# Generic noise bridge calculator

This calculator solves for some interesting results from the measurements using a generic noise bridge which is calibrated for increasing pF either side of null, +ve for 'inductive' pF, -ve for 'capacitive' pF.

 R (Ω) C (pF) Frequency (MHz) Cnull (pF) R0 (Ω) Rexp (Ω) Z (Ω) Y (S) Γ VSWR Reactor

SVN:

## 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;
• Cnull is the capacitance of the reactance control at null (from the manual or circuit diagram);
• R0 is the nominal characteristic impedance of the  transmission line used for calculation of Gamma and VSWR results.
• Rexp is an expander resistor in parralel with Zunknown 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 Zunknown in complex Cartesian format;
• Y is the parallel equivalent admittance of Zunknown 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 Z0=R0; and
• Reactor is the inductance or capacitance of the equivalent reactor and circuit Q.

Common values for Cnull, Re:

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