Created: Oct. 2024

-------------------------

Introduction

The MLA-30 / + active loop has been the subject of many reviews, but all were subjective [1 - 3]. There is one objective review [4], but the test frequencies and input termination were not specified. The unspecified test conditions make comparison difficult because certain parameters such as P1dB & OIP3 are frequency dependent and the gain is termination sensitive. Moreover, the gain measurement is confusing as the graph depicted negative gain between -46 to -12 dB, whereas the text reported +10 to +30 dB. 

To redress the previous reviews' shortcomings, we evaluated one unit of the MLA-30+. The gain measurement uses a dummy aerial [5] to interface with the pre-amp input (fig. 1). The dummy is necessary to simulate the loop's impedance. Our MLA-30+ sample suffered from leaky input blocking capacitors which have to be rectified with external 820 nF caps in series with the inputs (fig. 2 - 3). Without the external caps, the measured gain is erroneously 30 dB lower (fig. 4). The other measurements, P1dB and OIP3, are measured using 50 Ω source impedance because these measurements are insensitive to input termination. 

Results

The measured gain peaked at ~10 MHz due to resonance between the dummy and the preamp's input capacitance (fig. 5). Below resonance, the gain drops rapidly due to the dummy's high pass characteristic - this simulate the sensitivity reduction when the loop size becomes progressively smaller in comparison to the wavelength. Over 8 ~ 30 MHz, the gain exceeds 5 dB. A second peak at ~30 MHz is caused by the dummy's own resonance.

P1dB starts with a high value of -3.5 dBm at 5 MHz and gradually drops to -6.5 dBm at 30 MHz (fig. 6). We speculate that the decreasing P1dB is caused by circuit losses increasing with frequency.

The OIP3 shares the same trend as the P1dB of decreasing with frequency (fig. 7). The worst case value is -2.6 dBm at 25 MHz. Surprisingly, the OIP3 is only ~4dB higher than the P1dB, whereas simple BJT amplifiers typically exhibit a 10 dB difference. 

There are several striking differences between the previous results [4] and this work (fig. 8). Notably, the prior art reported 10 dB minimum gain vs. this work's  -30 dB. The previous OIP3 was 20 dBm vs. this work's -2.5 dBm. Hopefully, these discrepancies can be resolved when more experimenters replicate these measurements.

How does the MLA-30+ fare against the competition - LZ1AQ, M0AYF and PA0FRI? The MLA-30+ has the highest gain, but the lowest P1dB and OIP3 of all (fig. 7). The latter two results are not unexpected because of its low operating current.

References

[1] The SWLing post, "David reviews and compares the MLA-30 magnetic loop antenna", 2019, [Online] Available: https://swling.com/blog/2019/09/david-reviews-and-compares-the-mla-30-magnetic-loop-antenna/

[2] D. Day (N1DAY), "MLA-30 Magnetic Loop Antenna Review and Comparison", [Online] Available: https://hamsignal.com/blog/mla-30-magnetic-loop-antenna-review-and-comparison

[3] John’s Tech Blog, "Cheap Chinese Magnetic Loop Antenna (MegaLoop aka MAGALoop) MLA-30", 2019. [Online] Available: https://hagensieker.com/2019/07/24/cheap-chinese-magnetic-loop-antenna-megaloop-aka-magaloop-mla-30/

[4] M. Ehrenfried (G8JNJ), "Active antennas overview", [Online] Available: web.archive.org/web/20210306185317/https://g8jnj.net/activeantennas.htm

[5] Electronics, RF & microwave by 9W2LC, "Aerial simulation network / dummy aerial for active loops", [Online] Available: https://sites.google.com/site/randomwok/Home/electronic-projects/aerials/aerial-simulation-network-dummy-aerial-for-active-loops