Topband: Recycled ferrites for 160m

[Michael Tope]

Dan Zimmerman N3OX wrote:

>>
>>YES.  One VERY important caution. Antenna analyzers typically have stray
>>input
>>capacitance on the order of 10 pF!  The equivalent circuit of a ferrite
>>choke is
>>a parallel equivalent circuit, and for most chokes the capacitance will be
>>half
>>that value. Thus, the analyzer can introduce considerable error.
>>    
>>
>
>
>Jim, you're talking about the finished choke, right?
>
>Putting  -j11,000 ohms (8pF like my MFJ-259 @ 1.8MHz ) in parallel with
>30+j50 or whatever my test winding shows is basically irrelevant.
>
>If you try to measure the impedance of a finished choke, or sweep for
>resonances like I suggested, there's probably a problem.  It's also a
>problem to multiply excessively: "oh, I have 30+j50 with 1 turn through this
>bead, so if I put 100 turns I'll get 30000+j50000"  ;-)
>
>Then that -j30000 shunt capacitance of your winding matters a lot, and your
>-j10000 of the analyzer even more ;-)
>
>It's a good point and important.
>
>What do you think about signal generator + 1kOhm-5kOhm resistor + high
>impedance high frequency 10x scope probe?  That should be able to do OK from
>a ham-at-home perspective at measuring the impedance in a voltage divider
>setup, don't you think?
>
>
>73
>Dan
>

For measuring high series impedance, I recommend that you keep the 
circuit impedances low. Per the recommendation of K9YC, I would shunt a 
50 ohm resistor across the generator output to eliminate uncertainty 
about the generator output swing (the ~constant 50 ohm load will 
stabilize the generator output voltage). I would also use a relatively 
low impedance for the shunt element in the voltage divider. This will 
reduce errors caused by probe and stray fixture shunt capacitance at 
higher frequencies (and also the phase angle error if the DUT is 
reactive and not resistive).  Then its just a matter of getting enough 
dynamic range out of the scope and keeping the stray fixture C between 
terminals of the DUT as low as possible. For a 10K series DUT, and a 50 
ohm shunt resistor, the voltage ratio will be 1/200.  If you drive the 
DUT with +18dBm (5V pk-pk across 50 ohms), then the divider output would 
be 25mV peak-to-peak. That fine for a SpecA, but a pretty small signal 
for an oscilloscope (-28dBm), so one might need to strike a compromise 
and go with a higher divider shunt impedance.
 
73, Mike W4EF...............