[TowerTalk] Fw: who's "right" (a tuner question)

[Jim Thomson]

Date: Tue, 07 Dec 2010 09:48:14 -0800
From: Kevin Normoyle <knormoyle at 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