Rhombics
 

On Sun, 1 Oct 2006 17:50:47 -0500, "Richard Fry"
wrote:

no form of NEC and its Windows shells will show in a
_single_ evaluation whether in tabular or graphical form, the net values of
the surfacewave+spacewave elevation pattern at a user-specified distance
over real earth, for all elevation angles in a given azimuth slice?

I'm surprised that Roy didn't take credit for this where credit is
due.

The near-field table (which complete conforms to your constraints) can
be expressed in a spherical coordinate system. I've used this feature
often.

73's
Richard Clark, KB7QHC

Rhombics
 

Alan Peake wrote in
:

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?

I only ever had the use of one REAL rhombic. And that was on 60m in the
high arctic (gov't freqs). It was properly designed for the specific path
we were using and yes, it was VERY good. You could almost make your own
band opening with 5kw!

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.


--
Dave Oldridge+
ICQ 1800667

Rhombics
 

"Roy Lewallen" wrote in message
...
Antenna modeling tools aren't intended to model propagation effects. But
the stronger the signal radiated in the right direction, the stronger the
received signal will be. And the strength of the radiated signal in each
direction is what the antenna modeling program shows.


True, not argument there.

As for the "effect of capture area", the "capture area" of an antenna is
just another way of stating the gain. This information is what you get
from an antenna modeling program.

Roy Lewallen, W7EL


What I had in mind is the difference in "capture area" related to physical
size of antenna like rhombic vs. similar gain, say Yagi. It appears that
rhombics perform better than corresponding Yagi thanks to larger physical
size (5 - 10 wave lengths) interacting with the environment (more like
diversity effect?). It seems that rhombics perform better than calculated
gain/capture area figures.
We will have opportunity to do some tests of rhombics farm vs. some classics
soon. It will be interesting to see the results.

73 Yuri

Rhombics
 

Yuri Blanarovich wrote:

What I had in mind is the difference in "capture area" related to physical
size of antenna like rhombic vs. similar gain, say Yagi. It appears that
rhombics perform better than corresponding Yagi thanks to larger physical
size (5 - 10 wave lengths) interacting with the environment (more like
diversity effect?).

That's a commonly believed myth, but myth it is.

It seems that rhombics perform better than calculated
gain/capture area figures.

So do EH, CFA, Isotron, Rai-beam, and more other antennas that I can
count. That's because modeling programs just don't account for the
magical effects.

We will have opportunity to do some tests of rhombics farm vs. some classics
soon. It will be interesting to see the results.

Maybe. Even fairly simple RF measurements are very difficult to make
with even modest accuracy. Antenna measurements are more difficult yet.

Roy Lewallen, W7EL

Rhombics
 

On Sun, 01 Oct 2006 20:27:47 -0700, Roy Lewallen
wrote:


So do EH, CFA, Isotron, Rai-beam, and more other antennas that I can
count. That's because modeling programs just don't account for the
magical effects.

To add to the list, secret / undescribed / proprietary components that
impart the "special" features.

Owen
--

Rhombics
 

"Richard Clark" wrote:
I've been modeling for results at the horizon for as long as I've held
a copy of EL/EZNEC. This and the Brown, Lewis, Epstein data
confirm to within 1dB as I've reported on more than one occasion.
_____________

Even the demo version of EZNEC, used judiciously, can do that.

The link below leads to a screen clip of EZNEC output windows. EZNEC
parameters were set for the 1937 BL&E study for a 90 degree monopole, and
the r-f ground produced by ~113 radials each about 0.41 lambda long

EZNEC shows an h-plane field of 189.76 mV/m at 1 mile for 1 kw of applied
power. The BL&E paper shows about 191 mVm for those conditions (Figure 30).

The EZNEC calculation for these conditions differs from the BL&E paper by
less than 0.06 dB.

http://i62.photobucket.com/albums/h8...EC-2Emulat.gif

RF

PS: And many thanks to Roy Lewallen for making a demo version of EZNEC
available.

Rhombics
 

"Roy Lewallen" wrote in message
...
Yuri Blanarovich wrote:

What I had in mind is the difference in "capture area" related to
physical size of antenna like rhombic vs. similar gain, say Yagi. It
appears that rhombics perform better than corresponding Yagi thanks to
larger physical size (5 - 10 wave lengths) interacting with the
environment (more like diversity effect?).

That's a commonly believed myth, but myth it is.


Perhaps "myth" based on actual experiences. W6AM had a rhombic farm and
results in contests and comparison with locals using competitive antennas
seem to give it edge over calculations. I had rhombic once also and
experienced exceptional reports. I know it is anecdotal "evidence" but those
who used it, know the difference. Professional installations used them and I
have not seen them replacing it with the "same gain" Yagis.

It seems that rhombics perform better than calculated
gain/capture area figures.

So do EH, CFA, Isotron, Rai-beam, and more other antennas that I can
count. That's because modeling programs just don't account for the magical
effects.


I would not put rhombics in the same category, that is a stretch.
We saw the "infallability" of modeling programs using lumped inductors as
loading elements.

We will have opportunity to do some tests of rhombics farm vs. some
classics soon. It will be interesting to see the results.

Maybe. Even fairly simple RF measurements are very difficult to make with
even modest accuracy. Antenna measurements are more difficult yet.

Roy Lewallen, W7EL

Very simple RF measurement is when comparing two antennas, one hearing the
signals, while another one is not.
One example at VE1ZZ: he has few beverages for Eu. Regular one, phased pair,
staggered pair and the "weird" one terminated on a stainless steel hubcap in
the salt water, very similar to the Regular one. The SS beats all the others
most of the time. They are about the same length ~870 ft, can the modeling
figure out wasaaap?

I am just trying to find excuse for seemingly superb performance of
rhombics, attributing it perhaps to diversity effect by being stretched over
large area. I am not selling any, but have access to about 30 of them over
salt water marsh and will play with them.

73 Yuri, K3BU

Rhombics
 

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.
Alan

Rhombics
 

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:
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 :-)

Dr. Best, VE9SRB, in his 2001 QEX articles explained what
would happen. Based on his idea that 75w + 8.33w = 133.33w,
one could route the unused Rhombic power back to the source,
recycle it, and cause voltage superposition to multiply the
power up to a factor of 4. :-) To anyone who thinks I am kidding
about Dr. Best posting the above equation, it can probably
be verified by Google circa May 2001 on this newsgroup.

Dr. Best proved his assertions with the following power equation:

Ptot = P1 + P2 + 2[SQRT(P1*P2)]cos(A)

where A is the angle between V1 and V2. If we make the angle
between V1 and V2 equal to zero, we can take the P1 power
from the source and the P2 power routed back from the load
and increase our total power output by a factor equal to
2[SQRT(P1*P2)]. Who says there is no such thing as a free
lunch? :-)

Discussed by me in May 2001 was the fact that his term,
2[SQRT(P1*P2)] is constructive interference energy which must
necessarily be exactly balanced by 2[SQRT(P1*P2)] watts of
destructive interference energy or else the conservation of
energy principle is violated. At the time, Dr. Best did not
understand where the necessary destructive interference energy
was coming from. It comes from the Z0-match between the feedline
and the source and it works exactly like the thin-film layer on
non-reflective glass. Dr. Best's Ptot equation above is true for
A = 0 if and only if Ptot is being supplied with destructive
interference energy where A is probably equal to 180 degrees.
--
73, Cecil http://www.w5dxp.com