The umbrella antenna does not eliminate the need for ground
systems, as Earl points out.
It's also important to remember when the element to element spacing
is less than 1/4 wl, the efficiency of an endfire array (for a given
amount of distributed loss resistances) will decrease, sometimes
substantially.
When parasitic elements are close to the driven element,
array currents increase substantially. The system
requires a much better ground. It's very easy to get good F/B in a
close spaced array that depends on ground systems (or that has lossy
elements), actual gain is much tougher.
Keep in mind some modeling programs show earth connection
losses as zero ohms, no matter what "real ground" is specified.
73, Tom W8JI
w8ji.tom@MCIONE.com
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