**Next message:**Paul Stroud: "Re: QRP FD 1 C QRP"**Previous message:**JCoote@aol.com: "Help with Tektronics 492BP and 492"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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.

-73-

LB, W4RNL

**Next message:**Paul Stroud: "Re: QRP FD 1 C QRP"**Previous message:**JCoote@aol.com: "Help with Tektronics 492BP and 492"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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