Dear Crew:
What may have been lost in this discussion - but often provided by Roy -
is the difference between a true surface wave and a sky wave (from the same
vertical antenna).

An excellent ground screen at the base of a vertical antenna reduces the
losses and thus increases the amplitude of both the surface and sky wave.

A few to several hundred wavelengths from the vertical antenna, the
amplitude of the received surface wave is highly dependent on the
conductivity of the intervening earth. One could hedge this a bit for
frequencies above something like 5 MHz.

At distances from the antenna where the surface wave has become weak,
the sky wave dominates. As Roy has pointed out several times: the amplitude
of the sky wave is mostly determined by ground properties distant from the
antenna - much farther away than any reasonable ground screen will extend.

The sky wave effect may be demonstrated in NEC by effecting a 0.25 WL
vertical with a good set of radials over mediocre ground and then changing
the ground, at some distance from the antenna, to salt water. (Visualize
the antenna being in the center of a circular island and change the diameter
of the island.)

Broadcasters at MF, with some exceptions, are interested in maximum
surface wave and minimum sky wave. Radio amateurs, for the most part, are
interested in maximizing the sky wave. A good radial system assists the
radio amateur's goal by improving the overall efficiency. It is the ground
distant from the vertical antenna that determines the sky wave.

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Cecil Moore" wrote in news:JbR1g.62010$F_3.14225
@newssvr29.news.prodigy.net:

"John Ferrell" wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

The helix feature is found under the Wires window under the create
menu. But it is somewhat complicated. A lumped inductance load
will get you started.

The problem with feeding an impedance of 1.2-j1400 ohms is
getting power into the antenna without dissipating most of it
in the matching network.

A funny anecdote comes to mind in this connection. I was trying
desperately to load a very short helical dipole on 40M and ended up
hooking it to my old Dentron 1KW matchbox to see if that could do
anything with it. Damn antenna loaded up perfectly. SWR at on the line
to the matchbox 1.1 to 1, good power transfer at 25 watts from the
exciter. So I went to listen with it. NOTHING! NADA! Not even a dit,
even though the big vertical array just hummed with CW signals on the
band. So I leaned over to check connections and darn near fried my hand
on the case of the Dentron. THAT'S where all the watts were going. Damn
tuner was loading its own case!

--
Dave Oldridge+
ICQ 1800667
VA7CZ