Wideband Active Small Magnetic Loop Antenna

There are now extremely wideband software defined radios (SDR) where the wideband antenna is a natural choice. Wideband small magnetic loops (WSM loop) are used already 3-4 decades and I was curious to see what can be reached with them and to evaluate their usefulness as a wideband SDR input. The WSM loop should work in short circuit mode in order to reach flat frequency response in wideband frequency range. The antenna should be used with an amplifier since the loop current is very small. This amplifier must be with very low input impedance. A circuit diagram of active WSM loop antenna is shown on Fig.1. The antenna specification is given for 1m diameter circular loop with aluminum conductor with diameter 3.4 mm. Specification Diameter: 1 m, 1 turn Material: aluminum conductor with 3.4 mm diameter Loop inductance: 4 uH Antenna Factor Ka: 6 dB meters-1 @ 10 MHz (computed from the spice model) (1 uV/m input signal will give 0.5 uV output voltage) Flatness: Within 3 dBmeters-1 0.5 – 30 MHz; (computed from the spice model) Noise floor: >= 0.7 uV/m (computed from the spice model) Power supply: Remote, 13.5 V >150 mA Dynamic range: TBS; 1 dB Compression point >= 130 dBuV/m ( 5.6 V/m p-p output voltage, from the spice model)
Wideband Active Small Magnetic Loop Antenna - schematic

The construction of the loop should be considered with the following rule: the ratio of loop area to loop inductance should be maximized (see the Appendix) . That automatically means that circular shape with 1 turn is the best choice. The practical diameter is around 1 m with the conductor as fat as possible. The material might be copper or aluminum – actually the loop Q-factor is not important. The important factor is the low loop inductance. 1m diam. loop made from aluminum wire 3.4 mm gives inductance around 4 uH. I have used also 0.9 m diam. loop made from double foil FR-4 PCB material (Fig.3) with 1.5mm thickness and 20 mm width which reduces the loop inductance to 3 uH. The best results can be obtained with “parallel” and “crossed parallel ” loops (CP loop, see Fig. 5 , 13,14 , Appendix I,II). For urban locations where the noise level is much higher smaller loops can be used. This antenna will be used outdoors and the amplifier is placed in a small, IP55 secured, plastic box (Fig.2). These boxes are widely available on the market - any similar one can be used. The connecting cable between the antenna and receiver (RX) is shielded LAN cable FTP type with 4 twisted pairs. The signal and power use separate pairs. RJ45 standard connectors are used. These connectors are very cheap and reliable but the RJ connector should to be placed inside the box since it is not waterproof. There is no...

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