Peter G3RZP writes:
>A point to watch is the variety of vector impedance meter. The network
>analyser approach uses directional couplers to measure amplitude and
phase
>of the reflection in a 50 ohm system. If you measure a 2000 ohm load,
quite
>large changes in the load have relatively small changes in return loss,
so
>the accuracy goes to hell in a handbasket. Thus depending on the way in
>which the box works, it might be a waste of time. If you use a vector
>voltmeter, that's a different approach - and it needs a signal source.
Peter's comments are correct regarding network analyzers used to measure
impedance. A network analyzer measures the ratio between reflected and
forward power, and therefore is optimized for measuring how close to 50
ohms a given widget may be. Far from 50 ohms, and the accuracy
suffers. To be within a useful amount, the general rule is that you
should not attempt to measure an impedance who's VSWR with respect to 50
ohms is more than 10:1. 2000 ohms is much more than 10:1.
Fortunately, most instruments which are labelled "vector impedance
meter" are not based on directional couplers and ratio measurements.
The HP 4815A, available sometimes surplus, although I have yet to find
one for a pleasant price, actually has both a voltage and current
sensing transformer in the probe head. That is how most such impedance
measuring instruments worked until the early 1980s when they switched to
an auto-balancing bridge configuration. In either case, a true
impedance meter can provide accurate results over a wide range of
impedances.
Peter also makes good points on the values of the network's capacitors.
Merely demonstrating that you have achieved the correct load at the
operating frequency is necessary, but insufficient. You also must
demonstrate that you have the proper Q value in the network, and
assessing the capacitor values is a reliable way to do so.
73
Dave W8NF
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|