NEC-LIST: Definition of Gain

Hi

Definition of Antenna Gain

I would like to suggest some factors to consider in defining antenna
gain:

1) In order to win lucrative defence contracts manufacturers pour
hundreds of thousands of dollars into development of transmitters
compliant to the last fraction of a dB with the appropriate standards.
At HF for instance antennas have to cover the range from 1.6 to 30
MHz, sometimes with a tuning unit, sometimes by use of a wideband
antenna such as a conical monoole or log periodic. In this case such
standards to be met might be be MIL-STD-188-141B, or DEF-STAN-1045
which stipulate that the transmitter should deliver full power into a
VSWR of 1.3:1, thereafter allowing the power to be derated inversley
with VSWR. Other standards call for the transmitter to be fitted with
a line flattener to tweak the final antenna mismatch at the
transmitter end. From this veiwpoint the definition for antenna gain
should assume that the transmitter will be perfectly matched, and
should assume that the mismatch loss is accounted for separately.

2) The concept of free space gain is limited to antenna installations
offering path clearance, or ground/air or satcom applications. At
lower frequencies and antenna heights the received signal is the
combination of direct and reflected rays, and power gain only has any
meaning in the context of height above ground and groundconstants. I
therefore concurr with the objectives laid down by Jerry Burke in NEC2
and NEC4, which provide an "Average Gain", which I have found to be
both meaningful and useful. This parameter could also be used to
define an efficiency parameter which accounts for the ground
reflection losses.

3) For groundwave propagation gain has no real meaning. A parameter
is required which is consistent with Recomendation ITU-R P.368, which
provides ground loss curves generated by the associated ITU GRWAVE
program. I have tried doing this by comparing the fieldstrength at
ground level predicted using GRWAVE with the NEC2 prediction of field
above the ground at the same ranges. I have had limited success using
NEC2, but beleive a suitable parameter could be defined which is
consistent with the design objectives of NEC4.

4) Broadcasters & Maritime Radio Engineers may need to predict the
extent of the midnight fading zone. Broadcast administrations have
effectively agreed on Recommendation ITU-R P.1147 for prediction of
the skywave component. This uses the CMF for 1 kW input as a measure
of the antenna gain, which can easily be derived from power gain. I
therefore suggest that CMF for 1 kW matched input is used to define
the "gain" for groundwave output.

5) The concept of dipole moment recurs so frequently in papers that
this should be re-defined at the same time alongside the antenna gain.

Phil Ede
Radio Consulting Engineer

Phil Ede

148 Eldred Avenue
Westdene
Brighton
BN1 5EJ
United Kingdom

Telephone number International ..44 1273 552568
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email philede_at_fastnet.co.UK

For an illuminated CV see http://home.fastnet.co.uk/philede/cv_2000.htm
Received on Tue Mar 07 2000 - 04:26:50 EST