[Antennas] Return Loss Bridge Construction

2002年5月25日 17:26:13 -0400

de WB2CPN South Central Pennsylvania 2002年05月25日
So sorry, but this old telephone company man (retired by AT&T) would 
like to offer the real application of the term "return loss". It's
the loss, (attenuation), of the audio signal as measured in DB between
the 4-wire sides of a 4-wire to 2-wire terminating hybrid, plus any other
loss in the 4-wire circuit. If it was the perfect hybrid and perfectly
balanced, the loss between the receive side and the transmit side would
be infinate. In the real world some of the power which goes into the
receive side will come back out on the transmit side. No balance network
for the 2-wire side is anywhere close to perfect. Why care? Well, 4-wire
telephone carrier systems are engineered to have 23 DB gain. Now if the
return loss is less than that the circuit breaks into oscillation, i.e.
"sings". There is "Singing Point Test Set" that has a variable-gain
amplifier it it. When the test set is placed into the circuit and the
gain of the ampilifier is brought up to where the circuit "sings" we now
know the DB value of the return loss, or DB value of the attenuation across
the hybrid and other losses in the circuit. 
Now you can see where this was adapted to measuring the power going into
a circuit such as an antenna, and measuring the power being reflected
back. That can be expressed as Return Loss, not just VSWR, and often is
because it includes the loss of the transmission line.
Related note: If you're going to build a VSWR meter, you can give it a 
quick check by turning it around in the transmission line and see if the
new reading is a mirror of the first reading.
And, in case anyone asks, the term DB is a ratio; the ratio of P1/P2,
or V1/V2, or I1/I2. The reference can be just about anything; even weird
things like "Dyne per Square Centimeter".
 73 Clete