Better Detector for a Crystal Radio

After a significant improvement in performance was obtained by just adding an inductor to a simple crystal radio (Series-tuning for a Crystal Radio), focus was shifted to replacement of  the OA5 for increased signal output.

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.

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.

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 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.

Then, on a feeling that the minimalist converter would display identical behaviour, its output was next connected to ground.

The signals returned.

It was my first-hand experience of harmonic interference!

Series-tuning for a Crystal Radio

In order to improve the performance of my first crystal radio, series-tuning was attempted.

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.

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 too big for the crystal radio enclosure.

A small 1000 pf capacitor could take the place of the variable. Other small inductors were tried out in lieu of the loopstick.

A 200 μH moulded one, with radial leads, worked quite well even without a series capacitor.

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

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.

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, when it was connected across the OA90 diode there 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.

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.

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.

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

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.

Foxhole Radio detector variants

In the absence of a foxhole radio, a quick way to test out a safety-razor-blade-detector was to use it in place of the OA5 in my first crystal radio.

Detection was established, on the very first attempt, with a new (not blue!) blade and solid hookup wire for contact.  It was found that only printed areas of the blade were effective.

However, the local broadcast station sounded weaker with the blade than with the OA5. Results were equally bad with a pencil for contact.

A spare carbon brush of a mixer/grinder was then tried out. It worked fine on most parts of the blade's surface and the signal strength went up multifold, though not as strong as with the OA5.

The spring made it possible to vary the contact pressure for optimum signal.

A fixture was then homebrewed using junk-box parts. A piece of copper-clad board was used as the ground contact, enabling the blade to be moved around while adjusting the spring force to locate a 'hot spot'. A lock nut was provided to retain the setting. 

Trials confirmed easy/reliable/repeatable set-up and adjustment.

Other rusty/oxidized/plated steel parts were also tried out in place of the razor blade. The best of them all turned out to be a piece of broken hacksaw blade which even outdid the razor blade!

A 'pencil contact' was then homebrewed as a replacement for the carbon brush. Results were not that good and it was not as easy to locate a 'hot spot' with it as with the carbon brush.

A discarded extruded-carbon-block water filter was the inspiration for the next version. It served as the base cum detector contact. After removing the fabric cover, a self-tapping screw was used to secure one of the lugs on the block. The blade was lightly held in place using a rubber band.

Connection to the blade was made by sliding the other lug under it whilst a wrap of electrical insulation tape isolated it from the carbon block. Results with this 'giant', yet simple, detector were unbelievably good.

The performance of these blade detectors was considerably enhanced by connecting a run-down button cell in series (~ 0.1V with negative terminal to carbon brush).

Next, the button cell was replaced by a 'Ginger Cell' - a piece of ginger into which a brass screw (+ve terminal) and a steel one (-ve) were screwed in. Its open-circuit voltage of 0.5V dropped to around 0.1V in-circuit and it was as good as the button cell.

Later, a cat's whisker detector was made with a black oxidized-brass screw mounted on an old plastic pulley. 

The copper-wire cat's whisker was soldered to the plated screw and the rectifying spot located by adjusting the screw.

It worked quite well but was tricky to adjust and required frequent readjustment.

An adjustable RF Ammeter

This is an adjustable version of my homebrew RF ammeter

An old 10Ω, 5mA FSD moving-coil meter was used to display RF current.

A discarded plastic box was found to fit the bill for the enclosure.

RF current range is settable from 0.2A to 1A FSD through the 1kΩ screwdriver-slot potentiometer.