TopBand: Umbrella Antenna

[Tom Rauch]

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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