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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