[Towertalk] Low dipoles

[Chuck Counselman]

At 8:28 AM -0800 11/5/02, Al Williams wrote:
>First, Earth is said to be a pretty good reflector.

Yes, for horizontal polarization at low angles (i.e., near the 
horizon).  No, for vertical polarization at low angles.
So-so for any polarization at higher angles.


>Second, EZNEC reports for a half-wave 68' dipole 30' high at 7.04 mhz:
>                                              straight up
>no reflector         real ground (.005)       6.39 dBi
>70' refl on gnd      real ground (.005)       6.58  "


Unless you're using EZNEC-4 (based on NEC-4 rather than NEC-2), be 
aware that EZNEC (and any other program based on NEC-2, or on MININEC 
for that matter) does not accurately model wires within a small 
fraction of a wavelength of "real" ground.  "On ground" is a definite 
no-no.


I am not taking a position on the merit of putting a reflector on the 
ground, because I haven't (at least not recently) studied it. 
However I have studied a possibly related scheme, which is to insert 
a second _driven_ dipole one-half wavelength below the first one. 
The two dipoles have equal lengths; both are center-fed; and their 
feedlines are arranged such that the current in the lower dipole is 
equal to minus one-half times the current in the upper dipole. 
(Easily done.  Make each dipole's feedline one-quarter wavelength 
long; connect them in parallel to a common source terminal-pair; make 
the characteristic impedance of the lower dipole's feedline twice 
that of the other; and put a 180-deg twist in one line.)

IIRC, NEC-4 modeling with "poor" ground showed a gain improvement of 
2 dB at most elevation angles.  The modeling also showed that the 
half-wave spacing and the current ratio of minus-one-half were 
optimal.

IIRC, this scheme was discussed a few years ago on the NEC-LIST.  I 
was not the only person who studied it, but I don't recall who the 
others were.  I also don't recall how to "explain" or "understand" 
it; but if you have NEC-4 it should be easy to simulate it yourself 
and convince yourself that it works as claimed.

Obviously, unless you can suspend a dipole more than a 
half-wavelength above ground, you can't employ this scheme; but if 
you can, it appears superior to a single dipole at the same height 
(or lower).  OTOH, 2 dB isn't much reward for the trouble.

73 de Chuck, W1HIS