[Antennas] gain and directivity

[Dan Richardson]

At 12:02 AM 7/9/2002 -0400, Ed wrote:
>OK, let's get another thread started. I recently got into a discussion=20
>over the local 2 meter repeater. Some older hams and some brand new ones=20
>were talking about j-poles and gain and directivity as well as directivity=
=20
>in vertical base station antennas. The nearest I can figure is that no one=
=20
>agrees. My take was that the j-pole supposedly exhibits about 2.5 to 3.0=20
>gain and may be somewhat directional. I have personally experienced quite=
=20
>a bit of directivity in my vertical antennas at home. They are , of course=
=20
>, advertised as being omni directional. Apparently a lot of factors such=20
>as nearby objects and grounding affect this. I know a lot of this stuff is=
=20
>based on theoretical antennas in free space but it sure makes for good=20
>repeater fodder to listen to. What's your all's take and opinions. Please=
=20
>keep the flame throwers holstered as I just would like to hear the experts=
=20
>comments.


Ed,

A number of years ago I did a study modeling the j-pole using NEC. It was=20
prompted by attending a club meeting where the program speaker gave a=20
presentation on a j-pole that he had built. In his comments he mentioned=20
that he had noticed that there was a small directional gain. At that time I=
=20
had really taken no interest in peculiar in j-poles and my only knowledge=20
was based on what I had read in several amateur publications. "The j-pole=20
has about the same gain as a dipole, omni-directional pattern and etc." So=
=20
I thought that the reported direction "gain" was questionable.

I started by modeling a j-pole and comparing it to a =BD-wave dipole in free=
=20
space. This way I would be observing the antennas themselves without any=20
outside influences. What I gleaned was that the currents within the =BC-wave=
=20
matching section were not exactly the same amplitude nor did they have=20
exactly a 180=BA phase differential. This in turn resulted in a small amount=
=20
of radiation from the =BC-wave section. Not a great amount, but enough that=
=20
the elevation pattern of the j-pole was slightly higher than the reference=
=20
dipole (about 1-2=BA). Additionally there was a slight skewing of the=
 azimuth=20
pattern. The amount of skewing being greater for models which had a larger=
=20
spacing between the =BC-wave conductors. Stubs with a 2" spacing produce a=
=20
shift of about =BD dB that resulted in a front to back ratio of about 1 dB.=
=20
With a 5" spacing the F/B was about 2=BA. The additional radiation from the=
=20
stub section the j-pole resulted in a slight gain over the dipole, but it=20
was very slight and is only academic in that it would never be detected in=
=20
general operation. So based on the free space model I concluded that 1) the=
=20
gain of the classic j-pole was, for all practical purposes about the same=20
as a =BD-wave dipole and 2) what little skewing produced by the stubs=20
radiation again would be hard to detect.

More importantly is what I discovered when I modeled the j-pole over real=20
ground when mounted and connected to on a conductive mast. Using a typical=
=20
mast heights between 26 -30 feet there was a substantial increase in higher=
=20
angle radiation that when the j-pole was connected a conductive mast. This=
=20
is because the voltage (or current) at the shorted base section of the=20
antenna is not at exactly zero. This in turn couples some RF to the mast=20
which, of course radiates and raises the high angle radiation a lot. This=20
increase was more pronounced when the mast length was even =BD-wavelengths,=
=20
however, it was still substantial at odd =BC-wavelengths mast lengths.

Additionally, common mode current on the coax transmission line also raises=
=20
the higher angle radiation patterns. Depending upon the routing of the=20
transmission line the common mode current can and will have a detrimental=20
result on towards the horizon communications.

To recap my conclusions, based upon NEC models, for a *properly* built and=
=20
installed j-pole from my modeling is:

1- A j-pole has approximately the same gain and radiation pattern as a=20
=BD-wave dipole. (less that a dB difference)

2- The spacing between the stub elements (conductors) should be less that=20
2" for best the omni-directional pattern.

3- A j-pole should be insolated from a conductive mast or installed on a=20
non-conducive mast.

4- A common mode choke should be installed and the antenna's feed point.

Why some swear by and other swear at j-poles I feel is probably due to how=
=20
they were installed.

73
Danny, K6MHE