In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote:

In the most simplistic of terms, a rhombic consists of four
1/2-wavelength wires plus a lossy resistor which gets hot.

And, in this case, I think the "most simplistic terms" are a
misapplication of the way the term is usually used.

My recollection is that in practice, rhombics of the sort being
referred to have arms which are several wavelengths long. The total
double-arm-length of a wire rhombic is often 10 wavelengths or more.

In this sort of rhombic, the great majority of the transmitted power
is radiated before it reaches the termination resistor. There is little
power left to dissipate in the resistor. If the rhombic were
unterminated, and the forward-travelling wave were reflected at the
end of the rhombic, most of this reflected power would be radiated
before it reached the transmitter and were re-reflected.

Dissipating the remaining (small) amount of forward wave at the end of
the rhombic helps maintain a very high front-to-back ratio. This can
be advantageous both when transmitting (no back-spill) and when
receiving. I believe that rhombics were popular among U.S. government
radio sites for use at coastal sites, for precisely this reason - they
were very good at rejecting QRM from landside transmitters, and didn't
blast landside receivers with high power.

Long, terminated rhombics have another advantage - they maintain a
consistent directionality and feedpoint impedance over a wide range of
frequencies... rather wider than you can do with a resonant
standing-wave antenna such as a reflector/DE/directors beam.

Common sense prevails. No need to refer to Eznec. Even a drunken
old-wife would know which arrangement to choose, if only because it
saves the cost and fitting of a high power, non-reactive resistor.

If you're insisting that a "rhombic" may have arms of no longer than
1/2 wavelength, I'd agree.

Since that's not the only way to design 'em, though (and is not how
some of the better-known ones were designed), I think that your
conclusion is overbroad.

It's all a matter of serving your needs, whatever they may be. If
you're limited on space, and/or want a steerable beam, then a Yagi or
similar is probably the best choice. I certainly wouldn't try to put
up an effective HF rhombic on my roof!

If you've got oodles of space, want to listen (or transmit) only in a
single direction, need a lot of front-to-back isolation, and want a
broad bandwidth and consistent radiation patterns and a considerable
amount of gain, then a long-armed terminated rhombic may be a better
choice than the alternatives (e.e. a honking-big LPDA).

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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