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Project Notes

#375 Three Transistor ShortWave Receiver

Building a simple 3-transistor regenerative receiver for HF shortwave.

Build

Notes

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.

Regenerative Receivers

A Regenerative receiver introduces positive feedback in the RF receiver circuit, resulting in increased gain and selectivity. It was invented by Edwin Armstrong in 1914.

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

Circuit

I’ve redrawn the original schematic in EasyEDA for my study, available here.

Schematic

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

Parts

Qty Item Ref
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
1 Air coil L1

The Build

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

Build

Build

Build

Build

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
4.5-7 MHz 46
6-10 MHz 25
9-14 MHz 15
13-20 MHz 8
19-28 MHz 5
26-50 MHz 2

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:

HF_16m

Amongst others, two very strong signals for:

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:

17685

And here’s a quick demo of tuning in on All India Radio, 17895 kHz:

17895

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

About LEAP#375 RadioHF
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.