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

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

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

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

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
--

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

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
--

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
Not a bad idea. There's a diagram or three in Laport's "Radio Antenna
Engineering" so I'll have a bit of a read.

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

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