Foxhole Radio detector variant

Having not built a foxhole radio, the easiest way to get the feel of a safety razor blade detector was to use it as a replacement for 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 best results.

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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.

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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.

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A 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.

All parts were sourced from the junkbox. 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.

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.

It was roughly calibrated to read around 250 RF milliamperes maximum. The CW transmitter was then easily adjusted.
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Headlight bulb load quickfix

Recently a 13.8V 25A DC Linear Regulated Power Supply was in need of testing after its defective main rectifier was replaced. It had protective circuits for under-voltage, over-voltage, short-circuit and over-current.

A couple of 12V 55/60W headlight bulbs were used to load the power supply, but their very low cold resistance caused instant tripping.

A length of galvanised curtain wire (~ 1.5m long), connected in series, saved the day. The power supply did not trip and the bulbs emitted a feeble glow.

The connecting wire was then slid along the curtain wire. The illumination progressively increased and so did the load current. When the curtain wire went out of circuit, the bulbs were fully illuminated and drawing 23A from the power supply.
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A Simple ¼ λ Ground Plane for 2m

This simple ¼ λ ground plane antenna is based on G3OGR - OM F.G. Rayer's design for a 2m vertical antenna.

It's built-in at the top end of a 25 foot long, 2-piece telescopic aluminium pipe mast with the elements made of 3/8" aluminium tube.

The elements are flattened at the ends to facilitate sealing/mounting/coax connection and the joints are made water-tight/corrosion-resistant with epoxy sealant.

This antenna has been up for more than 10 years and still going strong.
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A Coax Collinear for 70cm

This 70cm Coax Collinear Antenna was homebrewed one evening in the year 2008.

The job was made simple, thanks to N1HFX - OM Mike Martell and his detailed article 'Build A 9 dB, 70cm, Collinear Antenna From Coax'.

All instructions were strictly followed.

A brass sleeve was soldered to the braid at the feedpoint and a SO-239 connector was used.

Both the ends of the antenna were waterproofed with epoxy.

SWR was close to 1.5:1 when checked at a power level of 50mW.

Signal strength with this antenna was 2-3 'S' points higher compared to that using a ¼ λ Ground Plane Antenna at the same height.
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