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Mode A Loop Antenna Model



Following the recent interest in mode A noted on this reflector, the
subsequent posting made by Bob, W7LRD, for his version of a horizontal
full-wave loop (including an ANS description, thanks Dan), and some
experimentation done by Mike, KF4FDJ, my interest was piqued.  I modeled
some versions of this antenna in NEC4Win and came to the conclusion it is
indeed a FB antenna for mode A.  Here's what I found:

At a height of 8' (1/4 WL) the loop is about 8.4' per side and input
impedance is about 155 Ohms at resonance.  The major lobe is straight up
with the signals at 30 degrees -10 dB from the top.  Not very attractive!

At a height of 12' (3/8 WL) the loop is about 9' per side and the input
impedance is about 185 Ohms.  The major lobe is at 50 degrees elevation.
Signals at 30 degrees are -5 dB.  Better, but not great.

At a height of 16' (1/2 WL) the loop is about 9.15' per side and the input
impedance is about 128 Ohms.  The major lobe is at 32 degrees and the
signals at the top (90 degrees elevation) are -9 dB.  This looks like an
almost perfect pattern for LEO reception.  Raising the loop higher lowers
the major lobe but also introduces significant radiation off the top-center.


The azimuth pattern is a slight figure-8.  At 20 degrees elevation, the
sides are -10 dB from the ends.  At 45 degrees they are -3 dB.  At 60
degrees the sides are only 1 dB down from the ends--practically
omnidirectional.  This model would indicate a E-W orientation would favor
most RS-12/13 passes where max elevation is below 45 degrees.  However, a
N-S orientation would put the gain where you need it the most when the
satellite is just coming over the horizon.  RS-15 comes across at a angle,
so the placement v. gain is more random.  Comments from users?

I found little change in input impedance or pattern for various ground
conditions.  If you have very good soil (wet sand), reduce the height to
15'.  If you have very poor soil (rocks), consider a same-size loop laying
right on the ground.  Caution:  for average ground, using a reflector as Bob
mentioned will increase the overhead signal at the expense of the horizon
signals.  If you have a horizontal beam to use in conjunction with a
2-element loop (as Bob does), that will work extremely well.

73 es gd lk,
Jerry, K5OE
k5oe@aol.com




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