One can make quite accurate impedance measurements using the "three meter"
approach at very low cost. Essentially, you hook a capacitor (chosen to
have an impedance around 50-100 ohms at the frequency of interest), a
resistor (50 ohms), and your unknown all in series and drive it from an RF
source at the desired frequency (low power... a couple of volts is good).
You then measure the AC voltage across each component, and, with a bit of
math, can turn that into R, X, etc... (this is essentially what the
analyzers are doing).
Ideally, one would like a good detector diode(s) for all this, but, one can
use 1n914 diodes, especially if you have some more drive voltage available.
You also will need some calibration standards (nonreactive resistive load, a
short, and some reactive standards, which can be made from transmission line
of known properties)
This has been written up in the antenna compendia in several of the
volumes.. Modern computers make it MUCH easier...
A RS232 DVM, an RS232 controlled transceiver, and some relays controlled by
the printer port could make the whole thing automated.
If you have a LOT of motivation, you could also make very precise
measurements by using the sixport technique, but calibration is tedious and
the math a bit complex, however, six port networks are the standard by which
other techniques are compared. The NIST web site has a fair amount of
information on this.
----- Original Message -----
From: "Chris BONDE" <ve7hcb@rac.ca>
To: <towertalk@contesting.com>
Sent: Saturday, May 04, 2002 10:43 AM
Subject: Re: [Towertalk] Antenna Analyzer Suggestions?
>
> >The MFJ-269 has a 12bit A/D converter whereas the 259B only used
> >8 bits to digitize the detector voltages. From what I have heard this
gives
> >the 269 more accuracy at high and low values of impedance when
> >compared to the 259 (the 269 over-ranges at a much higher impedance
> >level than the 259).
> Is the MFJ the only analyzer or is it just superior?
> What other measuring devices can be used if the budget does not support
the
> MFJ269 plus stuff. (Close to CA$800 landed) (Or close to 3/4 gross
monthly
> pension)
>
> Chris opr VE7HCB
>
>
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