Horn Speaker for Crystal Radio

This horn speaker is an improvisation for my crystal radio.

Horn Speaker
It requires only 2 additional parts - a dustbin and a food container - with the crystal radio speaker itself serving as the driver.

In order to simplify the mounting, the speaker is made to drive from its rear while its front is sealed by the food container.

Horn Speaker details
Rubber based adhesive is used for assembly.

Audio output is considerably more than with the speaker in its normal enclosure.

Crystal Radio through a Speaker

This is the speaker variant of my crystal radio.

Crystal Radio through a speaker - Schematic
Output impedance of the order of a few hundred ohms necessitates use of a matching transformer for the speaker.

A power transformer, with its inherent thick conductors, is an  ideal substitute for a low-loss output transformer.

Trials with transformers rated 230V/24V-2A, 230V/24V-5A and 230V/24V-15A proved that the last one is the best.

Reception of the lone local 200kW 612kHz AM broadcast station, located 20km away, is quite good.

Antenna is a 60' random wire.

Better Detector for a Crystal Radio

After obtaining a significant improvement in performance by just adding an inductor to my crystal radio, the focus was on a replacement for the OA5 which would give a higher output.

OA5 diode
The base-emitter and base-collector junctions of germanium audio transistors like AC130, OC74, AC127, 2N61, AC188 and AD162 were tried out. However, they were all only as good as the OA5.

Germanium transistors
As luck would have it, when the 2N61 was being tried out, an accidental short between its base and emitter leads resulted in a tremendous increase in signal output. The other transistors too gave identical results with their base and emitter leads shorted.

Crystal Radio - Schematic
Shorting the base and collector leads gave similar, though not consistent, results.

With the OA5 in the crystal radio replaced by the base-emitter-shorted 2N61, the lone local broadcast station on 612 kHz is now loud enough to be heard faintly even at a distance from the headphones!

Harmonic Interference

One day, in spite of my Minimalist Converter being disconnected, harmonics of the local 612 kHz broadcast station were still being received in the shack receiver.

That was indeed surprising. Then, after some thought, realisation dawned that the Crystal Radio on the shack table could be the cause.

This was confirmed by disappearance of signals when the the ground connection to the crystal radio was removed.
Crystal Radio - Schematic
Then, on a feeling that the minimalist converter would display identical behaviour, its output was next connected to ground.

Minimalist Converter - Output grounded
The signals returned.

It was my first-hand experience of harmonic interference!

Series-tuning for a Crystal Radio

Adding series-tuning was an afterthought to improve the performance of my first crystal radio.

Series-tuned Crystal Radio - Schematic
The loopstick (90 turns of Litz wire, close-wound on a 2" length of  ¼" ferrite rod) and variable (a 500+500 pF PVC gang condenser) were wired on a bread board and connected in series with the antenna lead-in.

Ferri loopstick & PVC gang condenser
With the plates of the condenser nearly fully meshed, the only local broadcast station on 612 kHz was received with a considerable increase in volume.

However, both the loopstick and variable were too big for the crystal radio enclosure.

Trials with a fixed 1000pf capacitor, in series with various small inductors, were unsuccessful.

Ultimately, a lone 200 μH moulded inductor was found to work quite well, with the short random wire antenna itself providing the capacitance.

Moulded inductor
It was easy to solder it right at the antenna socket.

Inductor-tuned Crystal Radio - Schematic
This minor change gave a significant improvement in performance.

Minimalist Converter

A simple converter was homebrewed in order to listen to a strong 612 kHz local station using the 1.5 - 30 MHz shack receiver.

Simple Converter - Schematic
Since the 1.612 MHz received signal was noisy, a 2.5 mH RF choke was connected across the output - to no avail. Then, on an impulse, it was connected across the OA90 diode. The result was an enormous increase in signal strength.

Tuning up later, with the converter inadvertently switched off, an equally strong signal was found on 1.836 MHz.

A real effective tripler had been stumbled upon, making the oscillator redundant.

Minimalist Converter - Schematic
So out it went, leaving only diode and choke to do the job.

The result - a chance minimalist project!

Simple Low-cost Demo Repeater

This project is based on 3 of my earlier projects - 'Coupling a commercial VHF handheld to an external antenna' , 'Fox Hunt Attenuator' 'UHF on a VHF Rig'.

Parts required are a couple of Rubber Ducky Antennas, suitable connectors, a single OA5 diode and a metal enclosure.

Demo Repeater
Also needed are a MF local oscillator, a VHF local oscillator, 2 VHF handhelds & a UHF handheld.

The schematics are as shown below.

For in-band operation the 600 kHz fundamental crystal oscillator output is mixed with the incoming signal.

In-band Demo Repeater - Schematic
For cross-band operation the 290 MHz 5th harmonic of the 58 MHz overtone oscillator is used.

Cross-band Demo Repeater - Schematic
In-band & cross-band repeater operation is easily demonstrated using the handheld rigs.

Down-link signals are quite strong inside the shack.

However the range of the demo repeater is yet to be checked.