******* Copied from uk.radio.amateur newsgroup. ********

Reg spoke of the SWR meter as a resistance bridge. It is possible to
build a
meter that is a bridge
===================================

The so-called SWR meter is ALWAYS a resistance bridge (although it may
be described in other terms).

In the HF, 100 watt models, the little ferrite ring is a current
transformer. The turns ratio on the transformer allows the bridge
resistors to be changed to values other than 50-ohms.

For example a bridge resistor which is across the transmitter changes
to 5000 ohms. And a bridge resistor in series with the load changes
to 0.5 ohms. In both cases the power lost in the resistors falls to
the order of 1% of the power which would be lost in 50 ohm resistors.

The meter becomes far more power efficient. With 50-ohm bridge ratio
arms the power lost in the bridge would be 75 percent of Tx power
output.

The 0.5-ohm resistor does not exist. Very cleverly, the input
resistance of the one turn primary winding on the current transformer
becomes the 0.5-ohm bridge arm.

The DC seperation between primary and secondary windings on the
current transformer allows the diode rectifier and moving coil DC
microameter circuit to be operated very nicely all at ground
potential.
----
Reg, G4FGQ

The term 'meter' is incorrect. It does not 'measure' anything. It
merely 'indicates'.