[TowerTalk] NVIS antennas Re: dumbing down

[Tom Rauch]

73, Tom W8JI
----- Original Message ----- 
From: "Gary" <gaschafer at comcast.net>
To: "Jim Lux" <jimlux at earthlink.net>
Cc: "Tom Rauch" <w8ji at contesting.com>; <ersmar at comcast.net>;
<towertalk at contesting.com>
Sent: Saturday, July 23, 2005 12:52 PM
Subject: Re: [TowerTalk] NVIS antennas Re: dumbing down


>
>
> Jim Lux wrote:
>  > I think you mean a horizontally polarized antenna
doesn't radiate any
>  > ground wave? I can propagate a horizontally polarized
signal parallel to
>  > the ground perfectly well (which most TV broadcast
stations depend
>  > on).  Whether a horizontal dipole over ground does it
well is really the
>  > question, and I readily concede that an idealized
horizontal wire
> doesn't
>  > have any radiation at zero elevation. However, a "real
antenna" over
> "real
>  > ground" that isn't perfectly flat may radiate at zero
elevation angle.
>
> Jim,
> I think he does mean "does not propagate on ground wave".
Horizontal
> polarization will not support a ground wave signal. It
gets shorted out
> by the earth. A vertically polarized signal does not have
that problem
> and will follow on the earth surface.
>
> A TV signal does not propagate by ground wave. It is line
of sight with
> some atmosphere ducting and bending.
>
> There are sometimes disconnects in terminology when
talking about ground
> waves. In this case "ground wave" really means "surface
wave", which to
> some is different than ground wave where they might be
talking more
> about line of sight propagation.

That's right Gary. VHF surface wave or direct wave should
not be confused with HF ground wave.

At HF, the wave has enough area and conductivity is high
enough that conductivity becomes a real issue. Lower
frequency signals have the electric field "shorted" by the
earth, and we all know (except perhaps the guy Bill
mentioned who was talking about the coax and the magnetic
field getting through the shield) when we take the electric
field to zero the magnetic field goes to zero.

We have a huge polarization filter at HF, the earth. The
lower the frequency the better the filter. Ground wave by
definition requires the electric field not parallel the
earth.

I can't work five miles on 80 meters on surface or ground
wave with horizontal polarization. I can if I let the
feedline radiate or if something near the antenna
re-radiates a vertical component,  but even that vertical
component attenuates rapidly with distance on HF. 160 meters
is the only band below 15 MHz or so with substantial
non-skywave coverage. It might be 30 miles or more with big
vertical antennas and reasonable power over soil.

Of course groundwave can go a long distance at MF or HF over
saltwater with vertical polarization (but no distance at all
with true horizontal polarization).

73 Tom