Dual rhombic antenna
 

I picked up a Oct 1973 issue of 73 & noticed a interesting article on
a mast mounted rotatible very
hig gain 435mhz antenna, cheap and easy to built. Ive been trying to
build a high gain antenna to
recieve ATV in the 903-928 band & seems this maybe the one. The claims
for
this antenna seem streached abit, they claim 26dB gain from just wire
and a few resistors
and claim its 435mhz design can work up to 890mhz, theres even a quad
rhombic shown
also. Sounds to good to be true, so i wandering if anyone has had any
experience with
actually building & using one of these antennas. I would like to build
some kind of high gain recieve
only antenna to see if its possible to get a decent signal from some
of the ATV repeaters.
Seems here in western N.C. the ATV activity is located mostly in this
located band.
Thx N4aeq

Dual rhombic antenna
 

On 21 Mar 2006 13:23:26 -0800, wrote:

The claims
for
this antenna seem streached abit, they claim 26dB gain from just wire
and a few resistors
and claim its 435mhz design can work up to 890mhz, theres even a quad
rhombic shown
also. Sounds to good to be true, so i wandering if anyone has had any
experience with
actually building & using one of these antennas. I would like to build
some kind of high gain recieve
only antenna to see if its possible to get a decent signal from some
of the ATV repeaters.

Hi OM,

It's as good as any other.... Good being highly qualified, especially
as the construction details are so shy of description here. Barring
those details, rhombics are not a mystery.

If it works on the 440 Band, you can scale it down for the 900 band if
you want it shorter, or just use the instructions provided. How good
it performs remains to be tested. You would have to test anyway to
confirm any antenna claim for gain.

73's
Richard Clark, KB7QHC

Dual rhombic antenna
 

Also I would check to make sure the gain is listed as 26 DBd instead of DBi.
Back in the day, allot of companies inflated their specs by stating the
DBi gain instead of DBd.
DBi being gain over an isotropic and DBd being gain over a dipole.

--
RoD
KD0XX
PG-6-29404


"Richard Clark" wrote in message
...
On 21 Mar 2006 13:23:26 -0800, wrote:

The claims
for
this antenna seem streached abit, they claim 26dB gain from just wire
and a few resistors
and claim its 435mhz design can work up to 890mhz, theres even a quad
rhombic shown
also. Sounds to good to be true, so i wandering if anyone has had any
experience with
actually building & using one of these antennas. I would like to build
some kind of high gain recieve
only antenna to see if its possible to get a decent signal from some
of the ATV repeaters.

Hi OM,

It's as good as any other.... Good being highly qualified, especially
as the construction details are so shy of description here. Barring
those details, rhombics are not a mystery.

If it works on the 440 Band, you can scale it down for the 900 band if
you want it shorter, or just use the instructions provided. How good
it performs remains to be tested. You would have to test anyway to
confirm any antenna claim for gain.

73's
Richard Clark, KB7QHC

Dual rhombic antenna
 

wrote:
I picked up a Oct 1973 issue of 73 & noticed a interesting article on
a mast mounted rotatible very
hig gain 435mhz antenna, cheap and easy to built. Ive been trying to
build a high gain antenna to
recieve ATV in the 903-928 band & seems this maybe the one. The claims
for
this antenna seem streached abit, they claim 26dB gain from just wire
and a few resistors
and claim its 435mhz design can work up to 890mhz, theres even a quad
rhombic shown
also. Sounds to good to be true,

You guessed it. This antenna has been thoroughly debunked in the 33
years since the original article.

I don't have detailed references, but the story goes that ground gain of
6dB was added on *twice* to the claimed original free-space gain
figure... which was optimistic in the first place.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Dual rhombic antenna
 

Stargatesg1 wrote:
Also I would check to make sure the gain is listed as 26 DBd instead of DBi.
Back in the day, allot of companies inflated their specs by stating the
DBi gain instead of DBd.
DBi being gain over an isotropic and DBd being gain over a dipole.

At 26Db there isn't enough difference between dipole and isotropic to
matter.

