[Antennas] Re: gain and directivity

[Dave Kelley]

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


The relationship between gain and directivity is based on the fact that
any power that is supplied to your antenna system by your transmitter
must be radiated or lost as heat.  The fancy term for it is "conservation
of power;" that is, all of the power entering the antenna system must
be accounted for.

If that input power were radiated in such a way that it spread out evenly
in all directions, then you would have the theoretical "isotropic radiator."
For practical reasons, it's virtually impossible to build an isotropic 
radiator, but the concept is a useful one in antenna theory.  If, on the
other hand, your antenna causes some of the power to be concentrated
in one or more directions, then the power radiated in other directions 
must be correspondingly less.  Remember that you are radiating a finite
amount of power, so if more power is radiated in one direction, less
power can be radiated in other directions.

The concentration of radiated power in one direction is the property
known as "directivity."  If an antenna is directive, then it will radiate
more power in the preferred direction than a theoretical isotropic 
radiator would.  Likewise, the directive antenna will radiate less power
than an isotropic radiator in the undesired directions.  The power is
being taken from the undesired directions and being applied to the
desired direction.  The amount by which the power radiated in the
desired direction exceeds the power that would be radiated in that
direction by an isotropic radiator is called the "gain."  So, the more
directive an antenna, the more gain it has in the desired direction.

Your J-pole, although it is "omnidirectional," is not isotropic.  The
J-pole (theoretically) radiates uniformly in all directions toward the
horizon, but radiates very little above and below the antenna, with 
the radiation falling to zero in the directions along its axis.  The term
"omnidirectional" generally refers to those directions pointing toward
the horizon, whereas "isotropic" refers to all directions (toward all
points on a sphere centered on the antenna) -- up, down, forward,
backward, left, right, and all points in between.

In a real installation, an antenna is surrounded by potentially
reflective objects, such as houses, trees, cars, and other items.
This causes pattern distortion by reflecting some of the radiated
energy in various directions.  Thus, although a J-pole would be
essentially omnidirectional when used in the middle of the desert,
it takes on some additional directivity if installed next to a house.
The reflected radiation may add to the direct radiation (that coming
from the antenna) in some directions and subtract from it in 
other directions.  Thus, more power will be radiated in some 
directions and less in others.  The antenna and its environment
therefore form a kind of "superantenna" that has directivity
and therefore gain in one or more directions.

I hope this was helpful, if not too long.  :)

73,
Dave NB4J

--
Dave Kelley, NB4J
Lewisburg, PA
Union County