"Roy Lewallen" wrote in message
...
Peter wrote:
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.

I have one and find it extremely handy for a variety of tasks such as
measuring the length or velocity factor of coax lines or the impedance of
ferrite cores. But ironically, the one thing it's almost completely
useless for, at my QTH, is analyzing antennas. Induced fields from TV, FM,
and AM broadcast stations are so high that the MFJ isn't able to detect
its own signal. This isn't unique to the MFJ -- I've had the same problem
with a very expensive HP digital impedance meter, and revert to my old GR
bridge with a tuned receiver for a detector when I want to actually
measure an antenna's impedance. For me the MFJ is worthwhile just for the
other functions it provides, but it might or might not be for you. Just
keep in mind that, depending on your QTH, it might not be able to actually
measure antenna impedances.


Roy Lewallen, W7EL

Roy

All comments noted. I do in fact live within 2km of an AM transmitter
although I believe it's not that high powered and I also have an FM
transmitter within 5km, therefore I'm interested in the details of your
location in relation to the various transmitters.

Regards
--
Peter VK6YSF

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

Peter wrote:

Roy

All comments noted. I do in fact live within 2km of an AM transmitter
although I believe it's not that high powered and I also have an FM
transmitter within 5km, therefore I'm interested in the details of your
location in relation to the various transmitters.

Regards

Roughly 10 - 15 miles from here is a ridge about 800 feet higher than my
location. On top are virtually all the TV and FM, and a number of the
AM, transmitters for the Portland Oregon metropolitan area. A single
strong transmitter is likely to be enough to confuse an antenna
analyzer, though. Filtering is possible, of course, but you have to take
care to design filters to have minimal effect on measured impedances
while still filtering effectively. This can sometimes be challenging
depending on the strength of the interfering signal and how far it is
from the measurement frequency.

I also have to be careful when working on audio circuitry -- on several
occasions I've tracked down "hum" as demodulated TV video, and I end up
having to include VHF decoupling and shielding to audio circuits. Don't
know much about the nature of HDTV signals, but it might not be so bad
if it doesn't have as much energy in the AF range when demodulated. It's
bound to drive untuned antenna analyzers bonkers, though.

Roy Lewallen, W7EL