[TowerTalk] COUNTERPOISE for Dipoles

[Tom Rauch]

> This is common practice in this part of the country.  Lots of people in
> the rural areas put a reflector under their 75 and/or 160m dipoles.  As
> Tom says, the point is to make the signal go straight up, not out, so as
> to have a good signal on the local nets.  It will decrease long haul
> signals and DX.  This is also why a full wave loop works so well on lower
> bands.  It radiates straight up at the resonant frequency.

Low full wave loops have almost zero gain over a dipole. Any 
advantage is in the minds of the owner.

The reason they have almost no gain is the "focus" the energy by 
nulling signals slightly on a line through the plane of the loop. 
Since earth already produces a null at that angle, the loop can not 
have gain because there is no energy to remove and redirect.    

The same is true for conventional loops or quad elements. If placed 
at a mean height that causes a vertical null, they have minimal gain.

If you want more gain straight up than a dipole, use an extended 
double zepp and a large counterpoise system below it. You won't 
get the full 3 dB because it also tries to null in an area where 
radiation is reduced, but the nulls it does form are more complete 
and some are elevated where there is radiation.

> reflector length single wire, a little less than a quarter wave under the
> dipole does a great job, especially if you don't have a clue where the
> earth's reflective level is.

Only because it reduces earth loss. The earth itself forms a large 
reflector. The more you reduce the loss in the earth, the more 
efficient the antenna is and the better focused it becomes.

Think of the earth as the screen reflector in a dipole backed by a 
screen. The better the reflector the more gain because less energy 
is turned to heat. When the dipole is close to earth, the reduction 
of loss can improve the signal at every angle...not just straight up.


73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com