Hello,

Are vertical J-Pole VHF antennas (essentially) omni directional in coverage
?

Thanks

On Jun 17, 9:46 am, "Robert11" wrote:
Hello,

Are vertical J-Pole VHF antennas (essentially) omni directional in coverage
?

Thanks

Yes it is omnidirectional. This antenna is merely a 1/2 vertical end-
fed with a 1/4 wave matching stub.

Frank
K3YAZ
Tucson

On Jun 17, 11:54 pm, RHF wrote:
On Jun 17, 11:54 am, wrote:
In article .com,

" wrote:
On Jun 17, 9:46 am, "Robert11" wrote:
Hello,

Are vertical J-Pole VHF antennas (essentially) omni directional in coverage
?

Thanks

- - Yes it is omnidirectional.
- - This antenna is merely a 1/2 vertical end-fed
- - with a 1/4 wave matching stub.
-
- It's not a good broadband antenna for SWL.
-
- --
- Telamon
- Ventura, California

WHY ? ~ RHF
.
.
. .

Because it uses a 1/4 wave matching stub section to transform the low
impedance coax input to the high impedance of an end-fed halfwave
("Zepp") at around 5K ohms. Because a 1/4 wave section is only a
quarter-wave at a narrow band of frequencies, the impedance match gets
poor too far from the design frequency (although I think odd harmonics
work find, so 3/4 or (2n+1)/4 wave all work exactly the same. But
that's a comparatively small percentage of total frequencies, so it's
somewhat of a narrow-band antenna.

As far as the radiation pattern, it is omni-directional in the
horizontal plane if it's used vertically. The energy is somewhat
"squashed donut" so there is some gain toward lower angles of
radiation vs. higher angles. It's a fairly OK antenna where moderate
gain and a relatively narrow band of frequencies are acceptable.

Hope this helps.

-- ross

On Jun 19, 1:33 am, Ross Archer wrote:
On Jun 17, 11:54 pm, RHF wrote:





On Jun 17, 11:54 am, wrote:
In article .com,

" wrote:
On Jun 17, 9:46 am, "Robert11" wrote:
Hello,

Are vertical J-Pole VHF antennas (essentially) omni directional in coverage
?

Thanks

- - Yes it is omnidirectional.
- - This antenna is merely a 1/2 vertical end-fed
- - with a 1/4 wave matching stub.
-
- It's not a good broadband antenna for SWL.
-
- --
- Telamon
- Ventura, California

WHY ? ~ RHF
.
.
. .

Because it uses a 1/4 wave matching stub section to transform the low
impedance coax input to the high impedance of an end-fed halfwave
("Zepp") at around 5K ohms. Because a 1/4 wave section is only a
quarter-wave at a narrow band of frequencies, the impedance match gets
poor too far from the design frequency (although I think odd harmonics
work find, so 3/4 or (2n+1)/4 wave all work exactly the same. But
that's a comparatively small percentage of total frequencies, so it's
somewhat of a narrow-band antenna.

As far as the radiation pattern, it is omni-directional in the
horizontal plane if it's used vertically. The energy is somewhat
"squashed donut" so there is some gain toward lower angles of
radiation vs. higher angles. It's a fairly OK antenna where moderate
gain and a relatively narrow band of frequencies are acceptable.

Hope this helps.

-- ross- Hide quoted text -

- Show quoted text -

RA - TYVM for the Explanation ~ RHF

"Robert11" wrote in message
...
Hello,

Are vertical J-Pole VHF antennas (essentially) omni directional in
coverage ?

No.
A J-Pole is not an isotropic radiator.

It is "omnidirectional" only if you consider signals that intersect the
antenna in the plane that is at a right angle to the major axis of the
antenna.
The closer you get to having the signal intersect the antenna at the end
the the less sensitive the antenna is. i.e. it is less sensitive to
transmissions that are from below or above the antenna.

You will get an idea of the plot the antenna's free space pattern if you
imagine an orange with two major parabolic dimples 180 degrees apart, the
centers of which are in line with the major axis of the antenna.

In practice the antenna is considered an omni because they are mounted
vertically to communicate over the surface of the earth with other radio
units on the surface of the earth .In day to day operations the surface of
the earth is very close to the geometric defintion of a plane.

Dan
AI8O