SND implications of variations on the untuned small loop at MF/lowHF

Three recent articles developed an explanation of the YouLoop-2T at MF/lowHF:

The first and third articles explained the concept of signal/noise degradation (SND) statistic, and gave graphs of the behavior of the subject antennas.

This article draws together those SND plots for two antennas, and some variations to the configurations.


Simple loop with transformer

Above, the “simple loop” with 0.5:1 ideal transformer. It could be implemented as a shielded loop (with transformer) with similar behavior (but improved common mode suppression).

Above is the calculated SND from 0.3 to 15MHz.



Above, the Airspy YouLoop-2T with ideal transformer.

Above is the calculated SND from 0.3 to 9MHz.

Shielded loop without transformer

A simpler loop is a shielded loop (without transformer).

Above is the detail of a simpler shielded loop using coaxial cable and presenting a load of 50+j0Ω to the loop gap. With 50Ω coax and 50Ω receiver, there are no standing waves on the feed line or coaxial loop section so the impedance presented at the loop gap is a broadband 50+j0Ω.

Above is the calculated SND from 0.3 to 15MHz.

Shielded loop with transformer

An alternate way to construct a shielded loop with flexibility to choose a loop load other than 50Ω is to insert a transformer at the loop gap of a basic shielded loop.

Above, a miniature ferrite cored broadband transformer or autotransformer could be used at the gap of the Simpler loop to provide broadband impedance transformation of a 50Ω receiver without the effects of standing waves and impedance transformation in the loop coax sections.

Above is the calculated SND from 0.3 to 15MHz with a 0.5:1 turns ratio transformer.

YouLoop-2T without crossover

Above is a variation on the YouLoop-2T where the top crossover unit is replace with a simple through connection of the centre conductor at the gap. This removes the 1:4 impedance transformation, but there remains some transformation on the loop coax sections which have standing waves.

Above is the calculated SND from 0.3 to 9MHz.

Signal to Noise degradation


Above is a chart comparing the simple loop and the simpler loop. The key differences is the load impedance for the simpler loop is 50+j0Ω whereas the simple loop is 12.5Ω which gives rise to the different SND response.

Some of the variations given above have some amount of transformation due to the loop coax sections so they aren’t simply 12.5Ω or 50Ω which gives rise to some small difference in their responses.

Other important configuration parameters

The load impedance presented to the loop proper has a great influence on SND response. The other key parameters are:

  • loop diameter; and
  • loop conductor diameter.

Loop diameter

Doubling loop diameter increases loop inductance and increases radiation resistance, and delivers a large improvement in SND (of the order of 5dB). Note that the frequency were it qualifies as a small loop is reduced. That is not to suggest it does not work where perimeter>λ/10, just that analysis based on small loop assumptions becomes invalid.

Loop conductor diameter

Doubling conductor diameter results in a small reduction in loop inductance and small improvement in SND (tenths of a dB).

Example larger loop

Above is the calculated SND from 0.3 to 15MHz of a 4m perimeter shielded loop of 12mm conductor diameter with 50Ω load.


Signal/Noise degradation with the small untuned loop is influenced by several key parameters:

  • loop perimeter;
  • loop conductor diameter;
  • loop load impedance;
  • impedance transformation due to standing waves on transmission line components;
  • departure of transformers from ideal.

The SND response is a compromise which can be tailored for the intended application.