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Created: Oct. 2025.
Introduction
Introduction
A clone of the LZ1AQ preamp is available on Aliexpress for ~USD11. Hence, if it turns out to be a dud, it won't be too painful :-). The preamp consists of a 65 x 30 x 1 (mm) FR4 PCB (fig. 1). We suspect the PCB consists of at least 3 layers because the bottom layer is mostly an earth plane and the top is so densely populated with components that there is little space for connecting traces. The RLC components are mostly 0805 size. There are four SOT-89 transistors with the marking "RE" - perhaps, a smaller version of the SOT-223 packaged PZT2N2222? The output transformer is a tiny 4 x 4 x 2 mm binocular core as opposed to the original's 10 x 6 x 4 ring. There are two anti-parallel diodes at the input, presumably to protect against either overdrive or static, though these diodes are absent in the original. The "A7" markings lead us to suspect BAV99.
Fig. 1: Photos of preamp with intact RF shield (left) and removed shield (right)
The components are enclosed by an RF shield made of stamped metal. The shield has a lid that can be pried open. Oddly, the lid obscures a power indicator LED, but this can be rectified by drilling a small hole on the lid.
Input to the preamp is through two SMA female jacks. These SMA jacks are connected to a slide switch to enable a choice of unshielded / shielded / Moebius configurations (fig. 2, top). However, the last configuration requires a crossover switch, but this is not supplied (fig. 2, bottom). More details on enabling different loop configurations can be found at this advert [1].
Fig. 2: A slide switch on the PCB enables a choice of unshielded / shielded / Moebius loop configurations in conjunction with a crossover switch (not supplied)
Material & method
Material & method
To measure the gain, the preamp input is connected to a dummy aerial (aerial simulation network) that replicates the impedances of a 1m diameter loop [2]. All other measurements omit the dummy aerial and are connected to 50 ohm sources via a 1:1 balun.
To give an idea how the preamp fares against the competition, it is compared against homebrew LZ1AQ [3], M0AYF and PA0FRI [4] preamps.
Results
Results
The evaluated preamp's operating current varies linearly with voltage (fig. 3) because it omits the original design's 10V regulator. If the voltage is held at 10V, the resultant current is quite similar the original's. According to the advert, the preamp can operated over 9 to 12V. However, if operated at 12V, the PCB is worryingly hot and the measured 190 mA current is far in excess of the advertised "156 mA max".
Fig. 3: The operating current rises progressively with voltage due to the elimination of a 10V regulator from the original design
The clone's peak gain occurs at 90 MHz vs. the homebrew's 30 MHz (fig. 4). The vast difference is due to the homebrew version incorporating an input LPF, but the clone does NOT. Additionally, the LPF absence has been confirmed by tracing the PCB connections with a continuity checker. Perhaps, the absent LPF enabled the vendor to claim "0.05 ~ 500 MHz" operating range! However, without the LPF, cheap SDRs may experience jamming from FM broadcasts. We speculate the clone's 90 MHz peak is contributed by the BAV99 diodes' zero bias capacitances and stray PCB inductances. Due to the clone's higher peak frequency, its HF gain is 2 ~5 dB lower than the homebrew's (fig. 5). Overall, the LZ1AQ clone has the lowest HF gain among all the competing designs. The lowish gain in the middle of the HF band may affect older receivers without sufficient amplification reserve.
Fig. 4: The clone's peak gain shifts to 90 MHz vs. the homebrew's 30 MHz due to the absent LPF in the former
Fig. 5: The LZ1AQ clone (dashed black trace) has slightly lower gain than its homebrew counterpart (solid black trace). Both clone & homebrew LZ1AQ have lower gain than competing designs.
The LZ1AQ clone has a common mode rejection ratio (CMRR) of ~18 dB at HF (fig. 6). This result makes it the poorest among the competing designs. It comes as a surprise to us that the clone's CMRR is significantly poorer than the homebrew LZ1AQ because the former's PCB looks professionally designed and without any obvious asymmetry
Fig. 6: The clone has a common mode rejection ratio (CMRR) of ~18 dB - the poorest among the competing designs
The LZ1AQ clone achieves P1dB >= 16 dB over HF; i.e. close to the homebrew one (fig. 7). The LZ1AQ design's best-in-class P1dB is expected because of their high operating current.
Fig. 7: The LZ1AQ clone (dashed green trace) achieves P1dB >= 16 dB over HF; i.e. close to the homebrew one (solid green). These two results are the highest among competing designs
The clone's OIP3 exceeds 26 dBm over HF; i.e. a few dBs lower than the homebrew one (fig. 8). We speculate that the lower OIP3 is due to the small output transformer being lossier. Nevertheless, the clone's OIP3 is still head & shoulders above the competition.
Fig. 8: The clone's OIP3 (dashed green trace) exceeds 26 dBm over HF; i.e. a few dBs lower than the homebrew one (solid green). Both clone & homebrew LZ1AQ are way better than the competing designs
In summary, the LZ1AQ clone is a near faithful copy of the original, except for the absent LPF. The missing LPF shifts the gain peak to 90 MHz and this may result in FM broadcasts desensitizing cheap SDRs. However, linearity parameters such as P1dB and OIP3 are reassuringly close to the original.
References
References
[1] https://www.aliexpress.com/item/1005008779151448.html
[2] "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
[3] "LZ1AQ active loop performance evaluation", https://sites.google.com/site/randomwok/Home/electronic-projects/aerials/lz1aq-active-loop-performance-evaluation
[4] "PA0FRI active loop evaluation", https://sites.google.com/site/randomwok/Home/electronic-projects/aerials/pa0fri-active-loop-evaluation
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