Re: NEC-LIST: Mast Radiator Ground losses

>Hi
>
>I have been working with mast radiators for ship-shore distress
>communications by groundwave on 2,182 kHz. For cost reasons the
>height of the antenna and radius of the earth mat may be as short as
>10m (26 degrees)
>
>The very poor ground encountered on one site led me to try to use NEC
>to determine the amount of power absorbed by the ground around the
>antenna. I tried to evaluate the E and H field losses [Watt, "VLF
>Radio Engineering", section 2.4] using the NE and NH commands to
>determine nearfield values over a matrix of points. I feel that this
>should enable me to determine surface impedance and loss per unit
>area, but I am unsure how to manipulate the numbers.
>
>I would be pleased for any suggestions.
>
>Thanks
>
>Phil Ede

Phil,

Electrically short antennas (VLF, LF and HF) is a topic that I have
some considerable experience. I have used Don Watt's method to
estimate ground losses for proposed VLF antennas, but I have no
experience regarding validation. The antenna was not put up.

HF camp/mobile antennas is a subject I have recently explored
(reference Communications Quarterly, Fall 1998, pp. 47-57). The
antenna we used for this study is popularly known as a screw driver
antenna, since the series tuned inductor is tuned by a screw-driver
motor. In fact I have an antenna that will tune to the 160M band. In
our experience, for an antenna height of 26 degrees, one can achieve a
radiation efficiency of about 40-percent. Your radials would be
elevated above ground level, as long as possible (ideally self
resonant) pointing toward the ocean in front of you.

An alternative method could be to employ an umbrella top loaded
antenna with tuned elevated radials (reference Carl E. Smith, "Short
Low Loss AM Antenna", IEEE Trans. on Broadcasting, 35, June 1989, pp.
237-240). I have extensively numerically modelled this antenna
(reference Conference Proceedings, 15th Annual Review of Progress in
Applied Comuutational Electromagnetics, Naval Post Graduate School,
Monterey, CA, 15-20 March, 1999, pp. 463-467). Employing all possible
tricks, such as configuring the tower to act like a shorted
transmission line, to effectively place an inductance between the top
of the tower and the umbrella wires, it should be possible to achieve
radiation efficiencies (for an antenna height of 26 degrees) of 75-90
percent. Again tuned elevated radials.

Regards, Jack

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Wed Mar 15 2000 - 17:31:21 EST