Yuri Blanarovich wrote:
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.
The same is true at VHF. All rhombics interact with ground, because
their length is many times greater than their height above ground. The
number of wavelengths above ground will change the details, but not the
basic fact.
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
Are you quite sure of that? In other words, do you have firm evidence
and reasons why?
or use VHF analogy being many wavelength away from the ground effects.
That analogy was your assumption, never mine :-)
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.
Maybe it does... but I tend not to believe such things unless they come
with good solid reasons.
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.
I don't ever see that as an "either-or" choice - I am not comfortable
until both viewpoints agree. If modeling and 'reality' observations do
not seem to agree, it means we're still missing some pieces of the
puzzle.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek