On Fri, 29 Sep 2006 16:56:07 +1000, Alan Peake
wrote:

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?
Alan
VK2ADB

The rhombic can deliver you a frequency agile antenna with gain, and
low angle major lobe if of sufficient length and at sufficient height.
Side lobes are not pretty, space requirements are huge at HF and the
antenna is not readily rotatable, construction is simple, but serious.

You are on hectares (doesn't sound as good as acres, does it?). Space
is not a big issue, and every ham that can accomodate a good size
rhombic should have one (or more) as a talking point. You could deal
with the fixed heading disadvantage two ways: place the shack in the
middle of the rhombic and switch feed / load ends, or go the whole hog
and erect a set of rhombics to cover your desired / preferred paths.

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m. Sag of 5% of
span is easily accomodated if the end heights are at 20m of more, but
becomes a problem as you lower the end height much. You could model
the effect of the combination of sag and low end height in NEC by
breaking the leg wires into several sections following the approximate
catenary (or parabola for ease). I haven't done it, but I suspect
uncertainty about the ground conditions and ground profile will
introduce more model error than modest sag.

Owen
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