Dear Dan:
I assume that you have DX in mind. If you just wish enhanced
coverage out to 1000 km, there are far better and much less expensive
solutions.
There are many references on and about this subject. I am working
on the history of, and engineering of, long wire antennas so I have read
many of the references - particularly the early references.
In my opinion, the most useful summery of design equations is to be
found in the first and second editions of my mentor's book: Antennas (by
Kraus). To my surprise, I have not checked the third edition. You
should have little difficulty finding a copy of this book.
A second reference is Chapter 4 of the first edition of Antenna
Engineering Handbook edited by Jasik. That chapter is written by
Laport. However, the table of design equations in Antennas and a
calculator may well be the most easy way to perform the design phase.
Obviously, using a modeling program will be a big help once the
equations get you in the neighborhood.
Most of the other references that are accessible are likely to be
too difficult to use because the information is provided on small
graphs.

That said, I doubt that a rhombic antenna at 3.5 MHz is cost
effective for amateur use. In order to have a take-off-angle of about
20 degrees at 3.5 MHz, you would need to use 200 foot supports (four of
them), unless you wish to be compelled to put lights on the towers. At
best, you would then have a narrow beam width, which might be positioned
to cover much of Europe or, alternatively, only some of Africa. The
towers will need to be substantial in order to support the 300 meters,
or more, of wire between the towers. The good news is that it will be
easy to have the antenna work over the entire band. However, it will
require some compromises to have performance at 7 MHz. (Think one
octave bandwidth before the pattern lets you down.)
The VOA has at least one huge rhombic that is usable starting at 4
MHz. Rhombics have been used at the lower end of HF. We are fortunate
to have one user of this forum who has a lot of experience with
rhombics. I am sure that he will have much to contribute.
In the 10 MHz to 30 MHz region, their cost is much lower and a
rhombic might serve a specific purpose.

In the days before the yagi and before curtain antennas were
popular, the rhombic was a big deal to commercial HF services. The
antennas that the rhombic replaced were generally more expensive and
often quite narrow band.
Today, if you wish a BIG signal when working 80 meter DX, think one
tall tower with a yagi. It will cost less and allow you to tailor the
direction of the main beam.

Do let us know what you end up doing. We will all learn a bit
more.\
73 Mac N8TT
--
J. Mc Laughlin - Michigan USA

"Dan Yemiola" wrote in message
...
Several related questions about Rhombics

Can any one point me to info on the design, building, care and feeding
of
Rhombics?

Real estate is not a problem here, but I would like to have the feed
points
for the antennas located close to my transmitter to keep feed lines
short.
Can any one point me to mutual interference info on building two or
more
80/40/20m Rhombics in close proximity to each other?


Imagine one antenna" main lobe oriented to Africa, another antenna's
main
lobe oriented to Europe.
would there be a quick way to reverse these antennas main lobe
directions
(180 degree reciprocal), from my shack?

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