[SOC] measuring load impedance: how?

[JMcAulay]

At 02:12 PM 05/03/2003 -0700, Lloyd Lachow wrote:
>   How does one measure the impedance of an antenna,
>or any other load, for that matter?
>

Lots of ways, a couple of which have been presented so far.  The Z-Bridge
of many years ago (Knight was a popular one) -- I don't think anyone builds
one any longer -- is a genuine Impedance bridge, providing a reading of
antenna Z when the bridge was balanced.  More useful is the so-called
"noise bridge" (Palomar Engineering, MFJ, and others -- also, building your
own is not a horrible task).  The noise bridge will give separate
indications of resistance and reactance of the load.  Although the accuracy
is pretty limited, it is good enough for most ham uses.  The MFJ Antenna
Analyzer has been reported favorably by many, but I've never used one.  HP
makes an Impedance Analyzer which can do a fine job, but it's very costly.
General Radio produced at least four RF Bridges, models 916, 916A, 1606,
and 916AL (for low frequencies).  These must be used with outboard RF
generators and detectors, but their accuracy of both resistance and
reactance measurement is outstanding.  The GR Bridges occasionally can be
found used for a hundred dollars and up, mostly up for one in reasonable
condition.  All of these are for testing only and cannot be used in-line
with more than a small amout of power applied.

The paper referred by Paul mentioned in-line VSWR meters, which measure
only Voltage Standing Wave Ratio.  Thus, they will only indicate whether a
good match to a predetermined non-reactive resistance exists (almost always
50 ohms).  But for many uses, that's good enough.  They are inexpensive,
and likely millions were made during the CB craze years ago.  Plenty of
these are available for a few dollars.  While they won't tell you much,
they will definitely indicate how good is your match to a load.  Depending
on the power level for which they are built, these can be used under full
transmitter power.  Many of the less sensitive CB units are not
satisfactory for QRP use, however.  

Delta Electronics makes a unit called an "Operating Impedance Bridge," a
piece of equipment which can be used under power which will provide direct
readings of load resistance and reactance.  Its principle of operation is
similar to that of the CB-type VSWR indicators, except that it has variable
reference elements so that balance is not limited to a single reference
impedance.  When one of these is available, rarely, it will go for several
hundred dollars.

Most of the full=power units, bridges or SWR meters, use a square
channel-type line with a sampling line parallel to the "hot" center
conductor for sampling standing waves.  Because the channel is pretty
short, sensitivity decreases at lower frequencies.  Improvement of LF
sensitivity can be made by lenthening the sampling section.  This can be
done with a large sheet-metal structure.  I made one of those years ago for
use at 150 kHz.  Steve Kershner, inventor of the Delta Bridge, was very
helpful with suggestions.  Alternatively, it may be easier to use a piece
of coax cable with a sampling line run between the coax dielectric and the
braid.  That can be made as long as needed.  

At lower frequencies, such as AC line or audio, other equipment will do the
job.  Both HP and GR equipment made equipment for such purposes, and
Heathkit offered impedance bridges for such measurements years ago.  eBay
is a sure source for this stuff.

73
John WA6QPL  SOC 263