[Elecraft] Rhombic antenna "gain"

[donovanf at starpower.net]

Hi Mike, 


Rhombic antennas -- with few exceptions -- were not usually designed 
for high gain. They were usually designed as a compromise between 
gain and typically one octave of bandwidth (e.g. 14-28 MHz). 


To compound the design compromises, the r hombic termination 
resistor throws away nearly 3 dB of whatever gain it might achieve. 
Because of this, high gain transmitting rhombics have very narrow 
beamwidth, typically 20-30 degrees. 


A high gain rhombic designed for 14-28 MHz -- competitive with a pair 
of stacked large triband Yagis -- might be 300 feet wide, 700 feet long 
and supported by four 100 foot towers. In order to achieve this gain, 
the rhombic beamwidth would be only about 25 degrees, requiring at 
least a dozen huge reversable rhombics to cover most of the compass. 


I visited many rhombic antenna farms many years ago (as far as I know 
they've now all been dismantled). They were typically at least one square 
mile sites with fifty to a hundred towers with heights of 50 to more than 
200 feet. Dismantled VOA Site C in Greenville, NC is a good example, 
a 1.5 square mile site with pairs of massive rhombics -- the biggest I've 
ever seen -- for diversity reception. 


To the extent these facilities are still operating (the vast majority are not), 
the rhombics were replaced by rotatable log periodic antennas, perhaps 
with higher power transmitters to make up for the slightly reduced gain. 
That approach replaces a one square miles ( in some cases much larger) 
with a few acres or perhaps 100 acres for very large site. 


This is a good reference: 


www.w8ji.com/rhombic_antennas.htm 


73 
Frank 
W3LPL 

----- Original Message -----

From: "Mike Markowski" <mike.ab3ap at gmail.com> 
To: elecraft at mailman.qth.net 
Sent: Friday, September 13, 2019 10:24:32 PM 
Subject: Re: [Elecraft] Rhombic antenna "gain" 

Can you expand on this, Ken, or if easier, a reference? I'm curious 
what tradeoffs are made. I used a rhombic at Ft. Monmouth, NJ before 
the Army base was closed in 2011, and used to boom into Europe and 
Russia. It was amazing. I also got copies of WWII manuals on rhombic 
construction while there. You know, just in case I became wealthy with 
tens of acres of land. :-) 

73, 
Mike ab3ap 

On 9/13/19 5:57 PM, Ken G Kopp wrote: 
> Rhombic antennas derive their well-known gain by "throwing away" some of 
> the design's gain. Ditto for the infamous "inverted vee". Use is made of 
> the lobes from the four wires while disregarding others. 
> 
> 73 ! 
> 
> Ken Kopp - K0PP 
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