Groups Feed

This, hopefully, will be a "new and improved" version of the simple superhet for reception of WRMI that was previously shown here.

Improvements will include wider range frequency coverage, better image rejection, and improved selectivity, achieved principally by changing from a single down conversion to an intermediate freque3ncy of 455 KHz, to a dual conversion up converting to 10.7 MHz as the first intermediate frequency.

With the use of a simple low pass filter as the front end, reception from the broadcast band to seven MHz or so should be readily accomplished. Images will be in the range above 22 Mhz so unlikely to present any issues, and a crystal filter at 10.7 Mhz will establish the initial selectivity ahead of the second IF.

Construction has begun. The Front end filter, mixer, and second conversion local oscillator are complete.

A TDA1072 again is used as the main device of the…

Heads Up, RadioBoard Friends

Wix (the service we use to host our site) is phasing out the old Forum app. Don’t worry — our discussions aren’t disappearing! They’ll be moving into a new space called Wix Groups.

What this means:

• You’ll still be able to read and post just like before, but in a slightly updated format.

• All our members and conversations will come along for the ride.

• The old forum will close for new posts after October 15, 2025, and Wix will remove it entirely on March 1, 2026.

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

A Germanium transistor may be configured as a diode, having its forward voltage as low as 0.1 V, by just interconnecting its base and emitter.

Basic Shunt-fed Crystal Radio

It makes for an ideal detector in a shunt-fed crystal radio.

I was a SWL in the mid-1970s and the antenna I used with a Philips transistor portable receiver was a long wire at a height of about 50 feet. One summer afternoon, the receiver front end (AF117) blew right after a loud crackle of static. 

Then again, my first tube homebrew CW rig had a 3-pin mains supply plug. The antenna was a straight dipole 50 feet high. Operating on a summer afternoon, I received a jolt through my Junker CW Key and survived to hear the crash of thunder from a nearby lightning strike. I immediately yanked the twin-line feeder and threw it on the floor (upper floor of my 2 storey house). After a few moments I was surprised to see the arc from the banana plugs to the cement floor.

Those days, in our sparsely populated area, my antenna was way above other surrounding structures. During thunderstorms…

A medium wave broadcast station antenna system generally comprises a ¼ λ vertical element

and earth.

It would not be wrong to infer that such an antenna would be ideal for a crystal radio.

Crystal Radio Antenna Arrangement

However, practical reasons necessitate use of a much shorter element, with a suitable loading

In a ham shack, either half of a 40m inverted ' V ' dipole antenna would come in quite handy

as a usable crystal radio antenna element.

Handy Crystal Radio Antenna

However, a suitable loading coil would be required to make it resonant at the desired frequency.

These are test results of a couple of low pass filters designated as 0 - 30 MHz and inexpensively available on AliExpress.

There are apparently at least two types - one is labelled 30M on the back and the other has a Rev 1 silk screened on it. The latter appears to be the better performer.

For testing I used a Red Pitaya StemLab 14 and set one of the outputs to sweep 0 - 50 Mhz. I set one of the FFT channels to look at 0 - 63 Mhz. I connected the generator output directly to the filter and connected a 30 dB 50 Ohm attenuator between the filter and the Red Pitaya input to simulate a 50 ohm dummy load. There is probably still some mismatch at the filter input, but I only had the one attenuator. Probably need to build another .....

Anyway, here are the…

Here's how to interface a piezo earpiece with a crystal radio.

Parallel-tuned, series-fed radio:

The earpiece is interfaced through a 33 kΩ shunt resistor. The resistor provides a path for the

detector current. The voltage drop across the resistor drives the earpiece.

Series-tuned, shunt-fed radio: