Bill, W4ZV wrote:
"I found my actual Delta loop dimensions...140'7" for each side and
243'7" for the horizontal leg. At 6' above ground, this resonated at
1830 kHz. I found EZNEC's results were resonant at a significantly
different frequency (>50 kHz I recall), so you either need to scale them
in EZNEC to resonate at 1830, or you need to find EZNEC's resonance of
the actual physical dimensions...I attributed this to modeling
errors...but the pattern and gain comparisons to a vertical over the
same ground, etc should still apply. Supported from the 175' level of my
tower by ropes, the vertical tips were about 60' apart and about 130'
high I recall.
"Maybe this is another case of modeling not representing the real world
accurately ................
====================
After Richard, K5NA asked me to see what effect a 170' tower used as the
support structure would have on the delta loop (in EZNEC), I was
surprised. You would think that a 170' grounded tower would be far
enough away from resonance at 1830 kHz that it would have very little, if
any, effect.
I did the model with the tower spaced only 6" from the delta loop. A
feedpoint Z (vertical polarization) of 208 + j0 for the loop by itself
jumped to 228 + j75 with the tower there. Very significant current was
flowing in the tower also. Gain was down almost 1 dB vs the delta loop
alone.
I moved the tower to 10' away from the delta loop restored most of the
lost gain, but the other effects hardly changed at all. Feedpoint Z =
225 + j71 and still boo-coo current in the tower. With the tower 10'
away, I had to shorten the total loop length by about 11' to bring it
back to resonance (222 + j0).
Although ~25% longer than resonance, the tower acted like a parasitic
reflector and had significant influence on the vertically-polarized delta
loop. At only 10' spacing, there was a 0.5 dB F/B on the "array".
Bill attributes the differences in modeling results and real world
results as "modeling errors". It's not known if Bill included the
support structure when he modeled the loop.
Thanks to the "King" (K5NA), I'll not make this modeling (human) error
again. Advice to modelers is, "when modeling vertical antennas
("antennae" are things insects have!) include any other vertical items
(like towers) that might be close by."
73, de Earl, K6SE
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