isosceles triangle. The points "A", "B", and "C" are at the
three corners. Point "D" is half way across the longest
(bottom) side between points A and C.
Point B should be supported 96 to 98 feet above grade. This
is one of the two high voltage sections of the antenna. If
you are planning to run significant power, take care to
adequately insulate the radiator from the support ropes
attached at this point. We used two six inch long ceramic
insulators between the radiator and support rope wherever we
attached rope supports. We had no instances of damage to
this antenna or supports due to "St. Larry's Fire".
Use #12 or #14 bare hard drawn copper wire for the radiator.
Wire segment AB = BC = 153 Ft.
Wire segment AC = (AD+DC) = 255Ft.
Rope segment BD = 84 Ft. (including 2 feet of insulators) to
help support center of long horizontal conductor. Point D
is the other high voltage point of the antenna. Keep the
region in the vicinity of point D clear of lossy dielectric
materials as much as possible.
Points A, D, and C should be 12 to 14 feet above grade.
Make tuning adjustments if necessary at both corners A and
C. Tuning adjustments should be proportional. For example,
if 6 inches is removed from segments AD and DC, then (0.866
* 6) or 5 inches should also be removed from segments AB and
BC to keep feedpoint impedance constant while changing
resonant frequency.
With the dimensions shown, the feedpoint impedance should be
very close to 50 ohms and resonance should be between 1.830
and 1.840 MHz. This loop has also been scaled for use at 80
Meters as the driven element of a 2 element parasitic array.
The 80 meter antenna proved equal to a local 4 square array
on the Tucson to Europe short path.
Feed directly with 50 ohm coax at either point A or C. A
ferrite bead sleeve type balun should be used at the
feedpoint to keep antenna currents from flowing on the
feedline. We used RG213 and 50 ferrite beads. Dress the
feedline directly away from the corner in the direction of
the support rope for a distance of at least 10 feet. Then
drop the feedline to ground and route at or below grade
level to TX.
73, Eric N7CL
--
------------------------------------------------------
Eric Gustafson N7CL | The mountains are high and
6730 S. Old Spanish Trail | the Emperor is far away.
Tucson, AZ 85747-9498 |
| You can't work 'em
INTERNET: n7cl@mmsi.com | if you can't hear 'em.
------------------------------------------------------
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