Hi Group

Can anyone give me a brief explanation of the theory of operation of a
commerical FM broadcast antenna. Specfically the Jampro JMPC (picture
at )http://www.jampro.com/fma/jampjmpc.htm It looks to me like a couple of
1/2 wave dipoles each folded into V and fed in phase using two gamma
matches. If this is true I am puzzled by the tap point chosen. It seems
further along the dipole than I am used to seeing, even considering that the
feed for the dipole is likely 100 ohm. The feed must be very inductive.
Polarization is stated as RH circular. How is this controlled?
WB6KCN
Eric

I was hoping someone else would jump in and answer, but
here goes anyway...

Eric Schumacher, WB6KCN, asked:

Can anyone give me a brief explanation of the theory
of operation of a commercial FM broadcast antenna.
Specifically the Jampro JMPC. Picture at:

http://www.jampro.com/fma/jampjmpc.htm

It looks to me like a couple of 1/2 wave dipoles
each folded into V and fed in phase using two gamma
matches. If this is true I am puzzled by the tap
point chosen. It seems further along the dipole
than I am used to seeing, even considering that the
feed for the dipole is likely 100 ohm. The feed
must be very inductive.

When the ends of a dipole are folded back, the radiation
resistance decreases and matching schemes have to become
more extreme. The Jampro antenna does not look that
exceptional to me. You're probably right about a certain
amount of reactance accompanying the feedpoint resistance.
This could be tuned out by a matching network hidden in
the feedline/support pipe.

Polarization is stated as RH circular. How is this controlled?

To my way of thinking, the circular polarization is due to
the horizontal spacing between the horizontal and vertical
dipoles and the phasing of their excitations. If that is
done in a certain way, the far-field radiations from the
vertical and horizontal dipoles will be in time-quadrature
at all azimuthal bearings, the necessary condition for circular
polarization.

If I look at this antenna in a certain way, it looks like
a highly truncated, half-wave section of a bifilar helical
antenna radiating in the radial mode with the two dipoles
simulating the currents flowing in a small segment of such
a structure.

My guess is that getting this antenna to produce good
circular polarization and a good match and uniform
azimuth coverage were the end results of a hell of a lot
of tweaking on the antenna range.

Jim Bromley, K7JEB
Glendale, AZ

My guess is that getting this antenna to produce good
circular polarization and a good match and uniform
azimuth coverage were the end results of a hell of a lot
of tweaking on the antenna range.

Jim Bromley, K7JEB
Glendale, AZ


Heck, Jim;

That's most of the fun:-)

73,
Chip N1IR

"K7JEB" wrote in message
news:GrQvc.45743$mm1.6092@fed1read06...
I was hoping someone else would jump in and answer, but
here goes anyway...

Eric Schumacher, WB6KCN, asked:

Can anyone give me a brief explanation of the theory
of operation of a commercial FM broadcast antenna.
Specifically the Jampro JMPC. Picture at:

http://www.jampro.com/fma/jampjmpc.htm

It looks to me like a couple of 1/2 wave dipoles
each folded into V and fed in phase using two gamma
matches. If this is true I am puzzled by the tap
point chosen. It seems further along the dipole
than I am used to seeing, even considering that the
feed for the dipole is likely 100 ohm. The feed
must be very inductive.

When the ends of a dipole are folded back, the radiation
resistance decreases and matching schemes have to become
more extreme. The Jampro antenna does not look that
exceptional to me. You're probably right about a certain
amount of reactance accompanying the feedpoint resistance.
This could be tuned out by a matching network hidden in
the feedline/support pipe.

Given the above analysis: Also note that one of the elements is quite
close to the supporting structure nearest its high current center.
--
Steve N, K,9;d, c. i My email has no u's.

Eric Schumacher wrote:
Hi Group

Can anyone give me a brief explanation of the theory of operation of a
commerical FM broadcast antenna. Specfically the Jampro JMPC (picture
at )http://www.jampro.com/fma/jampjmpc.htm It looks to me like a couple of
1/2 wave dipoles each folded into V and fed in phase using two gamma
matches.

Looks like a lindenblad antenna
(http://www.amsat.org/amsat/articles/w6shp/lindy.html)
with a original feeding.


Thierry
F4DWV

"hello" wrote in message
...
Eric Schumacher wrote:
Hi Group

Can anyone give me a brief explanation of the theory of operation of a
commerical FM broadcast antenna. Specfically the Jampro JMPC (picture
at )http://www.jampro.com/fma/jampjmpc.htm It looks to me like a couple
of
1/2 wave dipoles each folded into V and fed in phase using two gamma
matches.

