NEC-LIST: Definition of Gain

Dear George,

I must tend to agree with Hal Smith on this issue. Mis-match and
polarisation losses should be kept separate from unqualified gain
values and taken account of separately in the system budget.
Otherwise it is impossible to tell how much improvement can be had
from tweaking the match or re-aligning the antenna.

Mismatch loss

The discussion may seem a little academic if we only think of, say, a
horn with a 50 ohm coax flange as the input interface, but we need to
consider other situations.

If you include mismatch loss, what do you take as the reference
characteristic impedance of the feed? For example, consider a
resonant folded wire dipole, with say, an input impedance of 270+j0
ohms, fed from an open wire line. If I feed from an open wire line
with a characteristic impedance of 300 ohm, I have a mismatch loss. I
can, however, use a 270 ohm line, with no mismatch loss. Then, if the
transmitter has an output impedance of 300 ohm, in the latter case the
mismatch occurs there. Surely, if in the first case, we include the
mismatch in the antenna gain, in the second case, we should include it
in the transmitter efficiency, which is clearly daft.

My personal preference is always to qualify the gain, For example, if
I had a horn with a WG input and I wanted to include the mismatch loss
in the gain, I would quote the figure as "Gain referred to the
waveguide input flange." Likewise, if I wanted to exclude both
mismatch and ohmic loss, I would quote "Gain referred to the free
space interface", which is in fact the directivity.

Polarisation Loss

If you include polarisation loss in the gain, what happens when you
have mobile antennas (particularly on aircraft), where the
polarisation loss can vary according to platform orientation?

Tx and Rx gain

I think everyone is agreed that the transmit and receive gains cannot
be anything other than equal.

Antenna Efficiency

Another interesting point is how much of the efficiency is included in
the gain in cases where the former is dependent on the surroundings.
For example, consider a typical MF broadcast antenna, consisting of a
base fed vertical mast radiator. The efficiency is dependent on the
ground losses which are in turn dependent on the conductivity of the
earth. The designer has only limited control over this, by employing
an earth mat. The matter is complicated by the fact that the
propagation loss is also dependent on ground conductivity. Where does
the antenna gain 'stop' and the propagation loss 'start'?

It is usual to think in terms of directive gain and make a separate
allowance for all of the various losses, including those in the
ground, the conductors, the dielectrics, the antenna matching unit and
the feeders.

Regards,

Paul Carlier, BSc, FIEE
Consultant
FanField Ltd
Braxted Park
Witham
CM8 3XB
England
+44 1621 893500
pcarlier_at_fanfield.co.uk
http://www.fanfield.co.uk
Received on Tue Mar 07 2000 - 04:27:36 EST