A Tuned Passive Radiator

The problem with my Portable AM Crystal Radio was that it worked well only at a sweet spot on the shack roof.

The presence of the sweet spot was traced to overhead CATV cables acting as passive radiators.

Hence it was decided to create a tuned passive radiator to further improve performance.

One half of my 40 m inverted 'V' dipole antenna was chosen as the passive radiator. The series-tuned circuit was connected to the core of the coax at the shack end and earthed as shown.

Tuned Passive Radiator - Schematic
This setup enabled faint reception of the local 612 kHz 200 kW AM station on the portable crystal radio inside the shack.

After tuning the passive radiator to resonance, performance of the portable crystal radio was found to be quite good across the shack roof.

Shortwave Crystal Radio

It was my first attempt at building a shortwave crystal radio.

Shortwave Crystal Radio - Schematic
It's single-tuned and uses a 1N270 to drive vintage sound-powered phones.

The circuit was 'breadboarded' on an old fan regulator baseplate with the coil wound on a white plastic pill bottle.

Shortwave Crystal Radio
A 2-section variable capacitor, with slow motion drive, was used for tuning. The 2 sections were connected in series for an effective maximum capacitance of 180 pF.

Using a wire antenna (35' long) and earth resulted in 'local strong station breakthrough' and hum.

With earth disconnected, the breakthrough and hum disappeared and it was possible to tune-in to a faint shortwave broadcast. It was from AIR Chennai, on 7380 kHz, 300 km away.

Tuning further up, a weaker Oriental station was heard.

Severe fading was experienced with both the signals.

Similar results were obtained using a 1.5 m long telescopic antenna and earth.

Portable AM Crystal Radio

Here's the schematic of the portable version of my Crystal Radio.

Portable AM Crystal Radio - Schematic
It was wired on a piece of perforated board.

Two 4.7 mH moulded inductors, connected in series at the feed point, resonate the 0.75 m long telescopic antenna at 612 kHz (frequency of the 200 kW AM station located 20 km away).

Portable AM Crystal Radio
At first nothing was heard even on the shack roof. However, on moving around, a favourable spot was found where the reception was indeed quite good.

Minimalist UHF Crystal Radio

Here's the schematic of the minimalist version of my UHF Crystal Radio.

Minimalist UHF Crystal Radio - Schematic
It was wired on a terminal block.

Using this simple setup, digital signals from the nearby cellphone tower were received on the shack roof.

Compact Loudspeaker Crystal Radio

This is the compact version of my Loudspeaker Crystal Radio.

Trials on a number of assorted audio transformers, from the junk box, were not in vain. A push-pull output transformer, intended for a 1 W germanium transistor audio amplifier, gave excellent results.

Series-tuned Loudspeaker Crystal Radio - Schematic
The centre-tap on the primary is not used.

A discarded blister pack serves as the base.

Series-tuned Loudspeaker Crystal Radio
In spite of its small size, performance is as good as that of its counterpart.

My 'Homebrew Horn Speaker' and 60' wire antenna offer the perfect match for this crystal radio.

Makeshift Piezo Earphones

A makeshift piezo earphone arrangement comprises a stethoscope and a piezo transducer.

The sound emanating from the transducer is captured by the bell of the stethoscope, held against its face.

Piezo Transducer
This arrangement was recently used to figure out how to interface a piezo earpiece with a crystal radio. The transducer, a 1¾" telephone ringer, was salvaged from the junk box.

Series-tuned Moulded Inductor Crystal Radio

'A Series-tuned Air-core Coil Crystal Radio' and 'Another Series-tuned Air-core Coil Crystal Radio'  have been made simple by the use of moulded inductors instead of hand-wound coils.

Series-tuned Moulded Inductor Crystal Radio

Moulded Inductor
This makes it possible to use a compact plastic cosmetic jar as a housing.

Series-tuned Moulded Inductor Crystal Radio
There is no difference between the two with respect to performance. 

The local 612 kHz, 200 kW AM broadcast station comes in real loud with a 60' wire antenna and sound-powered phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 625 μA.

Another Series-tuned Air-core Coil Crystal Radio

This is the air-core coil version of my 'series-tuned crystal radio with alternative detector'.

Series-tuned Crystal Radio with
Air-core Coil - Schematic
It is enclosed in a cosmetic jar with the (500+500) pF PVC gang condenser mounted inside its cap.

Series-tuned Crystal Radio
There is sufficient clearance to close the jar even though the coil is wound on its body.

Series-tuned Crystal Radio - Coil view
The local 612 kHz, 200 kW AM broadcast station comes in real loud with a 60' wire antenna and sound-powered phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 750 μA.

'Gibson Girl' Crystal Radio

This series-tuned, shunt-fed crystal radio was built using parts salvaged from a vintage / battered SCR - 578 lifeboat radio (popularly referred to as 'Gibson Girl').

'Gibson Girl' Crystal Radio - Schematic
It consists of a baseplate, antenna tuning coil, antenna tuning capacitor and a terminal strip modified to suit.

'Gibson Girl' Crystal Radio
The only part required to be fished out from the junk box was a Schottky diode SB 860. It was mounted underneath the terminal strip.

The local 612 kHz, 200 kW AM broadcast station comes in real loud with a 60' wire antenna and sound-powered phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 750 μA.

Loop Antenna Crystal Radio

A loop antenna crystal radio is of interest on account of its selectivity and portability. But, with respect to signal strength, it is no match for a crystal radio using an external vertical wire antenna and earth.

If signal strength could be considerably improved with increased loop size, then its portability would be compromised.

As usual, 2 configurations are possible - parallel-tuned-series-fed and series-tuned-shunt-fed.

1. Parallel-tuned-series-fed version 

Parallel-tuned-series-fed Loop Antenna Crystal Radio
2. Series-tuned-shunt-fed version

Series-tuned-shunt-fed Loop Antenna Crystal Radio
The loop antenna was rigged up using a 4-legged stool as a makeshift frame. The stool was laid on its side on the table, making it convenient to wind the loop on its legs.

4-legged stool as Loop Antenna frame
Both the versions were tested with the plane of the loop oriented towards the local 612 kHz, 200 kW AM broadcast station.

Signal strength was the same with both, though a far cry from another crystal radio using a vertical wire antenna and earth.