Shortwave Broadcasting In The United States – A lasting memory for a teenager interested in electronics and radio in the 1970s and 1980s is the proliferation of broadcast stations covering the shortwave spectrum. Some of them were a bit surreal, like Albania’s Radio Tirana, proudly informing Western Europe that in the 1980s every village in the country had a telephone, but most stations were Voice of America and Moscow. were the main ideological pleasures of radio.
It’s a long time ago, the Cold War is a distant memory and citizens in East and West get their information from the Internet, but perhaps there are echoes of those days after the invasion of Ukraine. With most foreign news agencies expelled from Russia and their websites blocked, international broadcasters are opening up new shortwave services to deliver news. In Russia, owning a shortwave radio can again be a subversive activity. Let’s build one!
Shortwave Broadcasting In The United States
Once upon a time, everyone had a radio, and listening to the radio became a simple pastime. Families from the 1930s to teenagers in the 1960s and 1970s huddled around posh family radios with their laptops are the epitome of the 20th century. Although most of us will be listening to the radio here in 2022, we may no longer be doing so in the morning hours, and certainly not on shortwave. We have instant access to almost any content on the Internet, so there is absolutely no certainty that people have a radio. If these shortwave transmissions are restored, how can the intended audience receive them? Maybe it’s time to look at shortwave radio in 2022.
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If you don’t have a shortwave radio and you’ve been digging through all the junk in your family to find leftovers from decades past, the easiest way to get one is of course to buy one. AliExpress is full of “world band” radios under $20, and if you don’t mind waiting for shipping from China, this is the path of least resistance.
But the problem is that international events are fast-paced and you may not have the luxury of waiting three weeks or even ordering in a war zone. How to make one? Once again, the Silicon Labs series of single-chip radios are a very simple option. They provide a high performance shortwave receiver with minimal external components and are truly a marvel of integration. But then again, in a war zone and between chip shortages, they just might not be an option. So how can you build a shortwave radio receiver from existing electronics using parts you may already have on hand? We’ll first look at some possible methods, and then introduce some ready-made building blocks.
The best way to start is to look at what you already have. Electronic float and jet tuning such as battery-operated AM radios, car radios, or even a $10 RTL-SDR stick. All of these can be modified or modified to receive shortwave broadcast bands, often with off-the-shelf components.
Perhaps the simplest method would be to directly convert an existing AM radio. Thanks to [Phil M6IPX] for sending the suggested link for a way to do this. This involves changing the resonant frequency of the ferrite rod antenna coil in the radio, and I think relies on the harmonics of the local oscillator, but is fundamental to mixing. It doesn’t cover all radio bands, but it might for a while.
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The next method involves converting the shortwave signal from its original frequency to a signal that your existing radio can receive. Radio amateurs will be familiar with the receiver converter, a device that mixes the signal from the antenna with a fixed-frequency local oscillator to produce an intermediate frequency from their difference, and the technique should be relatively simple to use.
AM radio is tuned at about 1 MHz and can be used with a converter to cover just one of the many shortwave bands. [Phil] also suggested that a mixer with a 16 MHz crystal oscillator could be used to tune the 4.433 MHz PAL color burning crystal, which is common in AM radio with a 15 MHz (19 m) broadcast band and a simple micro oscillator. this. The same is true for the 5 MHz (60 m) band. If I were to make such a rough and ready converter for an AM radio, I would try to find an AM radio in the car to serve as an IF, as these radios are well shielded and have a convenient coaxial antenna input.
However, to receive shortwave, the RTL-SDR can be modified using a switch or converter. Direct sampling distortion bypasses the on-board receiver chip to route the signal directly to the SDR chip, and can be used by any SMD-solder capable one person can do, and another approach for those who don’t want to try it is to use a converter. 50 MHz oscillator. A few years ago I made such a converter using a CMOS chip as an input to the Square Inch Contest, but even simpler circuits can be found.
Finally, the simplest available shortwave radio is the direct-conversion receiver. Its principle is similar to the receiving converter, because the signal from the antenna is mixed with the oscillator signal, creating a difference between the two, and if the local oscillator is on the same frequency as the desired station, this difference can be. is fed to the audio amplifier and listened to. It requires three relatively simple circuits in oscillator, mixer and amplifier, and although it does not provide satisfactory performance for music radio, it is quite suitable for speech.
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Once you’ve turned on all the receive options and direct conversion receivers, it’s time to look at these building blocks. Without access to the global electronics supply chain, how do you make them from components you find in junk electronics?
There are many ways to build 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 the RF port, and diodes as mixers. Building variable frequency oscillators is a little more difficult than building them from basic components, but if you have a signal generator or even a Raspberry Pi with the appropriate software, you can use them instead.
Crystal oscillators are found on all types of PC expansion cards and other computer boards and provide a logical square wave at the output pin when supplied with 5V power. However, any Fast Ethernet port will have RF transformers, and small signal diodes can be found in most. various types of electronics. In addition to these parts, conventional discrete components such as transistors and passives may be needed, but these can be scavenged from many more sources.
A diode ring mixer is a very simple circuit that uses multiple frequency voltages and four diodes. It works by using diodes as switches that operate at the local oscillator frequency to alternately pass and block the signal frequency. The result is the intermediate frequency (IF), which is the difference between the incoming signal and the local oscillator. It is very easy to do with an Ethernet transformer and four signal diodes using the circuit shown. With a 100 Mbit Ethernet switch, it should be 100 MHz bandwidth. There are many ways to use this circuit with a suitable oscillator as a pick-up converter for AM radio or as a direct-drive receiver.
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For the converter, connect the output of the crystal oscillator to the local oscillator pin and connect the output to an AM radio, but for direct conversion use a variable oscillator and connect the output to a sensitive audio amplifier such as a microphone or phono amplifier. . The connection to an AM radio can be made directly to the antenna socket on the car radio or with a few turns of wire wrapped around the body of a portable AM radio. There is a problem with this circuit because it has no filter and therefore takes the sum and difference of the local oscillator and IF frequencies, but it should be good enough to capture shortwave broadcasting.
These aren’t the only ways to build a working shortwave receiver—after all, you can get anything from crystal sets to a trigger—but we think they’re probably the best ways to build a device that uses existing electronics. Maybe you have some ideas to add to the mix? Leave them in the comments!
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