Small untuned loop for receiving – a design walk through #1

This series of articles develops a simple design for a small receive only broadband loop for the frequency range 0.5-10MHz, and to deliver fairly good practical sensitivity.

Fairly good practical sensitivity is to mean that the recovered S/N ratio is not much worse than the off-air S/N ratio. Let’s quantify not much worse as the Signal to Noise Degradation (SND) statistic calculated as $$SND=10 log\frac{N_{int}+N_{ext}}{N_{ext}}$$, and lets set a limit that $$SND<3 dB$$.

Since Next is part of the criteria, let’s explore it.

External noise

ITU-R P.372 gives us guidance on the expected median noise levels in a range of precincts. Since most hams operate in residential areas, you might at first think the Residential precint is the most appropriate, but ambient noise more like the Rural precinct is commonly observed in residential areas, so let’s choose Rural as a slightly ambitious target.

Above is Fig 39 from ITU-R P.372-14 showing the ambient noise figure for the range of precincts. Readers will not that that are all lines sloping downwards with increasing frequency, so the external noise floor is greater at lower frequencies in this range.

In terms of achieving our 3dB SND target, we need the receive system NoiseFigure to be no higher than Fam.

Broadband amplifiers will tend to have gain and noise fairly independent of frequency, so an amplifier with low enough noise for the higher frequencies is better than needed at the lower frequencies.

Let’s look at the element that sits between the external noise environment and the amplifier input, the antenna.

Simple broadband antennas tend to fall into two categories:

• the short electric dipole; and
• the small magnetic dipole (loop).

The short electric dipole

A short electric dipole has a Thevenin equivalent source impedance of a small resistance in series with a very small capacitance. To capture much power from a short electric dipole, the amplifier must have a very high impedance.

Small magnetic dipole (loop)

A small loop has a Thevenin equivalent source impedance of a small resistance in series with a moderate inductance. To capture much power from the untuned loop, the amplifier must have a moderate impedance.

Small loop + amplifier

The article Small untuned loop for receiving – it’s not rocket science gave a simple equivalent circuit for a small untuned loop loaded by a moderate resistive load (the loop amplifier). The effect of the loop inductance in series with the total circuit resistance (mainly the amplifier input R) is an LR low pass filter (LPF), with gain falling with frequency above the break frequency.

The combination of the loaded loop as a LPF, and amplifier with constant gain and NoiseFigure is a block with gain and noise figure roughly proportional to inverse of frequency.

The graph above shows P.372 ambient noise (Rural), and an idealised LPF + amplifier with NF∝-log(f). Also plotted is the calculated SND for this scenario.

So this analysis suggests that it may be possible to make an antenna system with an untuned small loop and amplifier, having SND<3dB over the range 0.5-10MHz.

The next article will explore a practical loop geometry and amplifier characteristics to achieve the objective.

References

ITU-R. Aug 2019. Recommendation ITU-R P.372-14 (8/2019) Radio noise.