Rhombics
 

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

Rhombics
 

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?

Hi Alan,

Rhombics were the stars of antenna developement in the late 1920s for
RCA. For very long wavelengths, certainly few other practical designs
could achieve the same gains. For shorter wavelengths, other designs
replaced the Rhombic (poor return on real estate in comparison).

If the wire sags such that most of it is on the ground, you suffer.
This is a judgement call otherwise and sag is quite within the ability
to model if judgement demands.

The presences of trees and shrubbery is something all designs suffer.
Unless you are speaking of an antenna in a forest, the Rhombic would
probably do quite well (after all, it is wavelengths long, as are few
trees) until you start getting into short wavelengths.

Rhombics are few and far between these days. Reasons are principally
wavelength based in comparison to available real estate. Few have the
real estate for long wavelengths. If you are working at a short
wavelength, there is a better design to do the same job.

73's
Richard Clark, KB7QHC

Rhombics
 

Alan, the rhombic is a relatively inefficient antenna that 'may' give
you significant gain in point-to-point service, which is why it was
popular in the early days of commercial and governmental radio
services...
For ham radio it is less attractive due to the real estate it requres,
due to the cost of the poles (it usually needs to be supported at a
number of points along each leg to stop excessive whipping in the
wind)... Less attractive because it requires retuning of the matching
unit for small changes in frequency...Less attractive because it
confines your communications to a narrow angular beam...
Being a long ways from the other population centers of the world you
may be in a situation where you find it effective... Most of us
don't...

cheers denny / k8do

Rhombics
 

Denny wrote:
Alan, the rhombic is a relatively inefficient antenna that 'may' give
you significant gain in point-to-point service, which is why it was
popular in the early days of commercial and governmental radio
services...
For ham radio it is less attractive due to the real estate it requres,
due to the cost of the poles (it usually needs to be supported at a
number of points along each leg to stop excessive whipping in the
wind)... Less attractive because it requires retuning of the matching
unit for small changes in frequency...Less attractive because it
confines your communications to a narrow angular beam...
Being a long ways from the other population centers of the world you
may be in a situation where you find it effective... Most of us
don't...

cheers denny / k8do


I have to disagree with some of your statements Denny..

I used rhombic's over the years and they can be very efficient if
properly fed they do not require constant tuning. Their bandwidth can
be quit good. and you only need 4 pole if properly installed. I use
large rhombics before that went through 100 plus M.P.H. winds without
any problems. They are relatively expensive to install properly . the
matching situation can be handled easily with Baluns or open wire
feeders. only problem you may have is comming up with the terminating
resistor.. Their kinda hard to find cheaply today.

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.

If I had the room I'd consider rhombics for Sure.. W6AM used them for
years and as one who worked that station from many places on this earth
I can tell you he was never the weakest signal on any band.
73 Dave kc1di

Rhombics
 

I used a rhombic in the 70s while on contract TDY at Hill AFB, Utah [AGA5HI -
USAF MARS].

It was point to point from Utah to SE Asia [Vietnam War] and approximately 5
wavelengths per leg on 19 MHz and about 40 feet high [guess ... near top of
telephone poles].

It supported the SE Asia phone patch net [USAF MARS]. The USAF had LOTS of land
for the antenna.

We later switched to a Log Periodic as more versatile.

/s/ DD

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

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


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

73 Yuri

Rhombics
 

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

Rhombics
 

"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

Rhombics
 

On Fri, 29 Sep 2006 11:23:27 -0400, kc1di wrote:



Denny wrote:
Alan, the rhombic is a relatively inefficient antenna that 'may' give
....
I used rhombic's over the years and they can be very efficient if

Although the term "efficiency" seems to be used with gay abandon in
this place, the rhombic is not close to 100% efficient, a portion of
the transmitter power (approaching 50%) is dissipated in a
non-radiating loss.

That loss does not have an adverse impact on the gain in the desired
direction, and so it should not be regarded as a disadvantage of
itself.

Owen
--

Rhombics
 

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
--