[TowerTalk] COUNTERPOISE for Dipoles

[Tom Rauch]

> >Low full wave loops have almost zero gain over a dipole. Any 
> >advantage is in the minds of the owner.
> 
> Not as NVIS or straight up. Just like dipole, full wave loop adds some
> "stacking gain".

My initial impression was it would have little gain. I received an e-
mail a month or so ago, and that's what I said to the person who 
suggested I install a low loop for close in contacts during contests.

Since then I modeled it, and a full wave loop at low height indeed  
has almost no gain over a regular dipole. For all practical purposes, 
they are the same.

80 meters, perfect ground
 
Low dipole 8.79 dBi 
Low loop 9.26 dBi (gain over dipole .47 dB)
Low EDZ 10.98 dBi (gain over dipole 2.19 dB)

Let me explain why that occurs, because it will help people 
understand what is happening without using a modeling program.

Consider a loop. It is really two bent 1/4 wl long dipoles, one is 
center fed and the other is fed from the ends by the "bent" loading 
sections. These loading sections have phase reversal at the 1/2 
way point, and oppose each other, so they contribute nothing to far-
field radiation.

The stacking gain comes from two 1/4 wl long end-loaded dipoles 
being fed in phase. Stacking gain of two non-directional sources at 
that distance is just under 2 dB under ideal conditions.

The key to gain is energy MUST be removed from an area that has 
substantial radiation, or you get no gain. In other words, forcing a 
null where a null already exists will not create gain, because there 
is little energy available to be moved to a new direction.

If a horizontal loop is installed close to ground, at a height where 
the signal along the horizon and for some elevation above the 
horizon is nulled by the earth, we might just as well have used a 
dipole. The small 1.8 dB or so free-space stacking gain almost 
disappears, because there is little or no energy at the null angle to 
"move" to the main direction!

The same effect occurs when you mount a single element 
horizontally polarized quad at a mean height of 1/2 wl above perfect 
earth, or a multiple of that height. Since the height produces a null 
straight up, the 1/4 wl stacking height with it's poorly defined null 
makes no substantial change in pattern...or gain. Mount the quad 
element at 3/4 wl mean height so the earth causes a vertical lobe 
that peaks in the area where the stacking height forces a null, and 
you have maximum gain change over a dipole at the same mean 
height!

That gain is slightly less than the freespace gain of the quad 
element over a freespace dipole.

As you make the array longer and longer, the stacking null caused 
by the quad element has less and less effect because the 
elements try to form a null above the antenna at high wave angles 
where the stacking height has a null. That's why the advantage of a 
quad disappears when the quad is made long. The null from the 
long boom and multiple elements prevents the null from the 1/4 wl 
stacking distance from moving any energy to the main lobe. 

This technique is called pattern multiplication, and is what I used 
before I owned a computer to predict patterns.

> As team vertical 6Y2A etc. proved, verticals with two elevated and
> resonant radials placed over or next to "perfect" ocean ground are
> unbeatable. The closest inland situation would be to have few elevated
> radials over ground screen (plane, radials, dipoles, salty marsh). That
> also helps horizontal antennas by defining reflection surface, instead of
> mushy "RF sponge" that earth sometimes is.

It would make no difference if the connection was directly to the 
ground screen, or if it was to two radials over that perfect ground 
screen, except the two elevated radials would add some small 
amount of high angle radiation in some directions.

> There are two things that come to play with vertical antennas: the
> reflective "ground screen" and antenna's counterpoise (the other half). By
> using elevated radials over ground screen we take advantage of both and
> maximize the performance of the antenna. 

There is no reason to use elevated radials with a large ground 
screen underneath, unless you want to add high angle radiation in 
some directions. Efficiency is almost exactly the same for each 
system, except for the low angle signal that is removed to create a 
high angle signal from radiation from the elevated radials. 
Remember that even the radials from a four radial groundplane 
modify the pattern of a ground plane, because they don't perfectly 
cancel. Two radials is even worse. One radial is the ultimate in 
adding high angle radiation.

73, Tom W8JI
w8ji@contesting.com

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