Harmonic Interference

One day, in spite of my Minimalist Converter being disconnected, harmonics of the local 610 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

Adding series tuning to my first crystal radio gave significant performance improvement.

The loopstick is 90 turns of Litz wire close-wound on a 2" length of  ¼" ferrite rod and the variable a 500+500 pF PVC gang condenser with the 2 sections in parallel.

Location of the variable at the ground end was sacrificed for the sake of convenience of an outboard assembly in series with the antenna lead-in.

Minimalist Converter

A simple converter was homebrewed in order to listen to a strong 610 kHz local station using the 1.5 - 30 MHz shack receiver.

Since the 1.610 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 1.830 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 that, instead of the OA5, in my first crystal radio.

Detection was established on the very first attempt, using 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 real weak with the blade than with the OA5. Results were even worse with a pencil as 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 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 used instead of the blade. Out of them only a broken piece of hacksaw blade came close enough.

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 voltage of 0.5V open-circuit dropped to around 0.1V in-circuit. Performance was as good as with 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.

A Steampunk RF Milliammeter

A vintage 1Ω, 200mA FSD ammeter was there, so this steampunk project was taken up.

To start with, the 1Ω shunt and 30Ω series resistors were removed to restore the meter movement's original characteristics (10Ω, 5mA FSD).

The enclosure emerged from a scrapped multimeter housing and a sheet of acrylic. It was easy to work on the acrylic for the required cutouts.

All parts were available in the junkbox.

The circuit was wired up 'dead-bug' style.

The meter was then roughly calibrated to read around 500 RF milliamperes maximum.

Output of a vintage solid state 15W CW/AM/SSB rig was successfully peaked up using this meter.