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[TowerTalk] Meas resfreq with MFJ Analyzers (kinda long)

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Subject: [TowerTalk] Meas resfreq with MFJ Analyzers (kinda long)
From: (David J. Windisch)
Date: Sun, 9 Sep 2001 09:37:51 -0400
Hi, all concerned:

Hope this helps some one out there in radioland.

For some time now, I've been using an MFJ Antenna Analyzer (MFJ-AA) to
measure resonant frequencies and relative Q's of antenna elements and other
LC circuits.

Doesn't matter which MFJ Antenna Analyzer cuz they all work.   Others pls
try these techniques with sundry brands of AA and let us know if the
techniques are transferable.

So, how to couple the MFJ-AA to the circuit in a useful way is the

With lumped-LC, its easy: you put 'em in series with a resistor in the range
of 20 to 100 ohms, and connect the series-combination to the terminals of
the MFJ-AA.  At series-resonance, the MFJ-AA will read

The "dip" at resonance is easy to miss, so tune carefully, and experiment
with R values.   Sharpness of dip give a relative indication of Q.

If you don't like fuddlin' and flubdubbin' on the ground and hanging off the
side of the tower making measurements, the rest is probably only of academic
interest to you.

I make antennas from other antennas.   The current nightmare projects are:
converting a well-oxidized CC A3 3L tri-bander back to something useful, and
replacing the linear loading on a HyGain 402BA (397) 2L 40M yagi, with
lumped-L center-loads.

I needed to couple into the 1-1/8" al tube CC els without cutting 'em in
half, in order to set lengths,  so I sat the els on a wooden stepladder.  I
tight-wrapped a length of insulated hookup wire around the center of each el
as a coupling link, connected one end of the link to the MFJ-AA, and
experimented with # of turns and the series-resistor-value connected between
the other end of the link and the other terminal of the MFJ-AA.  Hot Dang!
I own this one :o)))

The HyGain was a different challenge.   I used EZNEC 3 (tu Roy!) to get an
approximate value of loading-reactance (650 ohms, 13.6 micro-H) for each
element, made the inductors from 1/4-in al tube, starting around 16 turns,
3-3/4" dia., 1/8" t-t spacing, installed 'em, and ended up using a 4T link,
of the same diameter as the loading-coil, in series with a resistor, to
measure the resfreq of each el.   The diffs in resfreqs were about 400 kHz
between near-dirt and 33 ft up, and around 600 kHz between dirt and 90 ft.
I ended up with around 11 turns.   16 turns resonated around 5 MHz.

For those who have read this far, and know some things about HyGain
antennas, care to guess why the final L was so much less than the modelled
L?   (It ain't due to problems with the Roytoy -- it has to do with the
mechanicals of the HyGain ;o))) )

73, Dave, K3BHJ

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