> All this talk lately of NVIS antenna work goes along with
some antennas and
> masts I have been building lately to enjoy casual,
portable hamming, with
> some emphasis on cheap antenna plans for a cheap emergency
80 trunk antenna.
> So far, results have been quite good using 18ga lamp cord
or speaker cord,
> using a 30' or less of the unzipped portion as a feedline,
to a 4:1 current
> balun and then connected to the mobile HF rig.
Anything can work, but best to use coax. Lamp cord and
speaker wire can have significant loss at radio frequencies.
> I once read that an old trick for boosting the efficiency
of horizontal
> antennas over questionalble ground was to run a series of
radials along the
> ground parallel to the antenna.
Better to call them a counterpoise or grid.
> For instance, if the antenna is a flat-top or inverted
Vee, run 4-5 radials
> parallel to it along the ground, spaced 10-15 few feet
apart.
True enough. The general rule of thumb is when conductors
are less than .05WL apart it looks like a solid sheet.
> What is an effective placement for such wires under the
antennas? Any rules
> of thumb for length and orientation?
Parallel with the antenna and anything ovelapping the
antenna length if possible, although shorter lengths will
work.
> If separate wires, do they need to be approximately 5%
longer than the
> driven antenna to act as reflectors?
No. First the 5% rule is a lame enough rule in freespace. It
is useless along the ground.
As a general rule in freespace a reflector has to be very
close to self-resonant at the lowest frequency to be used.
The reason for this is very simple. Any conductor reradiates
with inverted phase. It has a 180 degree flip in phase just
because it is reradiating. The optimum phase for a null in
line with the exciting and reflecting element is 180 degrees
plus spacing delay. That would occur exactly at resonance
since space delay equals optimum delay. When you lengthen
the element from self-resonance you add an inductive
component that delays phase of current. This means you
offset the rear null. A little of that is good, too much is
bad. My 40 meter yagi has a nearly perfect rear null with a
self-resonant reflector.
More important than that is what we are trying to do. The
earth is a large flat reflector. Think of the array as a
screen reflector using the earth as a screen. The earth is
lossy. The wires laid on the earth simply provide a low-loss
place for current to flow. It is NOT necessary, nor is it
really desirable, to make a screen or wide surface
"resonant". We don't do that with dish antennas, we don't do
that with screen reflector arrays. We just depend on
currents being spread out over a wide area (reducing loss)
and the fact the reflection itself inverts phase 180
degrees.
The last thing you probably want to do is put a resonant
wire right on the earth's surface because it would
concentrate electric and magnetic induction fields near the
wire in the lossy earth. It's far more effective to lay down
a screen or grid longer than the antenna length with several
parallel wires spaced (perhaps) .025 to .05 wl paralleling
the antenna to simulate a high conductivity screen.
My buried radial system on my rotating tower, about 60
100-150 foot long radials, makes a very effective reflector
screen for low dipoles on 80 and 40 meters even though most
efficient use of the wire would be with it all parallel to
the dipoles.
This is an old old trick people on 75 and 160 meters used (I
first learned of it in 1962 or 63), and when doing 3895kc
Gator Quarming competitions in the early 70's I experimented
extensively with NVIS antennas and grounds. Nothing was more
effective at laying out a regional blanket of RF than a
dipole about 1/8th wave high with a parallel screen or grid
that overlapped the dipole by a pretty good amount.
One or two resonant reflectors close to ground (which are
NOT freespace length) actually dogged out the signal a bit.
Your idea of a zig zag grid is probably a very good one.
73 Tom
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