There is a great deal of pure unvarnished BS around when
dealing with NVIS antennas, largely because losses or earth
effects are ignored. Some of the absolute worse signals I've
heard are on the Georgia ARES HF net. This is due entirely
to the thought very low antennas make good statewide 75M
communications. My mobile with a short vertical antenna
actually is often louder around the state than many home
stations with full size antennas!
> This NVIS configuration is included in the book "Near
Vertical Incidence
> Skywave Communication" published by World Radio Books. A
drawing and text
> is on page 73 where it is called the Jamaica and is said
to be used by the
> British Commonwealth Forces (military). It is given a
favorable efficiency
> but not too useful for the military due to requiring 6
poles and lots of
> room and wire.
I don't have that book, but anytime a horizontal antenna is
placed less than about .1 WL or so above normal soil with a
counterpoise or screen system losses will skyrocket.
If you want to use a horizontal antenna below ~ 1/4 wl
height, a good ground screen or grid of parallel wires will
greatly increase signal level. It does this by reducing
loss, NOT raising wave angle. The lower the antenna, the
greateer the increase in efficiency when a grid is added. At
about .05WL loss is in the area of several dB without a
ground screen or grid below the full area of the antenna.
Tuning the "reflector" for resonance when it is within a
small fraction of a wavelength above earth is a waste of
time. If the reflector is elevated .1 wl or more, it works
well. If it is below about .05 WL or so you are largely
wasting time trying to "tune" it.
You can see this effect in a real antenna if you measure
feed impedance. Impedance reaches a minimum and then starts
to climb as the antenna is lower further....whereas over a
good reflector impedance would keep dropping. At .1 wl or
less losses become increasingly terrible.
> My question is how useful is it for field day for sites
that have dedicated
> 40m phone and cw setups? The book's drawing is in terms
of wavelengths,
> however other articles in the book are confusing. One
article flat-out
> states that 40m is hopeless for NVIS. Others show graphs
of MUF (maximum
> usable frequency) with axis of 24 hour local time and MUF
for several Solar
> Flex Index e.g. 75,100,125, 150, 200. 40m is shown to
require the higher
> Solar Flex values else the signal will not be reflected
back to earth. As I
> have read the numerous graphs, NVIS at 40 meters is a
daytime operation only
> and is best during summer months and when the sunspot
cycle is NOT at it's
> minimum as it is now.
MUF over a very short high angle path would work, but don't
confuse it with MUF at low angles (long distances) which is
considerably higher in frequency. I think you probably want
to consider is critical frequency.
Daytime short hop is very much possible on 40 meters,
especially in summer at solar maximums. Some nights even 80
won't support short hop because critical frequency is too
low.
73 Tom
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