## Input fields

- R is from the bridge resistance dial;

- C is from the bridge reactance dial, +ve for 'inductive' pF, -ve
for 'capacitive' pF;
- Frequency is the frequency at which the null is observed;
- C
_{null} is the capacitance of the reactance control at
null (from the manual or circuit diagram);
- R
_{0} is the nominal characteristic impedance of
the
transmission
line used for calculation of Gamma and VSWR results.
- R
_{exp} is an expander resistor in parralel with Z_{unknown}
to extend the
range,
leave it empty if not used.

Note that the use of an expander resistor (say 200Ω) is popular with
these type of instruments, in fact some have a built in resistor with
switch. Whilst it does enable an expanded measuring range, it does that
at the expense of accuracy. Do a check by varying the inputs a little
in the calculator to determine sensitivity of results to small
measurement errors.

## Output fields

- Z
is the series equivalent impedance of
Z
_{unknown} in complex Cartesian format;
- Y is the parallel equivalent admittance of
Z
_{unknown} in complex Cartesian format;
- Γ is the complex reflection coefficient in polar format (ρ<θ°);
- VSWR is the Voltage Standing Wave Ratio that would occur in a
transmission line with Z
_{0}=R_{0}; and

- Reactor is the inductance or capacitance of the equivalent
reactor and circuit Q.

If you're clever, you will have worked out you can download
this page to your hard disk. If you're smart, you won't,
you will run it from the web site and automatically take advantage of
any updates.

Common values for C_{null}, Re:

- MFJ-202B 180pF, 200Ω
- Palomar R-X Noise Bridge 70pF

# Links / References

- Straw, Dean, ed. 2003. The ARRL Antenna Book. 20th
ed. Newington: ARRL. 27.25 -27.32.

Other tools.

© Copyright:
Owen Duffy 1995, 2021. All rights reserved. Disclaimer.