John:
Thanks for the reminder. In fact, I have a copy of the book
you mentioned, but it had slipped far down in the reading
pile. I skimmed it, quite late last night, and will start
reading it. It looks very good. Among the points I noticed
skimming last night...
- the author says that, based on reports from many stations
over a number of years, low power stations can confidently
expect to succeed with NVIS; the rub is that "low power" is
400w <g> and he says operators of 20w backpack radios often
have great difficulty...
My best recollection is the experiment that was undertaken by one of
the authors (Fielder, I think) with a point-to-point communications
in south-east Asia (Thailand, I think). The link was established in
mountainous jungle terrain over a distance of 70 miles, testing three
antennae - a vertical, a half-wave sloper and a dipole at quarter-
wave heigth. The only antenna that was consistently reliable over a
24 hour period was the dipole, then the sloper and the vertical was
only good for about four hours of communications. The diagram
showing message error rate versus time of day for all the antennae
knocked my socks off! Clearly, pactor/PSK-31 communications should
be considered in these instances. It was interesting to note that
they were using 20 watt radioes....
I must have missed that assertion that low power was in the 400 watt
range - that could be more of the peculiar military viewpoint - kind
of like the viewpoint that if one asprin will make my headache go
away, a whole bottle will do so much more quickly....
But I recall one article where he discussed the advantages of NVIS
but from mostly from a military perspective. For example, Feilder
asserted that radiodetermination of a station using NVIS is much more
difficult because of the near-vertical take-off of the transmitted
signal producing an omni-directional signal. Additionally, I am sure
he also mentioned that this characteristic is compounded by the use
of low power, in the 10 to 20 watt range, HF radioes. Looks like I
will have to re-read the book (for the fifth time!).
- the AS2259-type antenna that Keith N1XTK and I were using
does indeed, as I had recalled, rate pretty low compared to
other types
On the other hand, they are simple to erect and take down. On the
other hand, I bet that it you were to double the height of the
supporting pole, the results would be improved.
I had thought of building one of those antennae but substituting twin-
lead for the coaxial line, and using my Z-11. But since I can as
easily tune a balanced fed dipole, I just didn't see the point.
However, maybe a case could be made for a manual tuner. What do you
think?
- the author(s) suggest that the best antenna for NVIS may
in fact be ye goode olde dipole (or , if the angle is not
too great, an inverted Vee) at low fractions to the ground,
possibly with a reflector below at a measured distance.
Yes, I think that was the message that I had picked up. Indeed, go
to the University of Hawaii Amateur Radio Club site and take a look
at their recommendations with respect to using a three-wire reflector
underneath the dipole.
If you compare my main antenna to their recommendations, on
40 and 80 it pretty much fills the bill (with the exception
of the reflector element).
Again, your situation gave greater creedance to Feilder's and
Farmer's views and suggestions. It is this reason that in my
opinion, your experiments and their results (including the failure on
80 metres) were of greater import than perhaps you had first
realized.
This reminds me of an
observation in an article I read recently on NVIS for packet
radio, in which someone was quoted as saying that there are
a whole lot more NVIS antennas out there than most people
suppose!
I can think of one - a horizontal half-wave radiator located one-
quater above ground, with a three wire reflector, as recommended by
our friends in Hawaii. The half-wave would be connected onto one
side of a quarter-wave balanced line shorted stub, fed with a balun
closer to the shorted end and then an undetermined length of coaxial
cable back to the station. The location of the balun on the stub
will be determined by SWR measurments up and down the stub. One
would notice that the ratio of the balun would determine the relative
distance from the short, i.e. a 1:1 balun will be located closer to
the short than a 4:1 balun. The baluns can be of any type, either
coaxial (4:1 only), or ferrite/toroidal (4:1 or 1:1). If anyone has
difficulty in understanding this arrangement, think of a 2 meter J-
pole, that is oriented from the vertical to the horizontal, then bend
the bottom part of the "J" hanging down vertically, and then scale it
for 40 metres or 80 metres.
So, more to read -- this book looks like an engaging and
profitable read, so thanks to Jim VE3XJ!
I am fortunate that I have five 35-foot military surplus pop-up masts
to simultaneously support many NVIS antennas in my rear yard. I have
a few ideas to try and perhaps you and I and maybe others in this
part of the world can experiment on 40 and 80 metres over the
summer. I did a quick check with Mapblast, and I see we are not
quite 500 mile away via roads, so we are closer, I am sure, as the
crow flies. I don't know if I can get anything up for this Saturday,
but I sure would like to try. So let's keep in touch via e-mail.
Have you any interest in trying to maintain a pactor link? I don't
have any means of undertaking a portable/mobile PSK-31, and I am
couldn't afford the Kantronics box!
73
Jim, VE3XJ
