"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


Alan,
Rhombics perform better than what simulations and modeling show.
Put them up and see for yourself.

73 Yuri

"Yuri Blanarovich"
Rhombics perform better than what simulations and modeling show.
___________

I can vouch for that. In 1961 I was stationed at Dharhan AFB, Saudi Arabia,
and operated many times from HZ1AB there -- at the time the only licensed
amateur radio station in Saudi not connected with the royal family.

We used a Collins exciter driving a heavily modified BC-610 to a terminated
rhombic aimed down the eastern seaboard of the US. We had no trouble
reaching the States (and hearing 1,000s of DX-ers wanting to talk to us).

OTOH, military links I took care of using KWS-1s into 6-element rotatable
"beams" didn't do as well.

RF

Richard Fry wrote:
"Yuri Blanarovich"

Rhombics perform better than what simulations and modeling show.

___________

I can vouch for that. .............

Problem with the model??
Alan

More like model has a problem capturing or reflecting reality.
Modeling will calculate the pattern etc., but will not properly reflect the
interaction of the antenna design with propagation medium and
terrain/surroundings, like showing effect of capture area.

73 Yuri, K3BU

"Alan Peake" wrote in message
...


Richard Fry wrote:
"Yuri Blanarovich"

Rhombics perform better than what simulations and modeling show.

___________

I can vouch for that. .............

Problem with the model??
Alan

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.

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

Yuri Blanarovich wrote:
More like model has a problem capturing or reflecting reality.
Modeling will calculate the pattern etc., but will not properly reflect the
interaction of the antenna design with propagation medium and
terrain/surroundings, like showing effect of capture area.

73 Yuri, K3BU

"Alan Peake" wrote in message
...

Richard Fry wrote:
"Yuri Blanarovich"

Rhombics perform better than what simulations and modeling show.
___________

I can vouch for that. .............
Problem with the model??
Alan

Alan, VK2ADB wrote:
"It all looks very promising on the computer but I`d be interested in
real-world exerience."

Don`t worry. I`ve erected many rhombics. They were all astisfactory and
very forgiving. Most were about 4 or 5 wavelengths on a side (leg) and
they were about twice as long as wide. They all worked well over a wide
range of frequencies. It`s just a terminated transmission line with a
big spread in the middle so it will radiate. Matching is a cinch.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Alan, VK2ADB wrote:
"It all looks very promising on the computer but I`d be interested in
real-world exerience."

Don`t worry. I`ve erected many rhombics. They were all satisfactory and
very forgiving. Most were about 4 or 5 wavelengths on a side (leg) and
they were about twice as long as wide. They all worked well over a wide
range of frequencies.

How are they at VHF/UHF? For Field Day, I usually do 2M & 440 FM voice with
very large vertically-polarized yagis. What am I going to get with a
rhombic? Horizontal polarization, I'll bet, except maybe the whole rhombic
can be rotated 90 degrees to transmit/receive a vertically-polarized signal.

I really want to try some experimenting, since 5 - 10 wavelengths of 440 is
doable in my backyard using sticks guyed up with twine. What about UHF
television reception? This could get interesting.

Sal M. Onella wrote:
"How are they at VHF/UHF?"

Arnold B. Bailey in "TV and Other Receiving Antennas" wrote on page 528
specifications for a 200 MHz rhombic antenna. It may be scaled for
another frequency. For 20 MHz, multiply dimensions by 10. It is four
straight horizontal wires (no. 10 AWG) each 36 ft. long, separated 18
ft. at mid point, overall length 31 feet. Transmission line is 300-ohm
balanced twinlead. Reaistance at center frequency is 600 ohms as is the
termination resistance. Gain at centerfrequency is 14.5 dBd (or less).
Frequency bandwidth for 1 dB down is 30%. For 3 dB down, it is 100%.
Polar pattern shows lots of side lobes.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Alan, VK2ADB wrote:
"It all looks very promising on the computer but I`d be interested in
real-world exerience."

Don`t worry. I`ve erected many rhombics. They were all astisfactory and
very forgiving. Most were about 4 or 5 wavelengths on a side (leg) and
they were about twice as long as wide. They all worked well over a wide
range of frequencies. It`s just a terminated transmission line with a
big spread in the middle so it will radiate. Matching is a cinch.

Best regards, Richard Harrison, KB5WZI


Well, I'll give one a try. I can get four 15m poles at a reasonable
price (I've planted hundreds of pine trees on the property but the
tallest is only 25' so far and not where I need them - should have
planned that a bit better!!)
What sort of feed arrangement did you use?
Alan

Alan, VK2ADB wrote:
"What sort of feed warrangement did you use?"

600-ohm open-wire line. Many of the rhombics I erected were made from
U.S. Army WW-2 Signal Corps kits. These used three cables in the diamond
shaped curtain which came together at the end supports but were spread
apart by several feet at the side supports. This construction tended to
further reduce impedance variations as frequency changed. We didn`t
bother with this refinement with receiving antennas.

Actual antwenna driving point impedance tended to exceed 600 ohms so
width of the attachment point to the anteena was wide for the cable size
to appear as about 800 ohms and then the spacing tapered steadiy down
during the descent to the horizontal transmission line which was spaced
for 600 ohms.

The cable used in the kits was made from (3) AWG 12 Copperweld wires
twisted together. There was also stainless steel wire for a dissipation
line at the far-end of the rhombic. We had to replace this with larger
wire as we were broadcasting with 100 KW transmitters, far more power
than anticipated by the Signal Corps. Under some conditions and at some
frequencies, it is possible to dissipate up to 50% of the power fed into
the rhombic in its dissipation line.

It is possible to operate without a dissipation line or resistance. The
rhombic becomes bidirectional without the termination. I know from
experience during a period when the Signal Corps dissipation lines
melted from the broadcast power and our fan mail came from South America
as well as Central Europe.

Best regards, Richard Harrison, KB5WZI