Dave WD9BDZ

Dual rhombic antenna
 

To get 26 dB over a dipole would require rhombic leg lengths of about
125 wavelengths, or about 275 feet at 450 MHz. The overall length would
be around 500 feet. The beamwidth would be so narrow you'd have
difficulty getting it aimed toward one station, even if you were able to
turn it somehow.

Roy Lewallen, W7EL

David G. Nagel wrote:
Stargatesg1 wrote:
Also I would check to make sure the gain is listed as 26 DBd instead
of DBi.
Back in the day, allot of companies inflated their specs by
stating the
DBi gain instead of DBd.
DBi being gain over an isotropic and DBd being gain over a dipole.

At 26Db there isn't enough difference between dipole and isotropic to
matter.

Dave WD9BDZ

Dual rhombic antenna
 

This gain figure sounds very much like the 144MMz EME double rhombic
that was in the ARRL handbook for years. As I remember it was "attacked"
some years ago for claiming such a high gain.

I built one for 70cm some years ago and the results were disappointing,
or rather not as good as a 22 el loop quad. I didnt do much
experimenting with it sorry.

Cheers Bob VK2YQA

wrote:

I picked up a Oct 1973 issue of 73 & noticed a interesting article on
a mast mounted rotatible very

Dual rhombic antenna
 

Exactly what i wanted to know, i may hold off on this one and search
for a different type high gain uhf antenna. Only problem is all
the design programs i have found on the web for yagi or log perodic
antennas are for 6-8 elements maximum, @ 900mhz i would like to have
as many elements as a boom would support. So my next question would be
where to look for a multi element beam using atleast a minimum of
twelve elements?

Dual rhombic antenna
 

wrote:
I picked up a Oct 1973 issue of 73 & noticed a interesting article on
a mast mounted rotatible very
hig gain 435mhz antenna, cheap and easy to built. Ive been trying to
build a high gain antenna to
recieve ATV in the 903-928 band & seems this maybe the one. The claims
for
this antenna seem streached abit, they claim 26dB gain from just wire
and a few resistors

You might try modeling the antenna or rading a textbook that covers
real Rhombic antennas.

Antennas and Transmission Lines by John (jack) Kuecken is a great
place to start.

I think what you will find is that gain figure is extremely inflated.
It is unobtainable in the real world. I'd expect more like 15dBd
maximum gain if you can do almost everything perfectly, and you might
squeek another 3dB with two Rhombics in optimum configuration.
At the old VOA installations, the very large Rhombic antennas had less
gain than curtain antennas that occupied much less space.

I have a 300 foot tall support. I've been trying to model an Inverted V
or V beam with useful gain over a small Yagi without success. If
anyone has sucessfully modeled one, please let me know.

73 Tom


I've been

Dual rhombic antenna
 

wrote in message
oups.com...
I picked up a Oct 1973 issue of 73 & noticed a interesting article on
a mast mounted rotatible very
hig gain 435mhz antenna, cheap and easy to built. Ive been trying to
build a high gain antenna to
recieve ATV in the 903-928 band & seems this maybe the one. The claims
for
this antenna seem streached abit, they claim 26dB gain from just wire
and a few resistors
and claim its 435mhz design can work up to 890mhz, theres even a quad
rhombic shown
also. Sounds to good to be true, so i wandering if anyone has had any
experience with
actually building & using one of these antennas. I would like to build
some kind of high gain recieve
only antenna to see if its possible to get a decent signal from some
of the ATV repeaters.
Seems here in western N.C. the ATV activity is located mostly in this
located band.
Thx N4aeq

Hi Tucker

I had to re-look at the month that article was published to be sure it
wasnt an April 1 issue.

The antenna's beamwidth, with a circular cross section, will be close to
10 degrees maximum. It is likely that a 12 foot diameter dish would be
the minimum size that would provide that 26 dB Gain at 900 MHz. That gets
pretty difficult to make stable when also rotateable.

I would suggest that you start with a Yagi and try it. You'll soon know
if that gain is adequate. And the money spent on parts wont hurt your
pocketbook. I sure wouldnt expect you to jump into antenna design with a
beginning project of building a 26 dB Gain rotateable antenna.

Jerry