Medium Wave Radio Project : Active Antenna
Most medium wave receivers use a ferrite rod type antenna, as this keeps things compact and works fairly well. In a ferrite rod antenna, a tuned coil is wound around a rod of ferrite material, the rod increases the inductance of the coil due to the material’s high magnetic permeability, allowing fewer turns of wire on the coil for the same inductance compared to an air-cored coil.
A disadvantage of the ferrite rod antenna is it’s not really suited for picking up weak stations, the antenna is physically small and can’t be positioned up clear away from sources of noise, or just to get the best signal. It is possible to add a small coupling loop to the ferrite rod and connect this to an external wire antenna, but if we’re doing this, why not just make an entirely external antenna, doing away with the ferrite rod? This circuit aims to do just that.
The medium wave band runs from ~500kHz to 1.6 MHz, or in wavelength 600 to 200m. An outdoor tuned half-wave dipole antenna for these wavelengths is impractical. A physically shorter antenna than the half-wave dipole will have complex impedance and require matching to the broadly 50 Ohm circuits of a radio. Signals we want to receive will be weak, so some amplification would be useful.
I want a circuit that has high impedance input, to better match a small antenna or random wire, and a low impedance output to drive the following low impedance circuits. A JFET has a large input impedance and a low output impedance, a common emitter amplifier has an input impedance of a few hundred to a few kOhms, and a low output impedance - combining the JFET and a common emitter amplifier, I should get the characteristics I want.
Design is probably too strong a term for this, first off I sketched a JFET input circuit and followed it up with a common emitter amplifier, with all resistor and capacitor values guessed at.
Sketch of basic idea for the antenna converter
I then hunted around for parts I had on hand that might work and put together a simulation in QUCS.
I ended up adding a third transistor to get better overall characteristics, and to get the gain at about 3dB. On the medium wave band in the UK there are several very strong stations, I did not want to over amplify these at the risk of causing inter-modulation artefacts due to non-linear effects in the transistors. I can always amplify more in the IF stage of a super-het after some narrow filtering to select against the strong stations.
Simulating the circuit with 2 µV pk.pk input into 1 MΩ input impedance, gives 4µV pk.pk into 50 Ω, or -98 dBm out for -147 dBm in.
I was developing this circuit as I was talking about it it Twitter. In the end I wasn’t the first person to build it and characterise it, that honour fell to Philip Bragg.
I built the circuit up on a piece of protoboard with bench sweeping capacitors - I’d literally removed them from a random PCB a few days before building this board.
Shortly after testing the circuit, it stopped working. I suspected the JFET has died, and tried replacing it - my construction made this rather difficult, so I decided to build a second version on a piece of copper clad board to try to find why the first circuit had died, and also to see if I can improve the circuit in anyway.
Annoyingly, this iteration circuit oscillates, as can be seen on the tinySA.
Still having issues with the active antenna circuit. It’s still oscillating at around 600kHz even with the input loaded at 50 Ω. I’ve added some inductance (100uH) in the supply to the JFET and reduced the coupling capacitor between the JFET and the emitter follower. It seems to be the JFET oscillating. More work needed. Probably a new, neater rebuild. This build was supposed to help me work out what had died in the previous version.