"Roy Lewallen" wrote in message
...
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

I will have to try your configuration. I did C - L - C/2 - L -C, with
C=1000PF, L=2.2uH. According to SWCad, the gain is flat above 2 MHz, but
there is 120 degrees phase shift at 4 MHz, relative to the phase at 100 MHz.

Tam/WB2TT