MARTIN - G8JNJ

ECLECTIC AETHER - Adventures with Amateur Radio

Moebius loop antennas

Some initial notes relating to the performance of muti-turn Moebius loop antennas.


Main focus is based on the Youloop design published on the Airspy forum, but the versions used for these tests are not an actual commercially produced Youloop


Single turn of coax forming the loop with a crossover at mid position.




As a test, I built three loops of 500mm diameter, to dimensions as shown in the original Youloop drawing, but using 75 Ohm WF100 satellite TV coax consisting of a 1.00mm solid plain copper conductor, foam polyethylene dielectric, copper foil tape and a plain copper wire braid and a 1:1 transformer.

The loops were constructed as three different types, but as much as possible was kept the same in each loop.
   
  • Moebius type similar to the Youloop design with a coax crossover at mid point
  • Single turn screened loop with split in screen at centre point
  • Single turn loop just using inner of coax with screen removed

The loops were energised with a 1m diameter square wire loop mounted "face to face" with the loop under test.

All loops were mounted in the same position in the near field and were predominantly flux coupled.

Varying the spacing between the loops from 0.3 to 2m produced similar results.

There are some lumps and bumps in the response curves as I measured the loops indoors, and although I attempted to minimise interaction with other nearby objects as much as possible, some is still present, although it was noticeable that the single turn screened loop with split in screen at centre point gave the most consistent results.

Note that these results depend upon the near field measurements being an accurate representation of what is also occurring in the far field, and this may not be the case.

The test set-up is shown below.


Note:-

The amplitude response of the whole chain is pretty flat (+/-0.5dB) from 1MHz to 40MHz

The transformers have less than 0.5dB insertion loss (typically 0.3dB) from 1MHz to 40MHz and a return loss of 20dB over the range 1MHz to 36MHz.

Ignore the absolute dBm values on the vertical scale, I have not used a set calibration point and the dBm values changed with loop spacing.

All plots on each graph were taken with the same settings and as close as possible to the same physical loop position and spacing.


The first plot shows the measured levels of the three loops. The single turn loop with split screen gave the best average level across the frequency range, and the single wire loop was close behind. The Moebius type was typically about 3dB lower in level.


The second plot shows the Moebius loop relative level when the 1:1 transformer is swapped for a 8:1 transformer presenting a 400 Ohm load to the loop.


The next plot shows the relative levels of the loops terminated in 50 Ohms and additionally the Moebius type when terminated in 400Ohms.



Overall it looks like the Moebius type would possibly benefit from a different transformer impedance ratio, although it has been pointed out to me that using 75Ohm coax for the loop may cause a different impedance transformation ratio and various additional impedance mismatch 'ripples' to occur.

The next step is to build some loops from 50 Ohm coax, and make some signal level measurements using off-air signals as the source, in order to try and validate the near field measurements.

It may also be necessary to add a broadband pre-amplifier in order to bring the weaker signal levels up to a usable level.

To be continued