Small untuned loop for receiving – simple model with transformer gave a simple model for analysing a loop and and Towards understanding the YouLoop-2T at MF/lowHF applied that to the YouLoop-2T.

Above is the Airspy Youloup-2T. Try to put the two turns thing out of your mind, it is misleading, panders to some common misunderstanding, and so does not help understanding.

It would seem that many are quite confused by information from Airspy. The following quote from an online forum captures the confusion.

The main issue is that it has a massive self resonance at around 4MHz which causes a large variation in signal level and overall sensitivity across the 1-30MHz spectrum.

Above is Airspy’s published impedance plot looking into the supplied 1m feed line with the YouLoop-2T at the other end.

Keep in mind that this antenna is clearly intended as a receiving antenna. You might then ask is this plot directly relevant to the intended application.

It isn’t directly relevant, and as the quote shows, it confuses some readers.

## Receive system model

Let’s turn the thing around and instead of exciting it from the receiver end of the feed line, let’s excite it from the antenna.

Above is a Simsmith model of the antenna system in free space in receive mode.

- The generator source impedance is calculated as radiation resistance of the loop in free space (using the classic formula \(R_r=\frac{\mu_0c_0}{6\pi}A^2(\frac{2 \pi}{\lambda})^4\\\)).
- Loop inductance is taken to be 2.2µH, and its Q is set moderately low to capture the loss resistance of the inductor.
- An ideal 4:1 transformation captures the effect of the cross over connection at the loop gap.
- A 1m and 2m lengths of lossless coax models the feed line used by Airspy for their plot, it is lossless as I don’t trust the Simsmith loss model; and
- the transformer is assumed ideal.

In looking at the plot of power in the load (which is effectively the average antenna system gain), readers will note that it is a smooth curve (apart from a glitch where it seems Simsmith plotted at 0Hz), and there is no discontinuity around 4MHz as the quote stated.

## Error analysis

The assumptions used introduce some errors:

- lossless cable: very small, tenths of a dB;
- conductor loss: very small, tenths of a dB;
- transformer loss: very small, tenths of a dB;
- directivity: maximum gain in proximity of real ground could be up to 8dB more than average gain (partly due to the assumption of free space in the model).

## Conclusions

The Airspy plot doesn’t really help unsophisticated readers to understand the antenna system for receiving.

When you look at the average gain plot from the Simsmith model, you might understand that a seller might not want to publish that data because naive users might think that such low gain makes it unattractive.

Nothing here is to suggest that the loop conductor does not exhibit a self resonance but it will be well above 10MHz, so much so that the loop conductor can be treated as an ideal inductor below 10MHz.