Series-tuning for a Crystal Radio

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

Ferri Loopstick & PVC Gang Condenser
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.
Series-tuned Crystal Radio - Schematic
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 available fixed inductors, were unsuccessful.

Ultimately, a 200 μH moulded inductor was found to work quite well, with the 60' wire antenna itself providing the capacitance.

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

Moulded Inductor-tuned Crystal Radio - Schematic
Related post: My first Crystal Radio
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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!
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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.
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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.

My First Crystal Radio - Schematic
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.

Safety Razor Blade & Carbon Brush
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.

Safety Razor Blade Detector
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.

Pencil with spring
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.

Carbon block water filter
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).

Button Cell & 'Ginger Cell'
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. 

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

Cat's Whisker Detector & Screw
It was tricky to adjust, but its performance was quite good.

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Adjustable RF Ammeter

This is an adjustable version of my homebrew RF ammeter.

Adjustable 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.

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

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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 junk box.

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

RF Milliammeter - Schematic
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.

Related post: Homebrew RF Milliammeter
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Homebrew RF Milliammeter

The need for a QRP output meter was felt when the homebrew SWR meters / RF ammeters in the shack proved useless for peaking the output of a homebrew 7MHz 5W solid state CW transmitter. For their pointers barely moved.

Hence the decision to build a RF milliammeter on the same lines as my homebrew RF ammeter.

RF Milliammeter
All parts were sourced from the junk box. BNC sockets were used. The primary of the toroidal transformer is a piece of the inner conductor of RG-58/U coax. The secondary is wound with a length of solid hookup wire. A rubber grommet ensures positioning of the toroid.

Inside the RF Milliammeter
The small 1kΩ, 200μA FSD meter makes the unit quite compact. The enclosure of a defective 230V - 110V autotransformer came in handy, with the meter taking the place of the 110V socket.

RF Milliammeter - Schematic
It was roughly calibrated to read around 250 RF milliamperes maximum. The CW transmitter was then easily adjusted.

Related post: Homebrew RF Ammeter
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