Owen,

SWR meters with a sampling line.

The only experience I've had has been I once made one for HF. It was
of the type where a second wire is drawn alongside the inner conductor
of a short length of coaxial line of impedance in the same street as
the system it is to work with.

Operating frequencies covered the whole of the HF band. That is a very
wide band. Which indicates that line length plays no part in
measuring accuracy once calibrated.

To explain how the thing works it is necessary to return to what it
really is. It is a resistance bridge. All so-called SWR meters,
whatever the circuit or form of construction, are resistance bridges.

The bridge has 3 internal ratio arms. The 4th arm is the variable
transmitter load. If all 4 arms are of same resistance we have a very
sensitive arrangement suitable for QRP transmitters. However, 3/4 of
the TX power is dissipated in the 3 internal bridge arms.

For higher power transmitters it is necessary to use high ratios for
the ratio arms. In the case of meters which use a little ferrite ring
as a current transformer, a resistor of the order of 30 to 100 ohms
can be shunted across the current transformer secondary winding while
the primary winding has an input resistance of the order of 0.1 ohms
which forms the value of the ratio arm in series with the external
load. This 0.1-ohm arm is capable of carrying the load current of
several amps with only a small power loss.

The other two ratio arms can be a pair of high value resistors in the
same ratio as occurs via the current transformer. If the input
resistance of the current transformer is 0.1 ohms then the bridge
ratio is 50 / 0.1 = 500:1 where 50 ohms is the usual value of the
load resistance when the bridge is balanced and SWR = 1:1

The two high impedance arms can be capacitors in the same ratio of
500:1 which have zero power dissipation but have a minor effect on
accuracy. They introduce a small phase angle into the load as seen by
the transmitter through the meter. The error increases with
increasing frequency.

It will be seen that the take-off point is effectively the same for
both current and voltage.

Returning to the so-called sampling line.

There is a bridge configuration which is not quite so obvious. But
instead of a current transformer the current is picked off by means of
a short length of wire in parallel with the coaxial inner conductor by
virtue of their mutual inductance. The line is too short for
propagation effects to play a significant part.

Voltage is picked off at the same point by virtue of the capacitance
between the wire and coaxial inner conductor. The phase relationship
between volts and amps can be reversed just by reversing the direction
of propagation through the meter.

The bridge ratio is set partially by the ratio of the impedances Zo of
the additional wire and inner coax conductor. The length of coaxial
line affects only the bridge sensitivity and power dissipated in the
meter. As you must be aware, sensitivity falls of fast with decreasing
frequency and 160 meters was my favourite band. So the home-brewed
meter was soon discarded and I returned to ferrite rings.

I was left with the impression it was very easy to make and that
almost anything would work.

Hope you can understand the foregoing.
----
Reg.