[TowerTalk] MFJ 259 accuracy??

[John Brosnahan]

At 08:46 PM 10/25/98 EST, AA4NC@aol.com wrote:
>
>I have a question about the low frequency accuracy of the MFJ 259 antenna
>analyzer. I spent CQWW weekend doing some antenna work while KS4XG   did a
15m
>SOSB effort from here. Since the station was in use, I used my MFJ box to do
>the testing on a 160m inverted vee I was building. I spent most of an
>afternoon doing the up/down clip/add, etc. number and could never really get
>the antenna in the ballpark according to the 259. The best I got was a 2.5:1
>SWR dip at 1810 with the Z reading 125 ohms. The bandwidth was extremely
>narrow. It went off scale on both sides within 20 Khz. I finally gave up on
>the antenna, thinking there must be a bad connection or bad coax run. 
>
>After the contest was over, I had a chance to look at this antenna in the
>shack. Two Autek WM1 wattmeters and the internal wattmeters in an FT990 and
>IC736 showed the antenna to be quite resonant! The SWR curve looked
reasonable
>for an inv. vee with a 1:1 dip at 1830 and about 100 Khz bandwidth. I now
>suspect that I wasted my time all afternoon - the antenna was probably
fine as
>originally installed. I was just being fooled by problems in the MFJ 259. 
>
>I haven't had the 259 very long, but until now it seemed to track quite
>accurately with my other meters. I looked at some other antennas with it
>today, and everything looked fine. It's just this 160 inv. vee that it can't
>deal with. 
>
>Has anybody had any similar problems with their 259s ??


A common problem with all of the various analyzers out there.  Most likely
overload of the detector from a nearby AM station.  The AM RF just
biases the diodes causing a higher reading that what is real and what
is measured by an SWR bridge designed for higher power levels--such as
used in your TX.

Just a limitation of the technique of a low power device when used
with big antennas near large RF fields.  (My MFJ does this when
measuring a 4L 40M KLM Yagi because of a 5KW AM station 10 miles 
away.  How can I tell it is the AM station?  The MFJ meter bounces in
time to the music!)

The MFJ and others work well with some sort of tuned match where
the AM energy is reduced by the matching network (such as a Tee or
gamma match) but if there is no matching network (such as the dual
driven feed on the 4L 40M KLMs) then you can have problems.

73  John  W0UN


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