[600MRG] LF / MF Impedance Measurement continued

[Frank Lotito]

Most definitely, if you are looking for a way to spend this year's investment disbursement, purchase an impedance bridge. Go for it!  I agree, a so-so modification is far less valuable that spending a few extra dollars for and instrument where the ability you need comes directly from the factory.  However, if you want a quick and simple method requiring almost zero investment and almost no maintenance, try the "dark ages" approach alla EE Lab 202.  A Wheatstone Bridge /  a reasonably clean RF signal generator / X-Y scope Lissajous 
method of impedance matching / measurement.  For most of us, knowing our LF / MF antenna's impedance in R + jX terms is of little importance.  What we ultimately want is a "match," in other words, SWR=1.  To boot, this method is at flea power, maybe a tad more than laboratory grade $$$ impedance analyzers.  No license required.  

Building the bridge is a simple as obtaining 4 reasonably matched 47 or 51 ohm resistors (72 ohm if you prefer 72 systems), an X-Y scope, a 100 ohm carbon potentiometer and a capacitor substitution box (or an assortment of capacitors and a large multi-ganged broadcast receiver variable cap and a few clip leads.)   At the below the broadcast band freqs clip leads / wood baseboards are more than adequate.  If our signal generator is loaded with harmonic output, a 5 or 7 pole low-pass filter will clean it up enough so harmonic content does not befuddle the measurement.  

The output, as you know will be a straight line inclined at 45 degrees if the impedance result is 47 + j0, 51 +j0, or whatever characteristic impedance we chose for our bridge - when we test the 4th resistor, or a perfectly matched antenna system.  If we see an inclined ellipse whose axis is not 45 degrees, our impedance is not the desired characteristic impedance.  It is something different plus a reactive component.  Using our potentiometer in series with the capacitor substitution box at the bridge's "unknown terminals" we can determine both the reactive and resistive component if we want to know the antenna's impedance by varying R and C until we get an identical ellipse.  OR - if we replace the bridge resistor opposite the unknown terminals with the potentiometer / capacitor box and leave the antenna connected, we can vary R and C until we get an straight line inclined at 45 degrees.   

If we have the antenna's matching circuit / variometer installed we can use the trial and error method to obtain a 45 degree inclined straight line.  Here, a "picture is worth a 1000 words," as is, a computer generated list of R and X values.  

73 Frank K3DZ / WH2XHA