Owen Duffy wrote:
On Fri, 29 Sep 2006 16:56:07 +1000, Alan Peake
wrote:

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

The rhombic can deliver you a frequency agile antenna with gain, and
low angle major lobe if of sufficient length and at sufficient height.
Side lobes are not pretty, space requirements are huge at HF and the
antenna is not readily rotatable, construction is simple, but serious.

You are on hectares (doesn't sound as good as acres, does it?). Space
is not a big issue, and every ham that can accomodate a good size
rhombic should have one (or more) as a talking point. You could deal
with the fixed heading disadvantage two ways: place the shack in the
middle of the rhombic and switch feed / load ends, or go the whole hog
and erect a set of rhombics to cover your desired / preferred paths.

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m. Sag of 5% of
span is easily accomodated if the end heights are at 20m of more, but
becomes a problem as you lower the end height much. You could model
the effect of the combination of sag and low end height in NEC by
breaking the leg wires into several sections following the approximate
catenary (or parabola for ease). I haven't done it, but I suspect
uncertainty about the ground conditions and ground profile will
introduce more model error than modest sag.

Owen
--

Dollar for Dollar DB for DB i would errect a Curtain antenna. It
outperforms Rhombics in all aspects of design and construction.
Considering that the 4 poles of a Rhombic can be used to build an
Array that will cover the globe, have equal or greater gain that would
make the curtain antenna a better choice. I suppose thats why just
about ever shortwave station in the world uses them!

If you want an opinion of a station who has used and is still using
stacked rhombics ask Ian VK3MO. He will tell you thats his 6 wavelength
perside rhombic is a terrible general purpose antenna. Since his
antenna is fixed on new York he finds that quite frequently the
propagation path rarely comes in on the direct computed bearing. He
loses 10 to 20 db having his sharp rhombic. His rhombic does not have
sufficient azimuth diversity since the 3db horizontal beamwidth is so
narrow.

You also wont have the problem of finding a termination resistor, a
decent globar one anyway. See w8ji.com or look at TCI's web page. Its
one hell of a antenna. I am surprised nobody in ham radio has installed
one, considering the large number of 200 ft towers in the world. When
radio Switzerland closed down there was a special even ham station on
air using a rotable 300ft high curtain, oh what a signal 24 dbi
produces with a 100 watts! Now if you know of a stacked array that uses
4 mono band or other stacked antennas that cal deliver greater than
20dbi thats not a laser beam let us all know. I tried to come with
such a stack using 6 log periodics using 42 ft booms on a 200 foot
tower. It deliver such gain on the higher frequencies but fell short on
13mhz. The curtain will do it with a lot less expense.

Bob



Bob

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m.

Yes, it looks like 80m per leg is reasonable - 4 wavelengths at 20m
I did as you suggested in terms of modelling the sag and it didn't seem
to upset the pattern greatly.

I can put the antenna over the house but I'm not sure if the extra
feeder loss would outweigh the advantages. Might not be too bad with 600
ohm or greater feeder.
Alan

Dollar for Dollar DB for DB i would errect a Curtain antenna. It
outperforms Rhombics in all aspects of design and construction.

Well, I don't know much about curtains but VK3ATN apparently had one up
and preferred his rhombics. Don't know the full details though.


If you want an opinion of a station who has used and is still using
stacked rhombics ask Ian VK3MO. He will tell you thats his 6 wavelength
perside rhombic is a terrible general purpose antenna. Since his
antenna is fixed on new York he finds that quite frequently the
propagation path rarely comes in on the direct computed bearing. He
loses 10 to 20 db having his sharp rhombic. His rhombic does not have
sufficient azimuth diversity since the 3db horizontal beamwidth is so
narrow.

That's a problem, which is why I had thought of only 4 wavelengths per
side. NEC says it would be about 15 degrees wide on 20m.


You also wont have the problem of finding a termination resistor, a
decent globar one anyway.

I was going use it unterminated to begin with. Aimed at Europe, the
other end points at Central America so I'd be surprised if I had
problems covering both those areas at the same time.

Alan

"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.

feeders. only problem you may have is comming up with the terminating
resistor.. Their kinda hard to find cheaply today.

Might not be a problem for 100W - I'm on solar power and don't want to
have to fire up the generator to power a linear


Remember that they are very good point to point radiators and rx
antennas if you calculate things right for your desired path .. they
also can provide multi lobes to favored directions if desired. and that
is somewhat dependent on your operating Frequency and the size of your
Rhombic.

I found a strange thing with NEC in terms of multiple lobes. At 20m, NEC
gave me a nice main lobe with the antenna at 15m but at 10m, there were
two lobes about 30 degrees apart. I played with the height to see how it
changed the lobe pattern but the results didn't make sense. The two
lobes remained until the antenna went to a certain height, then one
extra millimetre in height, and it went straight back to one main lobe.
Alan

feeders. only problem you may have is comming up with the terminating
resistor.. Their kinda hard to find cheaply today.

Might not be a problem for 100W - I'm on solar power and don't want to
have to fire up the generator to power a linear


Remember that they are very good point to point radiators and rx
antennas if you calculate things right for your desired path .. they
also can provide multi lobes to favored directions if desired. and that
is somewhat dependent on your operating Frequency and the size of your
Rhombic.

I found a strange thing with NEC in terms of multiple lobes. At 20m, NEC
gave me a nice main lobe with the antenna at 15m but at 10m, there were
two lobes about 30 degrees apart. I played with the height to see how it
changed the lobe pattern but the results didn't make sense. The two
lobes remained until the antenna went to a certain height, then one
extra millimetre in height, and it went straight back to one main lobe.
Alan

Richard Fry wrote:
"Yuri Blanarovich"

Rhombics perform better than what simulations and modeling show.

___________

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

Problem with the model??
Alan

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

On Sat, 30 Sep 2006 16:31:29 +1000, Alan Peake
wrote:




feeders. only problem you may have is comming up with the terminating
resistor.. Their kinda hard to find cheaply today.

Might not be a problem for 100W - I'm on solar power and don't want to
have to fire up the generator to power a linear

Something that I saw done on commercial sites with high power
transmitters is to run a feedline down from the termination end of the
rhombic and run it underground for a while. If the loss is sufficient,
you don't even need the terminating resistor.

Owen
--

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