Howdy Rhombic fans,

OK on Ian's rhombic experiences on V/UHF, but that I believe would be
different situation, I.e. rhombic not working in conjunction with ground
reflections/effects. With "normal" HF rhombic we have a situation where
antenna is spread out over few wavelengths and interacting with ground.

Spacing two antennas few wavelengths can give us diversity effect by the
virtue of propagation and waves hitting them differently. Rhombic is kind of
antenna/feeders spread out over few wavelengths over ground. Perhaps there
is also some of traveling wave mode going on, like in a Beverage. You can't
simulate that or use VHF analogy being many wavelength away from the ground
effects. Don't forget that skywave polarization is all over and rolling
around.

I see big differences when trying to model vertical arrays destined to work
on ground and modeled in "free space", different pattern and just plain
ridiculous to do that. I know from my hardware experiments with Razors
(quad - yagi) that if I changed height of the boom and I reoptimized the
spacing/dimensions I would get different configuration. So my conclusion was
that antenna has to be designed for the height it is going to be used at.
Closer to ground, the more pronounced effect.

What I am trying to say and not sell anything, is that perhaps the ground
effect and rhombic's spread over it over few wavelengths might have
something to do with it's good performance.

I think that software modeling is a great tool, but I also know that it can
not capture all the variables and effects that are happening around
antennas, the ground and the sky.

All I am pointing out that based on mine and other's experience, rhombic is
a great antenna, performing perhaps better than modeling shows (W8JI web
site downplays it). If I get the chance, I will try to do some real life
tests and comparisons on HF. We have some 30 rhombics, some phased side by
side and will try to model and compare them with other antennas. BTW our
rhombics have a load resistor made of open (resistive?) wire stub, folded
few times back and forth. They were used with 50 kW transmitters.
see http://www.teslaradio.org/site_survey.htm

I am just trying to bring attention to possible discrepancy that perhaps is
worth exploring. If some believe in gospel of modeling and paper antennas,
than enjoy it. I prefer reality. When I wrote my observations about
propagation happening by ducting and refraction some 25 years ago, I was
ridiculed. Now the propagation experts are accepting it and playing
discoverers (only ON4UN gives me some credit :-).

Yuri, K3BU
Tesla RC N2EE


"Ian White GM3SEK" wrote in message
...
Alan Peake wrote:


Dave Oldridge wrote:

Properly designed, they have a good reputation for doing what the
theory says they will do. Just remember, though, that you're going to
have to sewer almost half your transmitted power into the terminating
resistor. But that's the half that would be going the wrong way,
basically.

But it would get there - eventually Long or short path. But I don't
know if both paths are ever open at the same time to the same extent. If
not, then it may not be a problem. What about running a transmission line
from where the terminating R would be, back to the feed point? Assuming
you can match it all that is.

No problem. Pipe the signal back from the far end into the shack, feed it
into a circulator, and add it to the outgoing signal. Cecil will explain
what happens to the power :-)


Replying to Yuri's point: from personal experience of using a rhombic 100
wavelengths long for 2m moonbounce, it had only about the same maximum
gain as a box of 4 mid-size yagis - and that is only while the moon is
passing through the very narrow main beam, which only happens for a magic
20 minutes on certain days of the month.

In other words, the rhombic did work, but the performance was nowhere near
as spectacular as we had expected from its huge electrical length.

What is undeniably true is that it *looked* spectacular! I've used many
kinds of antennas since then, up to an 85ft dish, but not one of them has
given me the same buzz as that rhombic. And there is the trap: buzz isn't
the same thing as performance.

We need to be very careful about applying dual standards. An unavoidable
feature of all very long rhombics is that the main beam is very narrow,
because the edges of the main lobe are sliced away by large numbers of
sidelobes that are not many dB down. If we saw that kind of E-plane
pattern in a yagi, we wouldn't hesitate to call it a "bad design"... so
what's "good" about the same feature in a rhombic?
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek