#375 Three Transistor ShortWave Receiver
Building a simple 3-transistor regenerative receiver for HF shortwave.
I found this design for a 3 Transistor Short Wave Radio by netZener. It is a restoration and update of an old Science Fair #28-110 kit from Radio Shack.
One of the disadvantages of a regen is its propensity to radiate RF back out the antenna especially when the regen is close to oscillation. I learned on Soldersmoke 164 that this was a particular risk for spies in WWII, as explained in the book Spycatcher.
In this circuit, the regen control comprises Q1 and it’s biasing components.
Some Other Regen Designs
- Regenerative Receivers - techlib
- A Short Wave Regenerative Receiver Project - tuberadio
- A Simple Regen Radio for Beginners - QST September 2000, pp 61-64
I’ve redrawn the original schematic in EasyEDA for my study, available here.
Variations in my build:
- used 2.2kΩ for R12 instead of 2.7kΩ (due to parts on-hand). Works fine, probably increases the AF amp gain.
- used a 2kΩ pot for R2 (due to parts on-hand). Means the regen control is not as sensitive, but it is workable
|1||Transistor NPN (2N3904)||Q1|
|2||Transistor PNP (2N3906)||Q2,Q3|
|2||33K Resistor 1/4W 5%||R1,R5|
|1||1K Resistor 1/4W 5%||R3|
|1||100K Resistor 1/4W 5%||R4|
|1||2.2K Resistor 1/4W 5%||R6|
|1||2.7K Resistor 1/4W 5%||R12|
|1||22K Resistor 1/4W 5%||R9|
|1||47K Resistor 1/4W 5%||R10|
|1||12K Resistor 1/4W 5%||R8|
|1||10K Resistor 1/4W 5%||R11|
|2||.01uF Disk Ceramic Capacitor||C1,C8|
|2||47pF Disk Ceramic Capacitor||C2,C3|
|1||15pF Disk Ceramic Capacitor||C4|
|1||.047uF Multilayer Capacitor||C8|
|1||.1uF Mylar Capacitor||C9|
|1||10uF Electrolytic Capacitor||C10|
|1||47uF Electrolytic Capacitor||C7|
|1||500 Ohm Potentiometer||R2|
|1||100K Ohm Potentiometer||R7|
|1||140pF Variable Capacitor||C5|
Semi-modular ugly construction:
- AF amp is mounted on copper PCB stock, ugly style
- detector and tuning is mounted on single-sided protoboard with M3 stand-offs
That’s almost finished. A couple of final changes:
- I finished soldering (I hadn’t yet soldered the audio out connection when I took the pictures above)
- I moved the coupling capacitor C8: in the pictures above, it is connected directly to R5, C6. But I subsequently took the short-cut: directly from L1/C5 to the R7 wiper.
Tuning the Tuning Circuit!
The original documentation provided the following recommendations for the air coil inductor:
|Frequency||Recommended Coil Turns|
First Test, ~16m
Great results for some stations around 16m with a 12 turn coil, approx 15mm diameter and 43mm long. The tuning range is from 12350 kHz (25m) to 22830 kHz (15m).
Here is the rig as tested:
Amongst others, two very strong signals for:
- All India Radio transmitting on 17895 kHz 10:00-11:00 UTC with 500 KW from Bengaluru
- Radio Free Asia transmitting on 17685 kHz with 250 KW from Tinian Island
Here’s a quick recording of me tuning in on Radio Free Asia, 17685 kHz. I’m using a Tecsun PL-660 to cross-check and verify:
And here’s a quick demo of tuning in on All India Radio, 17895 kHz:
Tuning Below 12 MHz
I’ve wound a couple of coils for a longer wavelength - aiming to pick up a very strong BBC World Service signal on 9740 kHz.
By tuning in to the radiated signal on the Tecsun PL-660, I can verify a tuning range of around 7 MHz to 12MHz. However I can’t seem to tame the regen: I can get a very faint signal on 9740 kHz, but it is overwhelmed by oscillation or noise (depending on the regen control).
.. time for some more study to learn how I might get this under control!
Credits and References
- 3 Transistor Short Wave Radio - original article by netZener
- Instructables - updated post by netZener
- Regenerative circuit - wikipedia
- Regenerative Receiver - electronics-notes
- Air Core Inductor Inductance Calculator
- LC Resonance Calculator
- Tuning A Regenerative Receiver - ARRL
- 2N3904 datasheet
- 2N3906 datasheet