I assumed that your VHF rhombic was perhaps high and maybe even rotatable.
I do not have measurements and evidence, just anecdotal evidence using one,
hearing from other users and based on some comments on the subject and
experiencing some discrepancies with modeling, just speculating that there
is something to it. If all those who never used rhombic, but modeled its
performance and are convinced that that's it, then I rest my case.
I will have chance to run some test and will report, if it doesn't annoy
K0TAR.
73 Yuri, K3BU
"Ian White GM3SEK" wrote in message
...
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