2019-01-07 TRF Receiver Experiment

2018-01-04 also FB group Building Transistor Radios

I thought that the ‘tinkerers’ who were tinkering with the loop antennas may have been getting a stronger signal because the loop antennas have a large area to capture the RF signals, so I decided to make a single channel AM Band receiver, and I chose KNX 1070 all news all the time as the station.

I wound sixteen turns of 24 AWG PVC insulated telephone wire around a small cardboard box, and taped the wire in place. I measured the inductance at 128 microhenrys, and used the resonance tool in the free app ElectroDroid to calculate how much capacitance the coil would need. I connected four 470 pF 1% silver mica capacitors in series- parallel to get that required capacitance and I connected the capacitors to the loop.

I added another four turns to the coil to tap off the signal to go to the base of the RF preamplifier. The schematic is in the photo. The supply is two 1.25V Ni-MH cells connected in series. I plan to connect a 4V PV solar cell to charge them.

The preamp is okay, and so is the audio amp stage after the detector diode. But I was getting poor sensitivity using a “1N34” diode from Asia, which tests as a Schottky diode, not germanium – it doesn’t have the leakage a germanium diode has. The volume was low, so I experimented with that detector. When I soldered a real germanium diode across that diode, the volume got much louder. It is performing up to my expectations now. I get a bit of harshness in the audio so I may add more capacitance after the diodes to filter out more of the high frequencies and not sound so harsh. The schematic shows the 5.1k connected to +2.5V but it is actually connected to ground.

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