[Collins] New Additions to the CRA Website

[Dr. Gerald N. Johnson, electrical engineer]

Rhombics can be arranged in arrays, stacked or side by side. I've not
seen any curtained. I suspect the VOA receive sites used rhombics for
the low gain directions and curtain arrays for the high gain (and
slewable) directions like the transmit sites. Its possible they may have
used curtains as shields from the eastern block country's intentional
interference.

Big rhombics tend to have some directional diversity that wasn't planned
for that makes them handle more variation in propagation path than
intended (and can make them suffer from multipath interference also).

LaPort wrote the major book on rhombics. Sometime in the past, he
designed a "low sidelobe" rhombic that was a pair of asymmetrical
rhombics in the same plane. He had arranged the wires to minimize side
lobes in the azimuth plane. And based on the measured width of the main
lobe, claimed 27 dBd gain. A 1296 MHZ version checked out at 17 dBd gain
at a Central States VHF conference antenna gain measurement a few years
ago. The next CSVHF conference proceedings has an article by me,
"Where's the other 10 dB?" Its in the cross polarized sidelobes in the
planes 45 degrees from elevation (each with peak power 7 dB down from
the main lobe) and azimuth and all the other 100 or so sidelobes plus 3
dB in the terminating resistor and the ground reflection in LaPort's
measurement. Remember each wire of the rhombic acts just like a long
wire, IS a long wire, that has many sidelobes. The rhombic arrangement
only uses the first lobe and ignores the rest. The three dimensional
pattern of the Laport dual rhombic looks like a dandelion seed head with
a short stem (the main lobe) 7 dB above the peak level of the sidelobes
nearest to the main lobe. 

The VOA high gain arrays were arrays of fed dipoles in front of a
reflector screen. Gain about 21 dBd. The columns of dipoles were fed
separately so that phase adjustments could slew the direction for
specific targets (either nations or relay stations).

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.