MLA-30 active loop noise reduction

The MLA-30+, a popular HF active loop, has a biastee which uses a switch-mode boost regulator to convert 5V from a USB port to 12V for supply to the pre-amp (fig. 1). Unfortunately, the boost regulator generates a lot of noise over 0.1-5 MHz (fig. 2). This noise appears at the biastee's 'To Receiver' port.

Fig. 1: MLA-30 biastee's internal photo & circuit. The boost regulator is indicated by red arrows

Fig. 2: The noisy output from the original biastee's 'To Receiver' port

Bypassing the boost regulator has been previously suggested [1], but this requires some delicate / dicey modifications to the SMD-populated PCB and also drilling the biastee enclosure to retrofit a DC connector. Moreover, the said prior art did not furnish any evidence of improvement.

It is arguably simpler to replace the original biastee with a passive one since the latter is widely available online at low prices. Alternately, the passive biastee can be easily homebrewed.

To test this hypothesis, we replaced the original biastee with a homebuilt passive biastee (fig. 3). The passive biastee is supplied from a linear power supply at 12V. Significant noise reduction was achieved over 0.1 ~ 5 MHz (fig. 4). Compared to the original (pink trace in fig. 5), the passive biastee (black trace) reduces noise as much as 50 dB at 3 MHz.

Fig. 3: Test setup using passive biastee

Fig. 4: Much quieter output of the passive biastee

Fig. 5: Noise output from original (pink trace) vs. passive (black trace) biastees in one graph. At 3 MHz, the noise peak is reduced by ~ 50 dB

Reference

[1] www.eevblog.com/forum/rf-microwave/mla-30-active-loop-antenna/

[2] Penang Radio Lab facebook post, 19 May 2024. Available: t.ly/7dWUf

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