Antenna analyzer

antennaimpedanceRF

I want to measure the impedance of an antenna (at 434MHz) and for learning/fun try to build a circuit to do so.

I found the return loss bridge
http://www.vk2zay.net/article/179

Can I measure a complex load (the imaginary part of the antenna) with this circuit?

Best Answer

Not directly, no. This will be a somewhat hand waving answer, but:

To do so you would need to make a primitive vector network analyzer. In effect, you'd take out the diode detector following the return loss bridge in that circuit (used to feed a voltmeter) and instead connect a suitable bandwidth oscilloscope. By measuring the phase (as well as magnitude) of the reflected signal vs that of the forward signal, you can calculate the complex impedance.

However, you may be able to determine it analytically using the scalar voltmeter measurement and sweeping across the frequency where it nulls up or down to the frequency of interest then comparing this to a model of how the circuit should behave to determine the unknown values. Or perhaps you could connect additional known inductance or capacitance and deduce a model from the resulting change in the scalar value.

In many practical applications, the desire is not to know the complex impedance, but rather to minimize the reactive component at the frequency of interest, making the antenna as close to a purely resistive load as possible. This would typically be done either by tuning the physical dimensions of the antenna, or by adding compensating inductors, capacitors, or even feedline stubs.