"Owen Duffy" wrote in message
...
"Peter" wrote in
. au:

Hi all

I'm looking at purchasing an MFJ269 antenna analyser and keen to hear
experience of others in this group regarding this or similar analysers.
Appears to be ideal if it is as good as MFJ claim.



Peter,

Effective exploitation of the capability of this type of analyser depends
on a sound understanding of transmission lines, and of the instrument's
own limitation. The availability of these analysers at low cost, and the
perception that they are a magic bullet has lead to a lot of pseudo
technical nonsense being proposed. The magic is more in the user's
capability than the box, and buying the box doesn't buy knowledge and
understanding... but the device can help develop knowledge and
understanding.

I have not used a '269 (though I have extensively used a '259B), but I
suspect that it does not display the sign of X. Estimation of the sign of
X in the '259 and similar instruments is an issue, and confuses many
users. There are propositions that sign of X is easily determined from
the slope of X with frequency at a point... but whilst that is true for
an ideal passive component, it is not true in general.

These instruments are often used in pursuit of the questionable goal of
resonance, and the instrument used to show resonance by observing X=0, or
X approximately zero, or a local minimum for X at some frequency on the
assumption that X changes sign at that point and that resonance of
something is indicated.

I wrote some notes entitled "In pursuit of dipole resonance with an
MFJ259B" at http://vk1od.net/blog/?p=680 that canvasses the behaviour of
the instrument in such an application, and flags the issues in
measurement. You may find them interesting.

Others have raised the issue that these instruments use a broadband
detector, which works fine so long as the internal oscillator has low
harmonic content and dominates the detector. If you let one of these
things time out, the oscillator is shut down, and if you see indication
on the meters, then energy from another source is of sufficient magnitude
to be concerned about the accuracy of measurements.

Owen

Thanks Owen for the information and your notes "In pursuit of dipole
resonance with an MFJ259B"
My intended use for the device is to get a bit of view on what's going on
with my various home brew antennas and matching devices and for my own
curiosity to compare theory with real world.

I must admit I had assumed that the 269 analyzer display the sign of the
reactance, I will have to reread the ad.

Regards


Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm