Rhombics
 

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

Rhombics
 

"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

Rhombics
 

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
 

"Alan Peake" wrote in message
...
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


Back in the 70s a friend and I put up a rhombic antenna in a pecan orchard
on 11M. If I remember correctly each leg was about 8 wl long. At first we
thought we had done something wrong but then we began regularing talking
with guy about 45 miles away west of Tifton Ga.that we could not talk to on
his quad. Apparently the beam width is so narrow it makes it impractical
except for point to point comunication.

Rhombics
 

Ian, GM3SEK commented thoughtfully on many points regarding the rhombic.

It is an excellent antenna for fixed point communications.
Unfortunately, the earth and moon are not stationary with respect to
each other. The rhombic may not be so re-directable as antennas like the
Yagi or corner reflector.

For transmitting, the rhombic has a lower maximum voltage than a
resonant standing-wave antenna. For high power, it`s a consideration.

The power remaining at the far end of the rhombic has been recycled by
some to eliminaate waste.

Cecil does an excellent job of explaining where the power in a
transmission line goes. Unfortunately, re-entrant rhombics tend to lose
some of their advantages of simplicity and bandwidth in the elaboration
process.

On the subject of diversity, it works by switching to a better receiving
system. Without switching, spaced antennas only provide a phased array
in which destructive interference happens during some signal conditions.

Where I worked, we used triple diversity receivers with 3 rhombic
antennas spaced about 10 wavelengths apart at the low frequency end of
the spectrum. A Crosby or Pioneer combiner selected the best of the 3
receiver outputs to feed a program line. Several receiving systems
shared the same 3 rhombics for simultaneous program relay.

Sidelobes are only a small problem when using triple diversity and
selectable sideband in reception.

Best regards, Richard Harrison, KB5WZI

Rhombics
 

Jimmie D wrote:
Back in the 70s a friend and I put up a rhombic antenna in a pecan orchard
on 11M. If I remember correctly each leg was about 8 wl long.

Isn't that an illegal antenna on 11m? :-)
--
73, Cecil http://www.w5dxp.com

Rhombics
 

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
 

Yuri, K3BU wrote:
"All I am pointing out that based on mine and other`s experience,
rhombic is a great antenna,---."

Agreed. I worked for an oil company on Tierra del Fuego. We frequently
had no contact with our office in Buenos Aires. I put up a rhombic
driven by a 20-watt Hallicrafters transmitter surplus from our Bolivian
operations. They had upgraded to Collins 30-K5 transmtters there.

The Argentine Post Office used a surplus 300-watt BC-610 into a dipole
near our camp. When I called the same station in Buenos Aires used by
the Post Office, they at first would not believe that I was as far away
as Tierra del Fuego.

Best regards, Richard Harrison, KB5WZI

Rhombics
 

Sorry I mis-spoke in my story about Tierra del Fuego. The power into the
rhombic and dipole was about the same. Despite the size difference
between a Hallicrafters HT-20, and the military surplus BC-610, both put
out about 100 watts on AM.

Best regards, Richard Harrison, KB5WZI

Rhombics
 

In article ,
Cecil Moore wrote:

Jimmie D wrote:
Back in the 70s a friend and I put up a rhombic antenna in a pecan orchard
on 11M. If I remember correctly each leg was about 8 wl long.

Isn't that an illegal antenna on 11m? :-)

Not back in the 70's........