Guys – N4RVR is working on getting an NVIS configured antenna setup at his QTH for regional work on the low bands. He sent along this email from K5KVH via an NVIS mailing list. Check it out for an idea of how to setup an NVIS antenna with a wire reflector near the ground to add gain to the antenna, and stay tuned for more NVIS info in the near future.
NVIS can be simple if you have the room to put up a full size dipole,
say at 20 or so feet above your yard.
If not full size, then you will need a tuner, and can put up as big a
dipole as you can manage, even if you have to fold the ends to an Ell or
U. Even 6 foot high dipoles work on 80m.
Put an insulated reflector wire under the dipole on the ground, and you
are set for NVIS. The reflector can be 5 per cent longer than the
dipole if you can manage that.
Feeding the dipole can be easier if you make the dipole of twin lead as
a folded dipole, and then string that from non conductive line that can
support the weight of 50 ohm 1/4 inch coax like RG 58.
If you have a husky tuner with big coil for 80m, you can feed the dipole
as one wire element fed at center with 450 ohm ladder line, 600 ohm
parallel line, or 300 ohm twin lead. For parallel lines like that as
feeders, you have to have a true current choke at the rig end. Use a
1:1 ferrite isolator, or a coil of coax to make a cable choke. Ferrite
isolators are easiest and are cores slipped onto a one foot of coax,
with suitable connectors on each end. This can even be used with
parallel lines, if you mount binding posts on one end of the one foot
coax. The Wireman sells suitable isolator kits, or complete units.
The trick with coax fed NVIS dipole, is that it is low to ground, and
thus you have a low impedance that some tuners have trouble matching.
Using two parallel 50 ohm coaxes, as quarter wave stubs, will allow the
low center impedance of the dipole to be transformed higher to match a
50 ohm coax feed line. The reason that a folded dipole is advocated,
is that its 300 ohm center impedance comes lower to about 60 ohms, when
it is lowered and installed close to ground, (20 feet or less). 60 ohms
is close to 50, making a coax feed line and tuner happy.
The added just above the grass reflector wire, turns your low dipole
into a 2 element beam pointed straight up, and this can make signals
“pop” out of the low background noise. Close to ground, your dipole
does not pick up low angle static or skip signals well, and makes things
quieter on 75m at night. However, you do not lose sky wave skip
entirely, and we regularly at Field Day on 75m phone, contact west coast
from TX, using only a six foot high dipole!
You will find making NVIS contacts around your state and nearby states a
fun new aspect of ham radio.
Another thing, with NVIS, it does not matter for NVIS mode, if the
dipole is N-S or E-W facing.
Hope the above is helpful for you to grab some wire, and jump in. Our
favorite cheap insulators for 100 watt antennas are empty vitamin and
medicine plastic pill bottles. Antenna wire can be no. 20 up, or what
have you. Full size dipoles are better made from 16 ga or larger wire.
Some years ago, I purchased a 500 foot roll of plastic insulated conduit
wire, which yielded several dipoles. That was 14 ga solid copper. It
stretches a bit the first time you use it, but don’t pull it too tight
and it does fine.
73,
Stuart Rohre
K5KVH
