I'm building a simple 2-el vertical array for 40M, with one element
driven against radials, and a passive reflector with an equal number of
radials. NEC predicts 2.7 dBi over lousy ground at 15 degrees, with
peak gain of 3.6 dBi at 25 degrees elevation, and F/B of about 8 dB.
We've got this set up in W6GJB's pasture, roughly 5 miles S of me, with
me centered on the main lobe, and I'm looking signal strength with my K3
reading relative dB (and with AGC turned off). Our signal is 35 dB
above the noise level with Glen's KX3 at 3 watts. Terrain is hilly
between us, and we have 16 radials on both elements.
We're making three measurements -- with the array as designed, with the
reflector shortened by 3 ft (which should make it director) but still
connected to the radials, and with the reflector simply insulated from
the radial plate.
What I hope to see if the antenna has the predicted directivity is 3 dB
difference between the designed array and the reflector floating, and
6-8 dB difference between the array as designed and reversed.
What I see instead is the same signal strength for all three
configurations within 0.2 dB. So the question is, why? A vertical plot
in NEC shows the F/B at all elevation angles, all the way down to 1
degree and up to 80 degrees.
73, Jim K9YC
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