TopBand: Interlaced Delta Loops

[Bill Tippett]

K6SE wrote:

>First, I modeled a single equilateral triangle delta loop with its bottom
>wire 8' above "good" earth.

Earl, I used a Delta from my first NC location for 1 year and then
from my current NC location for 1.5 years.  The "best" configuration
is a slightly "squashed" format, with the base longer.  The antenna
should be fed at 1/4 wave down from the apex with a 1/4 wave matching
stub of 75 ohm coax.  This "squashed" format only requires about a 120 ft
support and actually gives improved gain over an equilateral delta.  
I have trashed my model details since I converted to a new version of
EZNEC, but I recall something like 150 ft for each side and 240 on the
horizontal leg.

        I found by modeling very little change in pattern or
impedance by feeding this antenna right at the corner which was much
easier physically than 1/4 wave down from the apex.  I also "opened"
the apex to take advantage of my first NC tower @ 150 ft and my second
at 180 ft.  This actually improves the gain over a completely closed loop
by ~0.5 dB, by moving the vertical radiators farther apart and has no
other effects since the apex is at a current null anyway.

        You can also install this loop on a 90 foot tower by pulling
the base away from the tower.  In fact, this further improves the gain
in the direction the base is moved.  I believe this is what W3BGN is
using which works very well.  By opening the apex AND pulling the base
30 ft away to the NE from my 180 ft tower, I theoretically should have had
around 2.5 dB gain over a vertical.

        Now for the bottom line... I found the loop worked very well
broadside (NE and SW), but was terrible off the sides (as expected).
In December 96, I became worried about working VK0HI since Heard is
about 140 degrees from here, and I had a heck of a time working VP8SGP
in South Georgia whan I was at my former NC QTH.  I shunt fed my tower
and put down 20 1/4 wave radials.  Although I could not do instantaneous
switching between the two, it appeared that the vertical worked better in
ALL directions, even though the loop theoretically should have had 
more gain toward Europe.  After leaving both up for awhile, I finally
took the loop down completely and put down a full radial system under
my tower last summer.  

        If you have a 90 foot tower, you can take just ~540 ft of wire
and have a very respectable signal with a sloping/squashed Delta loop.
It should also be possible to use two at right angles with no interaction
if you open the feedpoint of the one not in use (and have the apexes open),
since the two resulting wires are nonresonant.  The latter does require
both loops be vertical so you would need at least a 120 ft support.

                                                73,  Bill  W4ZV

P.S.  The funny thing is that when I finally worked VK0HI, he was 
coming out of the Northeast and my original loop would have worked fine!


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