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
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