From: L. B. Cebik (firstname.lastname@example.org)
Follow-up on the right-angle upright delta loop, side fed
I did a little more work on the right angle delta loop as a superior vertical
radiator on 40 to the equilateral delta loop. As you recall, the right angle
loop is shorter by about 10' than the equilateral loop, allowing it to be
higher by that amount for a constant height top support. That is advantage
number one. The second advantage is a closer match (depending upon the height
of the horizontal base wire) to 50 or 75 ohm coax. The third advantage is
somewhat less sensitivity to the position of the feedline along the sloping
wire. And the fourth is somewhat greater gain at the angle of max radiation
in that flattened vertically polarized pattern prized by dxers.
The exact length of a right-angle delta depends on height. With the base at
35' and the apex at 65.4', the length at 7.15 MHz is about 1050/f or 146.8',
with the sloping sides 43' long each and the base 60.8' long horizontally.
With the base only 10' above ground, and the apex 40.2' high, the sloping
sides are 42.8' each and the base is 60.6' long, for a total of 146.2' of
wire, or 1045/f at 7.15 MHz.
At the higher level, you can feed the sloping wire anywhere from 2% to about
25% along the wire (but not at the bottom corner) with little change in gain
or input impedance. Both stay constant: gain 1.96 dBi; TO angle 16 degrees;
Z = 49.3 to 51.4 ohms with less than 1 ohm reactance (in the model, which has
no ground clutter). However, the least horizontal radiation occurs between 12
and 15 degrees along the wire--the 6' from the bottom corner region. (For
comparison, the equilateral delta had a gain of about 1.4 dBi, with a TO angle
of 15 degrees, and a 100-ohm feedpoint Z.)
With the base only 10' off the ground and ignoring ground clutter (which a
real antenna builder cannot do), the region of constant performance extends
from about 4% to about 20% along the sloping wire, with the least horizontal
radiation in the 13 to 16% range, again in the 6 to 7' from the corner
vicinity. Gain is about 1.54 dBi, TO angle is 23 degrees, and feedpoint
impedance is 77.5 to 79 ohms with negligible reactance. (For comparison, the
equilateral delta with a 10' high base wire has a gain of about 1.12 dBi, a TO
of 21 degrees, and a feedpoint Z of about 160 ohms.) The lower level right-
angle delta loop is a better match for 75 ohm coax, but 50 ohm coax will show
only about a 1.4:1 SWR, meaning insignificant loss from mismatch.
The reason to use the feedpoint position of least horizontal radiation is that
on either side of the region of best vertical performance, a secondary higher
angle lobe grows. It is not very significant, but it does permit high-angle
QRM/QRN to increase. If you use this antenna at all, it will be for its low
angle properties, not its gain, and you will want the best signal to noise
ratio possible on weak dx signals. Decreasing high-angle input decreases
domestic noise, whether QRM or QRN (although it will not eliminate the
neighbor's table saw noise). Hence, getting the feedpoint up to the 13% mark
or so is useful, but there is a +/- 2 to 3% region of virtually no difference
in both the high and low versions (meaning the region is there at all
intermediate levels for the bottom wire). You can also interpolate probable
feedpoint impedance from these limiting figures.
All models were made over medium quality ground (Somerfeld-Norton analysis)
with no ground radials, counterpoises, or other ground improvements. The
delta loop needs no radials.
Like the equilateral delta, the right-angle delta is not a sterling performer
on other bands, even when the feedpoint is moved to the center of the bottom
wire or to the top apex, with quite high values of resistance and reactance on
30 meters, medium high values on the WARC bands, and useable numbers on the
harmonics of 7 MHz. But it will do ok, especially on the uppermost HF bands,
if the bottom wire is at least 20' up. 80 meters, of course, is out for any
closed loop 40 meter antenna.
Remember: the loop gets a little smaller with smaller wire sizes (the
opposite of linear elements that get smaller with fatter wire). And, unlike
the equilateral loop that shows a small range of feedpoint Z change around the
loop, the low Z values occur only along the sides. At the apex or at the
center of the bottom wire, the Z goes to 200-250 ohm range on 40 meters.
However, the right-angle delta shines as a 40-meter 1 wl loop fed on the side
with a close-to-coax Z for maximizing vertical performance and dx work.
Hope this data is useful.
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