HP & Marconi microwave power meters
Adrian Godwin
a blown measurement head : I understand this is a common problem.
A replacement is pretty expensive : just the element is #238 from Marconi,
and a second-hand head about #100. Very few seem to appear on the amateur
market.
However, the heads for the HP meter (HP432A ??) are much more easily
available - I've seen guaranteed ones for #25 and unknown ones for #5.
Of course, the meters themselves (and the head cables) are pretty hard
to find ..
Are the actual sensor elements in these heads similar ? I want to know if
I can adapt the HP heads to suit my Marconi meter (perhaps with some
changes to the meter), either by building an interface box or by
transplanting the measurement cavity into my broken power head.
The Marconi heads are nominally 200 ohm, with a thermistor for temperature
compensation and an efficency correction pot mounted on the head itself -
I appreciate that the HP meters have the correction control on the meter.
Great accuracy is not particularly important - I could probably get
access to equipment to perform some sort of calibration and in any case
I'd find relative measurements alone useful if the absolute measurement
was OK to an order of magnitude.
I'd also appreciate comments on how these things work : they're usually
referred to as thermistor heads, so I assumed that the meter actually
measures resistance. However, the manual indicates that it measures voltage,
and there's no current source to the sensor, just a small offset voltage
that's used for zero setting. Is the same true for the HP meter ?
-adrian
(# above is used to represent pounds sterling, about $1.50).