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Author Topic: MFJ259B and a manual coil antenna  (Read 852 times)
KF7NUA
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Posts: 153




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« on: January 20, 2013, 09:53:24 AM »

When using a MFJ259B to adjust an antenna, specifically a mobile antenna that has an adjustable coil but not motorized, adjustment is by hand. To analyze a freq as in 14.200 do I adjust the coil to achieve lowest SWR without worrying about R and X or is there any other precautions or steps needed.  Huh
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WB6BYU
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Posts: 13005




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« Reply #1 on: January 20, 2013, 01:52:26 PM »

Adjust the antenna for minimum SWR at the desired frequency.

The analyzer is convenient in that you can tune up and down the band to see where
it is currently resonant, and that will give you an idea of which way to adjust it.

If the analyzer tells you the sign of the reactance (+/- X) then you can use that to
indicate the direction of adjustment, aiming for X = 0.  But this only works when the
analyzer is mounted RIGHT AT the feedpoint, or when you have a carefully-cut half
wavelength (or integer multiple thereof) between the feedpoint and the analyzer.
Because such antennas are likely to shift frequency due to a body standing nearby
(added capacitance), putting the analyzer right at the feedpoint isn't always
convenient.  (I remember laying under my Dad's car to try to keep from affecting the
antenna tuning while making adjustments.)

Or, if you know the actual coax length, you can use VK1OD's handy online calculator
to calculate the actual feedpoint impedance from the measured impedance and
coax length:

http://vk1od.net/calc/tl/tllc.php


But the simplest approach is to simply tune for minimum SWR at the desired frequency,
which probably means adjusting the coil, then moving away from the antenna while
taking measurements so your body doesn't affect it.  Tune the analyzer through its range
to find the current point of minimum SWR:  if that is too low, shorten the antenna.  If it
is too high, lengthen it (or add more coil.)

Don't expect a perfect match.

If you have a shunt coil across the feedpoint for impedance matching, then adjust both
the shunt coil and the loading coil alternately for minimum SWR.
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KF7NUA
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Posts: 153




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« Reply #2 on: January 21, 2013, 06:28:58 AM »

Dale - Thanks for the reply, I was doing it right, but I had a doubt and needed a confirmation.
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