top of page
The Radio Board.org
Forum Posts
vu2nan
Aug 01, 2025
In Crystal Radios
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 used while figuring out how to interface a piezo earpiece with a crystal radio. The transducer, a 1¾" telephone ringer, was salvaged from the junk box.
1
0
17
vu2nan
Jul 29, 2025
In Crystal Radios
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.
0
0
9
vu2nan
Jul 29, 2025
In Antennas, Accessories & Ground
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.Â
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 the static build-up on my 2m ¼ λ ground plane antenna would cause a whine in my 2m receiver as it dissipated through the front end coil to ground.
Conditions are totally different now, with my 2 storey house surrounded by high rises, cell phone towers and other structures with lightning arrestors. Problems of static build-up and lightning are a distant memory. I have not heard that static whine for years, even though I still use that ¼ λ ground plane antenna.
And my shack earth is just mains-supply-earth (third pin)!
Of course, if you live in the countryside, the lightning arrestor at the feeder entry-point, with a separate earth bonded to the mains-supply-earth, is a must.
While on the subject of bonding – a friend’s shack and attached bath had a separate safety earth, not bonded to the mains-supply-earth. His 2m antenna was mounted on a metal mast which was also separately earthed.
For reasons not known, the shack earth was open when the water heater developed a ground fault. The path of the fault current was through the floating earth wire, 2m rig, coax feeder braid, antenna mast and finally to ground, resulting in a coax cable fire. A parallel path also caused considerable damage to a Drake AC4 power supply.
Fortunately he had the presence of mind to pull the main breaker before he doused the fire.Â
73,
Nandu.
0
0
6
vu2nan
Jul 28, 2025
In Crystal Radios
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
coil making it resonant at the desired frequency.
Good results would be obtained with an element not shorter than 60 feet.
1
0
12
vu2nan
Jul 28, 2025
In Crystal Radios
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.
1
0
12
vu2nan
Jul 26, 2025
In Crystal Radios
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:Â Â Â Â Â
The earpiece is interfaced through a 2.5 mH RF Choke to prevent its capacitance from
short-circuiting the detector. The voltage drop across the detector drives the earpiece.
0
0
10
vu2nan
Jul 24, 2025
In Crystal Radios
1. Parallel-tuned, series-fed configuration:
Parallel-tuned, series-fed Crystal Radio circuit
The diode-clipped parallel-resonance voltage is output to the phones.Â
High-impedance diodes and phones are required to match the high impedance of the
parallel-resonant circuit.Â
This configuration is suitable for reception of distant, weaker stations.
 Â
2. Series-tuned, shunt-fed configuration:
Series-tuned, shunt fed Crystal Radio circuit
The diode-clipped series-resonance current is output to the phones.Â
Low-impedance diodes and phones are required to match the low impedance of the series-resonant circuit.Â
This configuration is suitable for reception of nearby, more powerful stations.
3. Series-tuned, shunt-fed variant:
Series-tuned, shunt-fed Crystal Radio variant
This is a variant of the above series-tuned, shunt-fed configuration.Â
Using low-impedance diodes and phones, it delivers identical performance.
0
2
29
vu2nan
More actions
bottom of page