Date: Tue, 07 Dec 2010 09:48:14 -0800
From: Kevin Normoyle <knormoyle@surfnetusa.com>
Subject: Re: [TowerTalk] Fw: who's "right" (a tuner question)
Interesting. w8ji also mentions that a bias misadjustment may cause excessive
harmonics and mentions the problem Sam refers to (frequency selective
loads..i.e. high Q). He says:
"Quiescent current (bias) in the RF amplifier section is adjustable. This
adjustment directly affects output signal harmonic content. Harmonics are worse
with low supply voltages, and with low impedance loads./Be sure you check the
harmonics as outlined below, with a 1/4 wl open-circuit stub!!/
Excessive harmonics can cause severe errors in measurement of
frequency-selective loads, even when dummy-load SWR tests appear perfect. Loads
most sensitive to harmonic-induced errors include, but are not limited to,
antenna tuners, tank circuits, very short resonant antennas, and distance to
fault and stub length measurements. If you notice something "funny" going on
with a stub measurement, it may be a fault of incorrect bias."
Tom wrote up a method for checking harmonics/bias at:
http://www.w8ji.com/mfj-259b_calibration.htm
-kevin
## excellent points. Part of the problem is..the MFJ is a microwatt output
instrument. In the old days, we would run a 200' long PTT control line to
the
ant...and just key the XCVR with a switch. [then stuff 10-50 watts CXR into the
ant....and measure swr / reflected power] The PTT control line can be made
of anything,
including a few extension cords plugged into each other, nose to tail. Of
course, the
vfo was on one freq, but that was good enough when tweaking a hair pin or
gamma match.
Jim VE7RF
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