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TopBand: Measuring impedance thru a filter

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Subject: TopBand: Measuring impedance thru a filter
From: owens@atd.ucar.edu (Chip Owens)
Date: Thu, 25 Jun 1998 10:11:16 -0600 (MDT)
To settle this question in my own mind I did a simple
test to see what really happens when you try to measure
impedance thru a filter.

First I made up a  test load consisting of a 49.9 ohm
resitor in series with a 3,300 pF capacitor. I measured the
impedance of this load using a network analyzer. Next, I put
a 4-pole LC bandpass filter between the analyzer and the load.
The filter passes the 160m band but cuts off sharply above and
below the band edges. Results are shown below. Reactance values
preceeded by a negative sign are capacitive. Those with no sign
are positive and represent inductive reactance.

                Load by itself                  Load via bandpass filter

Frequency       Resistive       Reactive        Resistive       Reactive
   kHz          ohms            ohms            ohms            ohms
__________________________________________________________________________

1570            49.57           -30.93          1.18            -118.18
1610            49.59           -30.05          1.92             -99.37
1650            49.53           -29.25          2.28             -78.69
1690            49.59           -28.67          3.17             -51.37
1730            49.58           -27.96          9.99             -18
1770            49.52           -27.38         69.94             -32.84
1810            49.55           -26.62         32.91              -9.11
1850            49.54           -26.14         65.82              26.88
1890            49.52           -25.56         93.42              21.19
1930            49.50           -25.03         52.12             -15.19

Below the 160M band-in the steep slope of the filter-one might expect to
see some wild changes in impedance. But, as the above data shows, even
within the filter passband there are still large variations in impedance.

The data support non-use of a bandpass filter when performing impedance
measurements on some unknown load. If the goal is to make antenna
measurements while minimizing the effect of strong nearby AM broadcast
stations it may be possible to use a notch filter that would notch out
the offending signal sufficiently to allow you to make the measurements.
I have not tried this, but it may be an idea for another experiment.

Chip Owens, NW0O
Boulder, Colorado


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