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