Small untuned loop for receiving set out a model for calculating the S/N degradation of an active untuned small loop antenna system.

The calculations in Small untuned loop for receiving – Trask noise and gain analysis might prompt the question of what is the optimal resistive load for an untuned small loop.

This article explores the topic for a simple model where the equivalent noise temperature of the amplifier is independent of source impedance.

## A simple model for a small loop

We can construct a simple model where the loop behaves as a fixed pure inductance, and its load is a fixed pure resistance.

This is a reasonably good model for a small loop, perimeter < wl/10, not too bad for perimeter up to wl/3.

The source impedance becomes the loop’s inductive reactance Xl which is proportional to frequency, and the load is Rl.

Above is a plot of the relative power developed in the load vs the ratio of Rl/Xl.

There is a maximum where Rl=Xl, and the power captured falls away either side. Continue reading Small untuned loop for receiving – optimal loop load resistance