[TowerTalk] COUNTERPOISE for Dipoles

[Bruce]

The main objective when using the NVIS concept is the take off angle, not gain.
More gain can be an advantage of course, but the very high take off angle you
get from a properly installed NVIS system is more important, when using it for
the close in communications its intended for, than gain alone, at the wrong
angle.

One of the most uncommon antennas of this type I worked with in my many
years of working with these in the US Army, was a NVIS log Periodic. It
looked a lot like a standard Log, only standing on the end, with the longest
element at the top. It looked like a normal LP with the short elements to the
ground, and the Low freq elements at the top. ( very funny looking, and
looked liked it was trying to beam into the earth). The way it was feed, each
frequency segment was the proper fraction of a wave length above ground.
  --------|-----------
   --------|---------
    -------|--------
     -------|------
       -----|------
_____GROUND________

If you want to see the NVIS concept  really work,  install a second 40 or 80m
dipole at 1/8 to 1/16 wave above ground. After this talk to some one within
300 miles on the high one, then the low one. The low one will blow the high
one away. The higher antenna is trying to put the signal towards the horizon,
while the low one is using the earth under it as a reflector, and beaming it up
for a great cloud warmer. Not intended for DX, but Great for close in work.

In the desert, since the dry sand is a insulator for RF, the antennas are often
buried in the sand since RF ground is several feet lower.

Many books, both civilian and military, have been written on the NVIS
( Near Vertical Incidence Skywave) concept. The best, and easiest  to
understand and NOT Classified is, "Near Vertical Incidence Skywave
Communications" It is written by LTC Fiedler and Maj. Farmer. It is
published by Worldradio Books.

                            73/DX, Bruce





Tom Rauch wrote:

> > I have often wondered about what would be the optimum counterpoise
> > (most improvement for least wire) for dipoles.  I'm thinking of several
> > wires parallel to the dipole laid on the ground under the dipole. Where is
> > the point of diminishing returns for number and spacing of wires? Would
> > parallel wires be more efficinet than a radial system centered under the
> > center of the dipole?  How long should the wires be for 80M? Is a
> > counterpoise worthwhile for dipoles at 1/2 WL high or higher?
> >
> > Tom  N4KG
>
> I first learned of this trick back in 1962 or so, when I saw the
> installation at W8PSX on 160 meters. He had by far the strongest
> 160 meter signal of anyone from his are, but ran no more power.
> He had a large counterpoise of parallel wires below his 160 meter
> dipole.
>
> When I did the same thing, people saw a definite improvement.
>
> It amazes me people use horizontal full-wave loops near ground in
> an effort to get NVIS gain, when the difference to a dipole is almost
> zero dB...yet ignore ground losses (which if removed can add a few
> dB to the signal).
>
> This enhancement occurs most profoundly with antennas mounted
> near earth, but also occurs in diminishing amounts as height
> increases to large fractions of a wavelength.
>
> I've never seen real world measurements of the change, but I can
> readily see the change in A-B tests of dipoles 1/4 wl high at any
> distance..not just in close.
>
>
> 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

--
--------------------------------------------------
Bruce Richards / WD4NGB /E30GA team leader/ ex HL9JV
DX Page http://www.qsl.net/wd4ngb
50 Miles North West of Nashville Tn

***** Eritrea DX-pedition ***********
http://www.qsl.net/eritrea/e3.htm



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