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