TopBand: Re: Sloped Beverage Terminations

[Frank Donovan]

Hello Brian!

Thanks for the note!

No, I have not personally checked Beverage sidelobe response with and
without sloped terminations; however, as you know, both AO abd EZNEC show
the improvement in models that may (or may not) be accurate due to
shortcomings in the NEC computational engine.

I was convinced to switch to sloping ends when I visited a large HF Over
the Horizon Radar facility several years ago, then later had an
opportunity to talk to the engineer who designed it's antennas. This
facility uses stacked Rhombics (2 high, 2 wide) in a salt marsh with over
one megawatt of RF feeding them.  The receive site, located about 40 miles
away, originally used the same setup on a dry rocky site sloping towards
the target area.   By the way...  There is never any doubt when the
megawatt transmitter is active...  a cloud of steam rises from the salt
marsh!

This receive Rhombic array always had problems with clutter from radar
return signals entering thru the many side lobes in the stacked Rhombics.
Extensive research was conducted in the 1970s and after extensive
theroretical and experimental work a new array was constructed to replace
the big receive Rhombics, with the peak sidelobe at -40 dB relative to the
main lobe.

The new array ???   A phased array of 128 Beverages!  Each Beverage is 220
feet long ( the nominal freq range is 8 MHz and up) and 4 feet high,
separated 4 feet from each other, with sloped ends. Grounding is extremely
difficult on this dry site, so after extensive experimentation they
selected chicken wire (a military specification control wire for a
particular Sears wire!) under the last 20 feet at each end of every
Beverage establish an excellent ground on bone dry soil.  Before
installation, a 250 ft x 1000 ft field was graded to be absolutely flat!

I accepted the work of the engineering team that designed that Beverage as
the basis for my own, and mine work extremely well!

73
Frank
W3LPL
donovanf@sgate.com


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