Topband: More on loop antennas

[Earl W Cunningham]

Just used EZNEC to model Tree's proposed half loop (50' up, 150' across,
50' down).  An interesting possibility!

To resonate at 1830 kHz, the two 50' vertical sections must be 179.4'
apart, with their tops connected with a 179.4' wire.  I used #14 AWG wire
for the entire antenna, so the horizontal dimension will be slightly
different if masts are used for the vertical sections instead.

If fed at the base of one of the vertical sections, the impedance is 29.2
+ j0 ohms.

If fed at a corner (junction of the horizontal section with a vertical
section), the impedance is 41 + j0 ohms.

If fed on the horizontal portion 13' from a corner, the impedance is 50 +
j0 ohms, or a perfect match for 50-ohm coax.

The gain in all cases is about 0.9 dB greater than a single 1/4-wave
vertical.  The azimuth pattern is an oval with max gain broadside to the
two vertical sections, so it acts like a pair of verticals fed in phase. 
The sides are down 6.5 dB from max.  Angle of max radiation is 24.6
degrees over "average" ground, or about the same as a single vertical. 
There is virtually no horizontal radiation, because the current in the
horizontal portion of the antenna cancels out.

Probably the easiest way to feed the antenna would be at the base of
either vertical section via a 1:2 unun.

73, de Earl, K6SE



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