[Antennas] Need a little advice

Sun Jan 6 17:18:22 EST 2008

At 08:06 PM 2008年01月03日, Ken KD4IEN wrote:
>I have a Hamstick dipole , and im thinking of mounting it under a 10 
>meter ground plane... about how much separation should I set the 
>antennas apart?

It looks like you should keep the 10m dipole at least 10 feet below 
the feedpoint of the 10m ground plane. The radials on the ground 
plane act like a parasitic element that is tuned to roughly the same 
frequency as the dipole. Depending on the exact tuning, it could 
appear as a reflector or director. Either way, it will tend to 
enhance the signal straight up or straight down from the dipole and 
slightly attenuate the signal on the horizon.
It appears that it is best to mount the dipole so that one of the 
drooping radials is 90 degrees from the dipole, when viewed from 
above, and the others at +/- 30 degrees. When one of the radials is 
parallel to the dipole, it causes some filling of the null off one 
end of the dipole.
With the length of the dipole at exactly twice the length of the 
radials, there is a slight enhancement on the horizon when the dipole 
is one wavelength (34.6 feet) below the feedpoint of the ground 
plane. Obviously, this isn't practical in your situation.
If you succeed in mounting these at 100 feet above ground, you may 
not notice much difference in the gain of the two antennas. In fact, 
the main lobe of the ground plane is at 4.5 degree elevation with 5.6 
dBi gain (omni). With the dipole 10 feet below the GP (90 feet 
high), it has 6.3 dBi gain in one direction and 5.5 dBi gain in the 
other at a 5.5 degree takeoff angle. (The pattern skew is due to 
the apparent asymmetry of the drooping radials with respect to the 
dipole.) With the GP removed, the dipole gain is 8 dBi bidirectional 
at 5.5 degree takeoff.
Of course, you can use the rotatable dipole to do some nulling of QRM 
off the ends on receive. And polarization definitely matters for 
local QSOs. Sometimes, there may also be some benefit to have the 
ability to switch polarization on DX paths.
The impedance of the dipole alone is about 69 ohms when 90 feet 
high. With the GP mounted 10 feet above it at 100 feet, the dipole 
impedance drops to 51 ohms.
BTW, I modeled both of the antennas as 0.1" diameter rods. It's not 
clear if there is any inductive loading of the 10m Hamsticks (since a 
1/4 wl is about 8.3 feet without any loading). If there is some 
loading, it may slightly modify the gain numbers, but the relative 
gains will probably be similar.
73, Terry N6RY