Comparing LZ1AQ, M0AYF, PA0FRI and MLA-30+ active loops

Introduction

We compared the performances of 4 popular active loops: LZ1AQ [1], M0AYF [2], PA0FRI [3] and the commercial MegaLoop MLA-30+ [4]. With the exception of MLA-30+, the other pre-amps are home constructed (fig. 1). However, our version of the M0AYF uses 2N2222A BJT instead of the original BC547. For the PA0FRI, we also use 2N2222A instead of the original 2N5109 due to the former's availability in our junk box. Both M0AYF and PA0FRI are constructed on the same PCBs are they have almost identical circuits. 

To our knowledge, this is the first time the 4 designs' most critical performances are graphically compared over frequency. 

Results

LZ1AQ’s gain peaked at ~ 29 MHz because it incorporates an input low-pass-filter that is intended to resonate with the loop at the HF top end (fig 2). The other 3 pre-amps, which do not have such filters, exhibit peaks in the 9~10 MHz range. The gain peaks occur due to dummy antenna resonating with the pre-amps' input capacitances (~80 pF). MLA-30+ has the highest gain of all, hands-down, because it utilizes a highly integrated op-amp / video amp, TL592B [5] as opposed to the others' single / dual transistor stages. The gain measurement uses a dummy aerial [6] to connect to the preamps' input terminals. 

The preamps' ability to resist blocking / desense by a strong interferer can be inferred from its 1dB gain compression (P1dB) specification. LZ1AQ has clearly the highest P1dB due to its high current consumption, while MLA-30+ is the lowest. PA0FRI and M0AYF lie in the middle (fig. 3). 

Immunity to spurious generated by two nearby interferers is given by the Output 3rd order Intercept Point (OIP3). Again, LZ1AQ exhibits the highest OIP3 due to its high current consumption, while MLA-30+ is the lowest. PA0FRI and M0AYF lie in the middle (fig. 4). 

Noise impinging on the coax shield can appear as a voltage between the preamps' earth and dual input terminals. The immunity against this noise is known as the common mode rejection ratio (CMRR). M0AYF & PA0FRI have significantly better CMRR than LZ1AQ because the former two are differential amplifiers; i.e. they have a shared emitter resistor and emitter inductor, respectively [7]. In  PA0FRI, the CMRR degrades as the frequency drops due to the decreasing inductive reactance (XL), but as the frequency increases, the XL also increases, hence resulting in it having the highest CMRR at the upper HF range (fig. 5). In contrast, M0AYF's emitter resistor can preserve the CMRR down to DC, but the parasitic cap shunting the resistor causes its CMRR to degrade gradually with increasing frequency. LZ1AQ, which is not a differential amplifier, can only rely on circuit balance to reject common mode noise. 

Finally, the performance parameters are summarized in a table (fig. 6). 

References

[1] C. Levkov, “Wideband active small magnetic loop antenna”, 2011. [Online] Available: http://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm

[2] Des, M0AYF, 'Active loop antenna for HF', transcribed by M0LMK. [Online] Available: http://www.m0lmk.co.uk/2015/02/14/active-loop-antenna-for-hf/Des, M0AYF, 'Active loop antenna for HF', transcribed by M0LMK. [Online] Available: http://www.m0lmk.co.uk/2015/02/14/active-loop-antenna-for-hf/

[3] F.H.V. Geerligs, PA0FRI, "Active loop antenna for reception", 25 Jul. 2023. [Online] Available: https://pa0fri.home.xs4all.nl/Ant/Actieve%20ontvangst%20antenne/Active%20loop%20antenna%20for%20reception.htm

[4] MegaLoop MLA-30+. Can be bought from various online shopping platforms.

[5]  M. Ehrenfried, G8JNJ, "Active antennas overview", [Online] Available: web.archive.org/web/20210306185317/https://g8jnj.net/activeantennas.html

[6] Facebook post of Penang Radio Lab, 27 Jan. 2025, [Online] Available: https://www.facebook.com/share/p/1C1cCGekDH/

[7] Facebook post by Penang Radio Lab, 26 Jan. 2025. [Online] Available: https://www.facebook.com/share/p/15nwpC7oEs/