RE: NEC-LIST:Groundwave gain compared to skywave gain

Duncan:

Your attached questions, although simple, are interesting. Here are a few
comments:

1. I dont recall seeing the Litva, et al. report on Beverage antennas, but I
did an analysis of the Beverage for use as an OTH-B antenna some years ago. At
that time, I had NEC-2, and I used a quarter-wave stub to approximate a ground
for the terminating resistor and also for the feed on the input side (at the
transformer). I looked closely at the low-angle gain for skywave as the issue
at the time was whether a long Beverage was better than a rhombic.

I used my XELEDOP to measure a Beverage at Nashua, NH, and reported on the
results of my reference monopole at that site at an ACES meeting (3rd?). The
NEC-2 predictions were very good for describing the low-angle response of both
the 7.5 ft. vertical monopole (see the ACES paper) and also the Beverage. So I
suggest you NEC model the antennas you are considering and use NEC-3 if you
can. If you cannot, then just use the old quarter-wave stub just above ground
to approximate the short to ground. The difference in accuracy between NEC-2
and NEC-3 or higher for the pattern should be negligible. The absolute gain
may be off a little, but not enough for your engineering purposes as I
currently understand them.

2. The gain of the Beverage at low angles will be a strong function of how
poor the soil is from an electromagnetic standpoint. For this reason, do a
parameter sensitivity study on gain at the take-off angle you care about,
using ground constants as the variable.

3. When I measured the Beverage with the XELEDOP, I did vertical lifts at
diferent distance. In that way, I measured the HF surface wave as a function
of distance from the antenna. That surface wave is the culprit from a RADHAZ
for fuel standpoint. In other words, what is the vertically polarized field
strength in the vicinity of where they are going to be doing the fueling.

Bill Vail, then of SRI International, did a study of the hazard from an OTH-B
transmit facility to an area where fueling could take place. This was done for
the Navy's ROTHR program office. You may be able to get a copy of his report
from SRI headquarters in Menlo Park, CA. Bill did a good job of that analysis,
and several aspects of it could be useful to you. I dont know if Bill is a
member of NEC-List or not.

Hope this helps.

Regards,

George

Original Message:
-----------------
From: Prof Duncan Baker dbaker_at_postino.up.ac.za
Date: Thu, 13 Sep 2001 17:27:56 +0200
To: nec-list_at_gweep.ca
Subject: NEC-LIST:Groundwave gain compared to skywave gain

Good day all.

Hopefully these are simple questions.

The report by J Litva et al. [1] contains a diagram (Fig 32 p. 72) of
the measured vertical radiation pattern for a pair of Beverages using
an aircraft, and a balloon. What interests me is that this diagram
and others suggest that the groundwave gain for vertically polarised
antennas may pretty well be the same, or at worst about 3 dB less than
the far field peak gain, where the peak gain is at lowish take-off
angles. Is this a reasonable assumption to make? For example, in
[2] the predicted skywave gain for a quarter wave dipole with 120
ground radials is given as 0 dBi at 2 MHz. What is the corresponding
groundwave gain? Should I use 0 dBi as a worst case, or 3 dB less?

 I am dealing with a case involving the risk of inadvertent ignition
of fuel vapour at a planned filling station close to an existing radio
communications site. Possible antennas include vertical monopoles
with reflectors, Beverages and vertical half rhombics.

Also, does anyone have gain figures from about 2 - 30 MHz for a 600 m
long Beverage, spaced about 3 m above the ground? Many years ago I
started rewriting and compiling a program to do the necessary
calculations, but was unable to complete the task.

Best regards,
Duncan Baker.

[1] J Litva et al., "Beverage Antennas for HF Communications,
Direction Finding and Over-the-Horizon Radars," CRC Report No. 1282,
Department of Communications, Ottawa, August 1976.

[2] JL Thomas and ED DuCharme, "HF Antenna Handbook," CRC Report 1255,
Department of Communications, Ottawa, April, 1974.

--
***************************************************************
Prof. Duncan C. Baker (Pr. Eng., FIEEE, FSAIEE, Sci. Nat.)
Dept. of Electrical Electronic and Computer Eng.,
University of Pretoria, 0002 Pretoria, SOUTH AFRICA.
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