Rhombics
 

Ian White GM3SEK wrote:
. . .
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.
. . .

As always, Ian put his finger directly on a fundamental principle.

The problem with modeling programs is that they don't model the buzz.

Roy Lewallen, W7EL

Rhombics
 

Yuri Blanarovich wrote:
. . .
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 . . .

NEC-based modeling programs don't have any problem simulating a Beverage
and its traveling wave operation -- they do quite well at it.
MININEC-based programs, no longer in wide use, weren't able to because
of the use of perfect ground during impedance and current calculations.

Roy Lewallen, W7EL

Rhombics
 

Ian White GM3SEK wrote in news:bVv
:

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?

Yes, they are much better as HF point-to-point antennas than they are as
general purpose. For that kind of service, you can pick your height and
rhombic size to specifically service the one path.

--
Dave Oldridge+
ICQ 1800667

Rhombics
 

Gene Fuller wrote:

Yuri,

I am a bit puzzled by your comment. Ducting and refraction received a
lot of technical and mathematical study back at least to the mid-1930's.
Terman's books show similar figures to those in your CQ article, and
they give numerous technical references from the 1940's.

You may have been ridiculed by the amateur radio community, but the pros
accepted that sort of propagation explanation many decades earlier.

73,
Gene
W4SZ

Gene, anyone who has been here a while has learned that Yuri knows all,
and has discovered all, well except for that other guy. How could you
possibly say otherwise?

tom
K0TAR

Rhombics
 

As far as I can tell, Terman describes ducting and refraction relating to
UHF. This was well known in VHF/UHF circles and I mention that in my
article. What I tried to point out that this is also happening at the HF
frequencies, down to 160m.

The closest thing would be Pedersen ray, but that seems to describe
relatively short refraction in the layer.

All the literature I was familiar at that time showed distorted pictures of
Earth and exaggerated height of layers and "bounces".

73 Yuri, K3BU

"Gene Fuller" wrote in message
...
Yuri Blanarovich wrote:

[snip]

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

Yuri,

I am a bit puzzled by your comment. Ducting and refraction received a lot
of technical and mathematical study back at least to the mid-1930's.
Terman's books show similar figures to those in your CQ article, and they
give numerous technical references from the 1940's.

You may have been ridiculed by the amateur radio community, but the pros
accepted that sort of propagation explanation many decades earlier.

73,
Gene
W4SZ

Rhombics
 

Gene, anyone who has been here a while has learned that Yuri knows all,
and has discovered all, well except for that other guy. How could you
possibly say otherwise?

tom
K0TAR

If that is all you learned, I pity you.
I don't know many things, especially what is eating you.
I simply describe my experiences and express my opinions. You could do that
too and argue technical points rather than spewing crap.
I think you might be confusing me with W8JI :-)

73 Yuri

Rhombics
 

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

Rhombics
 

On Tue, 3 Oct 2006 23:27:56 -0400, "Yuri Blanarovich"
wrote:

Terman describes ducting and refraction relating to UHF

Hi Yuri,

I have a couple of refraction citations made by Ladner and Stoner in
"Short Wave Wireless Communications," 1932. They dedicate the book
to C. S. Franklin.
"Refraction may produce an appreciable increase in signal strength
at longer distances and can be expect to produce fading effects
similar to those experienced with the longer waves from the
ionosphere."

How this is achieved conforms to classic ducting, although they
neglect to call it such:
"Under certain conditions theoretical curves are available
from which the field strength at an elevated receiver,
produced from an elevated transmitter, may be deduced."

73's
Richard Clark, KB7QHC

Rhombics
 

Yuri Blanarovich wrote:
I assumed that your VHF rhombic was perhaps high and maybe even
rotatable.

That thing was 600ft long! Some rotator, that would have needed... a
small private railroad maybe?


--
73 from Ian GM3SEK

Rhombics
 

On Tue, 03 Oct 2006 12:17:21 -0700, Roy Lewallen
wrote:

NEC-based modeling programs don't have any problem simulating a Beverage
and its traveling wave operation -- they do quite well at it.
MININEC-based programs, no longer in wide use, weren't able to because
of the use of perfect ground during impedance and current calculations.

Roy Lewallen, W7EL

Roy,

I think you'll find that Antenna Model (a MININEC based program) can
model a Beverage quite well. .

Danny, K6MHE