This is an active current loop antenna
Working of this active loop antenna
This active antenna is a low input impedance current mirror amplifier. The emitters of Q1 and Q2 form a balanced input and low impedance loading of the screened loop.
By loading the loop (ideal: short circuiting) with a low impedance, it becomes effectively aperiodic. I.e. its current, due to a received signal, is independent of the frequency of the signal. So this antenna is broadband with no need to tune. Therefore the current amplifier (Q1..4) has to be highly linear. This is done by setting the DC current to a relative high value.
The output of the current mirror is the connection point of the collectors of Q2 and Q4.
R3 is to get a stable working point, which can de set by R12 (set R12 so as to get the voltage at the output half way (~ 6 V with respect to common). Q5 and Q6 form a so called “super emitter follower”. This makes the output impedance of Q5 much lower as it would be without Q6.
Attaining a low output impedance, together with R11 will make the output close to 50 ohm, independent of the working frequency.
The loop has to be shielded in order to minimise the pick up of E-field signals, especially the near-E-field (man made noise!).
