Bill Horne writes:

The goals with NVIS are twofold:
- Avoiding nulls in local area coverage.
- Eliminating distant noise and QRM. If you're, after local coverage,
you also want that.

Local is a relative term. If the reflective layer is 200 km up, ranges
up to 480 km have a takeoff angle of 40 degrees or higher. A dipole a
quarter wave up above average ground is less than 2 dB down at that
elevation. At very low heights, the dipole is 3 dB down at 45
degrees. (It also radiates just a small fraction of the power you feed
it.)

As to why this would be the best design, it won't be for winning the
contest! Your brother may have another goal, like simply wanting to
test the design.

By the way, only part of the power going down is lost. The rest is
reflected. How much depends on height and ground characteristics.

73
LA4RT Jon

Thanks for reading this. I've just been through a very confusing
antenna trial, and I'd really appreciate some info from hams who know
more about them then I.

My brother, W3TDH, called me up yesterday and asked me to help test
some 80 and 40 meter antennas that his club is preparing for field
day. He told me that the club is going to serve their target coverage
area by using two dipoles, spaced 1/2 wave apart and fed in phase, so
as to maximize NVIS (Near-Vertical Incidence Skywave)radiation.

I'm not arguing with the design: antennas spaced 1/2 wave apart and
fed in phase always have maximum radiation at right angles to a line
bisecting both antennas. Ergo, two dipoles, horizontally mounted,
etc., will indeed produce their maximum radiation straight up and
down.

My question is: why would that be the best design? I thought that NVIS
radiation was a byproduct of having a radiator less than 1/2 wave
above the ground, and that it wasn't to be sought after, but rather
avoided. Leaving aside the losses due to 1/2 the power pointing
straight down, why would an antenna on 80 or 40 meters be most
effective by radiating almost straight up? Wouldn't the coverage be
improved by phasing the antennas so as to maximize radiation toward
the horizon?

73,

W1AC

--
Bill Horne, W1AC

(Remove QRM from my address for direct replies.)