Re: NEC-LIST: Wideband HF Antennas and Propagation

Michael:

Wideband HF was first studied and implemented successfully, to my
knowledge, by the MITRE Corp. in Bedford, MA. They established a test
link between a site in Florida and Bedford and published some
results. A figure of merit was the minimum power needed to close the
link vs time. Others have also implemented this technology, including
Raytheon and Harris. I am not sure of the current status of these
developments, but they all have to contend with the antenna and
propagation problems you mentioned.

In general, antenna characteristics dont change rapidly enough over 1
MHz to be much of a problem. But it is a good idea to start with
antennas that are relatively broadband. Such antenna problems are
greatly eased by using the travelling wave antennas, like the vertical
half rhombic, the sloping V and the Beverage. These antennas are
usually several wavelengths long at the operating frequency and
therefore their characteristics change slowly with frequency across
the band of the wideband emission (e.g., 1 MHz or so). For example,
the vertical half rhombic's gain decreases and its main lobe lifts to
higher elevation angles as the frequency is lowered. But this usually
is what you want as you try to match the pattern to the propagation
for changes of operating frequency with time of day. The dispersion of
amplitude or phase over 1 MHz is not a problem for such antennas.

The propagation, on the other hand, can be a problem--especially if
the 1 MHz in use at any one time is centered around a nominal
frequency where the path characteristics are changing within this
bandwidth. Part of this can be mitigated by appropriate selection of
the operating frequency relative to the part of the spectrum
supporting the path at any given time. This is a "real-time frequency
management" problem, when one tries to find the "sweet spot" in the
propagating spectrum. But assuming that you have a decent nominal
center frequency, the dispersion in the 1-MHz band of interest can be
corrected for by proper design of the modem. Phil Bello did some work
on this design problem for MITRE about 10 years ago, and I recall that
he published some of his results. I dont have a citation, but it may
have been in the IEEE MILCOM series.

There was an effort at the NTIA Institute for Telecommunication
Sciences (ITS), Boulder, CO, on modeling wideband HF propagation and
noise. Jim Hoffmeyer and John Lemmon were the principal investigators,
and they also published some papers on this subject. Their goal was to
develop a simulator for wideband HF analogous to the narrow band HF
simulator developed at ITS many years before by Clark Watterson. They
made good progress on this. Jim has now retired, but John is still at
ITS working on different topics. He contributed a paper on wideband HF
noise to the U.S. URSI Commission E session I organized at the U.S.
National Radio Science Meeting (USNC/URSI) held in Boulder last month
to honor my former colleague, Dr. A. Donald Spaulding (who made many
contributions to the study of radio noise).

I hope this answers your questions and gives you a few leads in the
event that you want to follow this up further.

Regards,

George
Received on Tue Feb 15 2000 - 21:52:37 EST