ARLA/CLUSTER: Cover very short distances on HF.(NVIS reports by Tom (W6QJI)

[João Gonçalves Costa]

Hi, 

I've read discussion in this session about the inability to cover very short
distances on HF. There just has to be something wrong or overlooked. The
two things that come to mind are a sick rig, or bad coax, at one end or
excessively high NVIS Critical Frequency (f0F2). 

F0F2 can be seen at Near-real-time F2 Layer Critical Frequency on the Internet. F0F2 --- the
highest frequency at which NVIS propagation is supported by the ionosphere -
rarely exceeds 6 MHz, in the northern hemisphere, at this point in the solar
cycle, this time of year and the time of day. And, it does vary considerably
from day to day. The global curve set is updated every five minutes While
the curves shown on the site are not brick walls, 40 meter NVIS, with f0F2
at 6 MHz, is unlikely.

Absent rig troubles, 75 Meter NVIS should work very well over the ranges
indicated.

A suitable NVIS antenna is a dipole with the feed point at 15 feet AG and
the dipole ends at 2 feet AG. The length will be a little longer than
468/f, so suggest you cut it a little longer then tweak it on. The
bandwidth will be about 100 Khz. The feed point Impedance will vary with
ground conditions - but usually very close to 50 ohms - no tuner required.
The impedance will go up, if the ends are raised.

W.Wa. Section Mgr., N7NVP and I have developed a series of NVIS designs
which we presented at the NW Comm. Academy. Ed and I have used these
antennas extensively. I my case, and I'm sure Ed's experience is the same,
coverage is solid from the antenna out in a circle with a -3db radius of 600
miles - a little farther at night. A CD, containing Power Point
Presentations of some of our NVIS antenna work, is available via the ARRL.

Vertical antennas are hard to achieve at 75, Some loading has to be used,
to keep them reasonably short, and an efficient ground is very difficult.

Ground Wave --- with the stations only two miles apart, ground wave with
just about anything made of wire for an antenna and 100 watts, should
produce a readable signal on 75 or 160. 

In the 1980's we ran a social net on ten meters with members checking in at
distances 30 or so miles. We had antenna parties where we built 10 meter J
poles. When we all got the J Poles up, we had solid copy every night. So,
maybe 10 meters and a simple J Pole antenna would get it done. 

Finally, Mark Yordy, W7BBO, and I played with a full wave loop on 160 lying
flat on the grd. It was 20 or 30 feet too long from 468/f. But, after we
got it on 1.965, we got excellent reports all over the NW with a 100 watt
mobile rig.

I hope these thought are helpful.

73 de Tom (W6QJI)

tjsand  wavecable.com

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