Owning A ShortWave Radio Is Once Again A Subversive Activity

A constant memory of a teenager fascinated by electronics and radio in the 1970s and 1980s was the proliferation of propaganda stations covering the shortwave spectrum. Some were slightly surreal, such as Albanian Radio Tirana, which would proudly inform Western Europe in the 1980s that every village in the country already had a telephone, but most stations were the more vocal ideological gladiators of the Voice of America. and Radio Moscow.

This is a long time ago, as the Cold War is a distant memory and the citizens of East and West receive information from the Internet, but there may be an echo of those times after the invasion of Ukraine. As most foreign news agencies have been expelled from Russia and their websites blocked, international broadcasters launch new shortwave services to get the news through. Owning shortwave radio in Russia could again be subversive. Let’s build one!

What happened to the portable radio?

A typical short-range radio.
Typical radio for a small world group. Donald Trung Quoc Don (Chữ Hán: Countries list) CC BY-SA 4.0 International.

There was a time when everyone had a radio and listening to the radio was a universal activity. From families from the 1930s, clustered around ornate family radios, to teenagers from the 1960s and 1970s with their portable devices, this is a defining image of the 20th century. Although many of us are still listening to radio here in 2022, it is likely that we will no longer be doing it in AM and certainly not on shortwave. On the same subject : Analog Devices Launches High Dynamic Range RF Transceiver for Challenging Mission-Critical Communications Applications. We can get instant access to almost any content online, so in no case will certain people have a radio. If these shortwave broadcasts start again, how can their target audience take them? Maybe it’s time to take a look at 2022 shortwave radios.

If you lack a shortwave radio and dig up all the rubbish in your family that hasn’t found a relic from decades past, then the easiest way to get one is, of course, to buy one. AliExpress is full of “world band” radio stations starting at anywhere under $ 20, and if you don’t mind waiting for delivery from China, then this is the path of least resistance.

But there is a problem, international events are evolving rapidly and it may not be a luxury to wait three weeks or even be able to order one in a war zone. How can you do that? Again, there is an extremely simple option in the series of single-chip radios from Silicon Labs. These provide high performance shortwave receiver with a minimum of external parts and are truly a marvel of integration. But again, in a war zone and in the midst of a chip shortage, they may simply not be an option. So how can you make a shortwave radio using what parts you can get your hands on? We will first look at some possible paths and then present some of the easily accessible building blocks.

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Where to start?

The best way to get started is to look at the things you may already have. Such electronic flotsam and jetsam as battery-powered AM radios, car radios or even $ 10 RTL-SDR sticks. Read also : Military Communications Market Size to Reach USD 62.96. All of them can be modified or converted to accommodate shortwave radiation strips, often with easily accessible parts.

Probably the simplest possible method may be to directly modify an existing AM radio. I am indebted to [Phil M6IPX] that he betrayed me link to method instructions for this. This involves changing the resonant frequency of the coil of the ferrite rod antenna in the radio and I guess relying on the father’s harmonic of the local oscillator and not the basic one to perform the mixing. It does not cover all broadcasts, but it can be done in steps.

Block diagram of the receiving converter
Block diagram of the receiving converter

The next method is to convert the shortwave signal from its original frequency to one that can be received from a radio you already have. Radio enthusiasts will be familiar with the transducer, a device that mixes the signal from an antenna with a local oscillator with a fixed frequency to produce an intermediate frequency of their difference, and it should be relatively easy to use this technique.

The AM radio is tuned to about 1 MHz and can be used with a converter to cover only one of the many shortwave bands. [Phil] again proposed a 16 MHz crystal oscillator module to be used with a mixer to tune 15 MHz (19 m) broadcast band to AM radio and a publicly available 4,433 MHz PAL color crystal with ordinary transistor oscillator can do the same for the 5 MHz (60 m) band. If I were making such a rough and ready AM radio converter, I would try to find a car AM radio that would serve as my IF, because these radios are well shielded and have a convenient input for a coaxial antenna.

My one inch converter board
My one inch converter board

Meanwhile, the RTL-SDR can be modified to receive shortwave by modification or by using a converter. Hack for direct sampling bypasses the on-board tuner chip to transmit signals directly to the SDR chip and can be performed by anyone with good SMD soldering skills, and for those who do not want to try it, an alternative approach is to use a converter with a 50 MHz oscillator . A few years ago I made such a converter using a CMOS chip like my participation in the Hackaday Square Inch race, but there are even simpler schemes.

Finally, perhaps the simplest shortwave radio to use is the direct conversion receiver. Its principle is similar to the receiving converter in that the signal from the antenna is mixed with that from the oscillator to get the difference between the two, and when the local oscillator is at the same frequency as the desired station, this difference can be applied to an audio amplifier and listened to. It requires three relatively simple circuits in oscillator, mixer and audio amplifier and although it does not provide acceptable performance for music radio, it is good for speech.

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The Nitty Gritty: Parts and Chains

Once you’ve set everyone on fire for receiving converters and direct converting receivers, it’s time to look at these building blocks. On the same subject : Harris introduces two waveforms to improve RF-7800V VHF handheld software-defined radio. How can you make them from the components you will find in e-waste without easy access to the global electronics supply chain?

There are many ways to create oscillators and mixers, but for our purposes the components we are interested in are crystal oscillator modules for the local oscillator, broadband RF transformers for RF connection and diodes as mixer elements. Variable frequency oscillators are a little harder to build but can be made from the most basic componentsbut if you have a signal generator or even Raspberry Pi with appropriate software you can use them instead.

Crystal oscillators are ubiquitous in all types of PC expansion cards and other PCBs and provide a square wave at the logic level of the output pin when equipped with 5 V. Meanwhile, each Fast Ethernet interface will contain an RF transformer and small signal diodes can be found in many different types of electronics. Beyond these parts, normal discrete components such as transistors and passives may be needed, but again, they can be extracted from a wide variety of sources.

Mixing circuit with diode ring
This is the main mixer with a diode ring made of garbage. It’s not perfect, but it works.

The diode ring mixer is a very simple circuit using several RF transformers and four diodes. It works by using diodes as switches operating on the frequency of the local oscillator to pass and block the frequency of the signal. The result is the intermediate frequency (IF), which is the difference between the input signal and the local oscillator. It can be very easily done with an Ethernet transformer and four signal diodes using the diagram shown. With a 100 Mbit Ethernet transformer, it must have a 100 MHz bandwidth. There are many ways in which this circuit can be used with a suitable oscillator as a receiver converter for AM radio or as a receiver with direct conversion.

For the converter, simply connect the output of the crystal oscillator module to the local oscillator pin and supply the output to the AM radio, while for direct conversion use a variable oscillator and connect the output to a sensitive audio amplifier such as a microphone or amplifier. The connection to the AM radio can be either directly to the car radio antenna socket or by a few turns of a wire wrapped around the body of a portable AM ​​radio. There is a problem with this circuit because it has no filtering and thus captures both the sum and the frequency difference of the local oscillator and IF, but it should be good enough to attract shortwave radiation.

These aren’t the only ways to make a working shortwave receiver – after all, anything from a crystal set up can be forced to work – but we think they’re probably the best ways to make one using electronics that probably at hand. Maybe you have some ideas to add to the mix? Leave them in the comments!

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