A Tuned Passive Radiator

The problem with my 'Portable AM Crystal Radio' is that it works well only when close to an overhead CATV cable working as a passive radiator.

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

Series-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.

Identical performance was obtained when a parallel-tuned circuit was used.

Parallel-tuned Passive Radiator - Schematic

Related post: Portable AM Crystal Radio
_______________________________

Shortwave Crystal Radio

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

Shortwave Crystal Radio

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 - Schematic

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 60' wire antenna and earth resulted in 'local strong medium wave station breakthrough' and hum.

With earth disconnected, the breakthrough and hum reduced considerably and it was possible to tune-in to a faint shortwave broadcast. It was from AIR Chennai, on 7380 kHz, located 270 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.

On a later occasion the same stations came in louder, with no breakthrough and hum, using a 20 m inverted 'V' antenna.
_______________________________

Portable AM Crystal Radio

Here's the portable version of my 'Series-tuned Crystal Radio'.

Portable AM Crystal Radio

The prototype was wired on a piece of perforated board.

M0UKD's 'Loaded Quarter Wave Antenna Inductance Calculator' gave a loading coil inductance of the order of 10 mH at 612 kHz (the frequency of the 200 kW AM station located 20 km away). Hence two 4.7 mH moulded inductors were connected in series at the feed point, to resonate the 0.75 m long telescopic antenna at that frequency. 

Portable AM Crystal Radio - Schematic

At first nothing was heard even on the shack roof. However, on moving around, a favourable spot was found, where the reception is indeed quite good. The spot is close to an overhead CATV cable working as a passive radiator. The cable runs exactly in the direction of the station like a Beverage antenna.

Encouraged by the results, the final version was built using the plastic front panel of a discarded radio. In this version OA79 diodes were used with equally good results.

Portable AM Crystal Radio - final version

This radio works quite well, unaided by passive radiators, up to 5 km away from the transmitter.

Related post: A Tuned Passive Radiator
_____________________________

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.

Related post: Some thoughts on UHF Crystal Radios
_______________________________

Compact Loudspeaker Crystal Radio

This is the compact version of my 'Loudspeaker Crystal Radio'.

Compact 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.

Compact Loudspeaker Crystal Radio - Schematic

The centre-tap on the primary is not used.

A discarded blister pack serves as the base.

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.

Related post: Alternative Detector for a Shunt-fed Crystal Radio
_______________________________

Makeshift Piezo Earphones

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

Stethoscope

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

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

Moulded Inductor

There is no difference between the two with respect to performance. 

Series-tuned Moulded Inductor 
Crystal Radio - Schematic

Another Series-tuned Moulded
Inductor Crystal Radio - Schematic

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

The performance of a parallel-tuned version is also quite good with a headphone current of 300 μA.

Parallel-tuned Moulded Inductor
Crystal Radio - Schematic

Related post: Another Series-tuned Air-core Coil Crystal Radio
______________________________