makes sense to me, but that is not talking about 'antenna' reciprocity, that
is the whole system. the antenna ends where it attaches to the feed line.

"Antonio Vernucci" wrote in message
...
While reasoning on antenna systems reciprocity (i.e. same gain / losses
for transmit and receive), a case occurred to me which, at first glance,
does not look to be reciprocal (whilst it obviously is in reality). I
consider that case fairly instructive, and I then propose it to you for
your comments.

Let us consider:
- an antenna having a resistance of 150 ohm (no reactance).
- a coaxial cable having 50-ohm characteristic impedance and a nominal
attenuation of say 2dB. For calculations simplicity, let us assume that
its length is a multiple of half wavelengths.
- a transceiver having 50-ohm impedance (both RX and TX).

During receive the SWR on the cable will be 1:1 (the RX impedance matches
the cable impedance), and cable attenuation will then stay at nominal
(i.e. 2 dB).

During transmit, the antenna impedance mismatch will cause an SWR that
varies along the cable, ranging from 3:1 (at the antenna-end) to about
1.9:1 (at the TX-end), and the TX will so see an impedance of about 95 ohm
(or 26.3 ohm if you will). Due to the SWR, the cable will then show an
extra attenuation that does not occur during receive. The ARRL Handbook
graphs quantify such extra attenuation as about 0.8 dB.

The different cable attenuation between receive and transmit may
erroneously lead someone to considering the system to be non reciprocal.

My simple explanation of why the system is instead reciprocal is the
following:

- during receive the 150-ohm source (i.e. the antenna) which sees a 50-ohm
load will be subjected to a mismatch loss that can be easily calculated to
be about 1.25 dB.

- during transmit the 50-ohm source (i.e. the TX) which sees a 95-ohm load
will also be subjected to a mismatch loss but of lower value, that can be
easily calculated to be about 0.45 dB.

The mismatch loss difference, i.e. (1.25 - 0.45), is perfectly compensated
for by the extra cable attenuation of 0.8 dB. This witnesses why the
system is reciprocal (even having based calculations on a quite simplified
model of reality).

73

Tony I0JX, Rome Italy