While designing a 2L reflector Yagi for 6M, I played with the reflector
dimensions, going from 0.2 WL to 5.0 WL at a spacing of 0.15 WL.
I made two very interesting discoveries:
1 - parasitic lengths LESS than 0.3 WL are nearly 'INVISIBLE'.
2 - parasitic lengths MORE than 0.5 WL produce decreasing
gain and F/B but NEVER become 'invisible', truncating
at 2 dB gain and F/B of 4 dB with 0.15 WL spacing.
This corresponds to 10.5 ft spacing on 20M, 7 ft spacing
on 15M, and 5.25 ft spacing on 10M. I expect the effect
would drop off (SLOWLY) as spacing is increased.
This suggests that any LOWER Frequency antenna WILL
interact with any higher frequency antenna if they are in
fairly close proximity (<0.5WL).
Tom N4KG
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