Besides the limited dynamic range of the FM link, the audio is quite pleasant for occasional listening. The diagram is similar to the ADA active differential antenna described at This way, the notch filter does not remove the 'Alpha' beeps around 12 kHz.

A low-noise, large bandwidth opamp LT is used as an instrumentation amplifier with very large input impedance. The connector for the upper and lower antenna rod are on the left. The dynamic range of this 'HiFi' wireless audio link isn't spectacular, but for occasional listening it's ok init was replaced with a new FM link. The receiver also runs on battery power to eliminate coupling noise from the house to the VLF rx site. With the same instruments, measured capacity between loop conductor and aluminium tape shielding was pF here in the circuit splitted in two pF capacitors. It is small and portable but with great performances: up to 15 dB more sensitive than Easyloop and an incredible 8 Hz to 20 kHz bandwidth flatness, with only 50 x 50 cm space. Reception taken 30 m away from home, in the middle of the country IdealLoop is represented by the yellow square. Furthermore the high sensitivity at 1 Hz made him affected by microphone effect: was enough a breath of weak wind, or someone who was walking nearby, to saturating the input, due to the Earth's magnetic field.įor this reason, the input circuit has been adapted to the current form, by starting the band flatness from 8 and not from 1 Hz. But field trials have suggested to amend specifications: the loop was perfectly flat but the intrinsic noise of the device under 10 Hz was very high. The idea of having a loop totally flat from 1 Hz was intriguing. This means that none of the mentioned parameter is extremely critical, but the respect of all it helps to have an excellent final result. Since, however, the most critical part as sensitivity is always the lower part in frequency, we made a good choice with the AD The noise of the sound card input is much below the noise level of the amplifier : at 10 Hz there are still 12 dB between the two levels. At frequencies above the OP27 is less noisy. If someone asks you 'How can best be described a good VLF loop? Soundcard background in in green. Many turns mean: more voltage across the loop, but higher resistance then higher thermal noisehigher impedance, lower self resonance. If we had an ideal preamplifier free input noise the reasoning would be over herebut it is not yet : loop impedance and therefore the turn number influences the noise of preamplifier inputthen the noise figureand then the final sensitivity of the device itself.įew turns mean lower voltage received, low impedance, but low resistance, than low thermal noise and high self resonance frequency.

In this context, the sensitivity limit is represented only by loop thermal noise.

Bbb4 vlf receiver