Reg Edwards wrote:
Cec, you have YOUR explanation and I have MY explanation.

Mine is a lot simpler. The Heath HM-15 has two pickup elements.
If you install a Z0 resistor load at one end it "picks up" the forward
wave. If you install a Z0 resistor load at the other end it "picks up"
the reflected wave. The two pickup voltages are rectified and compared
through a calibration procedure.

The parts that came with the HM-15 kit in the 50s-60s included two
72 ohm resistors. RG-ll was very popular at the time. If one wanted
a 72 ohm SWR meter, one installed the 72 ohm resistors. If one wanted
a 50 ohm SWR meter, one installed the 50 ohm resistors. A switch could
be installed that switched between 50 ohms and 72 ohms calibration.

This serious ambiguity also applies to your weird contraption. ;o)

Actually, the Heathkit design concept is easier to understand than
is the bridge explanation or the toroid-pickup/phasor-addition
explanation. The first SWR meter I built in the 50s, used two
lengths of insulated wire shoved under the braid of the coax.
It worked but, at the time, I had no idea why it worked. Heath's
little slotted line pickup device was pretty slick. I sometimes
see them for sale at hamfests.
--
73, Cecil http://www.qsl.net/w5dxp