"If a folded dipole performs like a normal dipole, why doesn't a horizontal
loop perform like a rhombic ?"
If constructed as a rectangle, a loop can exhibit lobes and gain similar to
a rhombic on frequencies where the legs are "long" with respect to the
frequency wavelength. I use an 80 meter horizontal loop which is a rectangle
with the corners at 30 to 40 feet high. The north/south legs are longer then
the east/west legs and the southern corners are higher then the northern
corners. When fed at one of the corners, the loop has definite lobes, with
gain, above 20 Meters- - 20-17-15-12-10 Meters. The pattern changes with
frequency and tends to go from a wider corner lobe on 20 meters to narrower
corner lobes on 10 Meters. Changing the corner which is fed also moves the
direction of the major lobes(My pattern comparisons are done with a
rotatable yagi). Naturally, the maximum take-off angle drops as the
frequency is raised. I recently changed the feed-point to maximize the lobe
towards Asia rather then Europe with good results, as predicted by the
antenna model. Again, this is with a specific design loop. As indicated
earlier, random loops will give random patters. My loop becomes a
"sky-warmer" on 40 and 80 Meters with very little directionality - more like
a "low dipole." I also feed the loop with a 9:1 balun at the feed-point.
This brings the loop SWR down to less then 3:1 on 20 through 10 Meters.
Loops can yield interesting results, depending on the shape and height of
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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