Looks like a lindenblad antenna
(http://www.amsat.org/amsat/articles/w6shp/lindy.html)
with a original feeding.
Thierry F4DWV

Not to me. It looks like the lindenblad elements are fed IN phase, right?

Differences
JMPC Lindenblad
-two bent half waves -four straight half waves
-Elements in two planes -elements in four planes
-symmetry around the -symmetry around the center
center of the two of the four antennas
elements for 180 for every 90 degrees
degrees of rotation of rotation

similarities
circular polarization
Perhaps fed in phase elements
Weird.

--
Steve N, K,9;d, c. i My email has no u's.

Thanks for the insight guys. There were some pearls in there.

Eric


"Steve Nosko" wrote in message
...

"hello" wrote in message
...
Eric Schumacher wrote:
Hi Group

Can anyone give me a brief explanation of the theory of operation of a
commerical FM broadcast antenna. Specfically the Jampro JMPC (picture
at )http://www.jampro.com/fma/jampjmpc.htm It looks to me like a
couple
of
1/2 wave dipoles each folded into V and fed in phase using two gamma
matches.

Looks like a lindenblad antenna
(http://www.amsat.org/amsat/articles/w6shp/lindy.html)
with a original feeding.
Thierry F4DWV

Not to me. It looks like the lindenblad elements are fed IN phase, right?

Differences
JMPC Lindenblad
-two bent half waves -four straight half waves
-Elements in two planes -elements in four planes
-symmetry around the -symmetry around the center
center of the two of the four antennas
elements for 180 for every 90 degrees
degrees of rotation of rotation

similarities
circular polarization
Perhaps fed in phase elements
Weird.

--
Steve N, K,9;d, c. i My email has no u's.

Thierry, F4DWV, writes:

Looks like a lindenblad antenna
(http://www.amsat.org/amsat/articles/w6shp/lindy.html)
with a original feeding.

I think there is definitely a kinship there. Folding
back the dipole "legs" makes the dipoles less directive
and it becomes possible to use only two dipoles rather
than the four of the Lindenblad.

Of course, when I look at a Lindenblad in a certain
way, it appears to be a highly truncated, half-wave
section of a quadrifilar helical antenna radiating
in the normal (radial) mode. You have to tilt your
head just right to see it.

Jim, K7JEB

Here is a description of this antenna by Peter Onnigian, P.E., who developed
and patented the design:

+ +
(Onnigian) Shunt Fed Slanted Dipole Antennas
The slanted dipole antenna in its present configuration was developed and
patented in 1970. It consists of 2 half-wave dipoles bent 90°, slanted and
fed in phase.
The slant angle is critical as it is the factor which determines the ratio
of vertically and horizontally polarized radiated power. The phase point
center is at the feed insulator on the dipole support arm. When fed through
a vertical support pole on which the antenna was mounted during initial
development tests, the axial ratio varied less than 1 dB.
The commercial adoption uses a horizontal boom containing a step
transformer. This boom supports two half-wave dipoles in which the included
angle is 90°. The two sets of dipoles are rotated at 22.5° from the
horizontal plane. Two opposite arms of the dipoles are delta matched to
provide a 50 ohm impedance at the radiator input flange. All four dipole arm
lengths may be adjusted to resonance by mechanical adjustment of the end
fittings. Shunt feeding, when properly adjusted, provides equal currents in
all four arms resulting in excellent azimuth circularity.
+ +

(RF) The 1 dB axial ratio of this configuration is reduced by the adjacent
transmission line(s), and the tower structure needed to support it (as with
all sidemounted radiators).

NEC-2 patterns for this design, and three others commonly used in FM
broadcast transmit antenna arrays are shown in paper 10 at the link below.
Paper 6 shows examples of sidemount antenna pattern distortions from the
line and tower.

RF

Visit http://rfry.org for FM broadcast RF system papers.

______________

"Eric Schumacher"
Can anyone give me a brief explanation of the theory of operation of a
commerical FM broadcast antenna. Specfically the Jampro JMPC ...

On Mon, 7 Jun 2004 08:02:18 -0500, "Richard Fry"
wrote:

Here is a description of this antenna by Peter Onnigian, P.E.,
who developed and patented the design...

...The slant angle is critical as it is the factor which
determines the ratio of vertically and horizontally
polarized radiated power....

... The two sets of dipoles are rotated at 22.5° from the
horizontal plane....

There you go - that's the secret sauce. It wasn't obvious
from the catalog photos that the dipoles were inclined at
an angle.

Good info, Richard.

Jim, K7JEB