Owen Duffy wrote:
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
--

Dollar for Dollar DB for DB i would errect a Curtain antenna. It
outperforms Rhombics in all aspects of design and construction.
Considering that the 4 poles of a Rhombic can be used to build an
Array that will cover the globe, have equal or greater gain that would
make the curtain antenna a better choice. I suppose thats why just
about ever shortwave station in the world uses them!

If you want an opinion of a station who has used and is still using
stacked rhombics ask Ian VK3MO. He will tell you thats his 6 wavelength
perside rhombic is a terrible general purpose antenna. Since his
antenna is fixed on new York he finds that quite frequently the
propagation path rarely comes in on the direct computed bearing. He
loses 10 to 20 db having his sharp rhombic. His rhombic does not have
sufficient azimuth diversity since the 3db horizontal beamwidth is so
narrow.

You also wont have the problem of finding a termination resistor, a
decent globar one anyway. See w8ji.com or look at TCI's web page. Its
one hell of a antenna. I am surprised nobody in ham radio has installed
one, considering the large number of 200 ft towers in the world. When
radio Switzerland closed down there was a special even ham station on
air using a rotable 300ft high curtain, oh what a signal 24 dbi
produces with a 100 watts! Now if you know of a stacked array that uses
4 mono band or other stacked antennas that cal deliver greater than
20dbi thats not a laser beam let us all know. I tried to come with
such a stack using 6 log periodics using 42 ft booms on a 200 foot
tower. It deliver such gain on the higher frequencies but fell short on
13mhz. The curtain will do it with a lot less expense.

Bob



Bob