Tam/WB2TT wrote:

I am happy with mine. Only thing I can add is that it eats batteries, and my
unit does not like to work below 13 Volts. They do sell a tunable bandpass
filter for the lower frequencies. I tried a high pass filter - don't bother,
it messes up the phase.

If you do want to measure antenna impedance with something like the MFJ
and have trouble (as I do) with local broadcasting and other RF, I
recommend using a "half wave" filter. This type of filter has reasonable
out-of-band attenuation characteristics and doesn't disturb the
impedance measurement too much. (Over a limited frequency range it
mimics a half wavelength of transmission line.) The impedance
disturbance will be the least when the filter is designed to
approximately match the measured impedance.

It's simply two cascaded pi sections, with each element having the same
value of X. For example, a lowpass looks like this:

---.---L---.---L---.---
| | |
C 2C C
| | |
----.-------.-------.---

For example, a filter with Z0 = 50 ohms (one which mimics a half
wavelength of 50 ohm line) has XL = XC = 50. One designed for 7 MHz
would have L = 1.14 uH, C = 455 pF. You can wind the coils on small type
2 or 6 powdered iron cores, and use the closest standard value (e.g.,
470 pF) for the capacitors. This filter will cause little impedance
disturbance at 7 MHz and, for load impedances near 50 ohms, at lower
frequencies also. A highpass looks like this:

---.---C---.---C---.---
| | |
L L/2 L
| | |
----.-------.-------.---

Again, XL = XC = the filter Z0. Make filters according to your
particular measurement needs. You can check the amount of disturbance
the filter causes by making a known impedance out of a resistor and
capacitor or inductor which is about equal to the measured impedance,
and measuring it with and without the filter.

If you can read German, there's a marvelous book describing how to make
a wide variety of measurements with an antenna analyzer, with tricks,
tips, and details. It's _HF-Messungen mit einem aktiven
Stehwellen-Meßgerät_ by Gerd Janzen, DF6SJ. It's available directly from
him at Hochvogelstraße 29, D-87435 Kempten, Germany.

Roy Lewallen, W7EL