Radio Hacks

1270 Articles

Half Crystal Radio, Half Regenerative Radio

April 24, 2023 by 11 Comments

A rite of passage in decades past for the electronics experimenter was the crystal radio. Using very few components and a long wire antenna, such a radio could pick up AM stations with no batteries needed, something important in the days when a zinc-carbon cell cost a lot of pocket money. The days of AM broadcasting may be on the wane, but it’s still possible to make a crystal set that will resolve stations on the FM band. [Andrea Console] has done just that, with a VHF crystal set that whose circuit also doubles as a regenerative receiver when power is applied.

The key to a VHF crystal set lies in the highest quality tuned circuit components to achieve that elusive “Q” factor. In this radio that is coupled to a small-signal zero voltage threshold FET that acts as a detector when no power is applied, and the active component in a regenerative radio when it has power. The regenerative radio increases sensitivity and selectivity by operating at almost the point of oscillation, resulting in a surprisingly good receiver for so few parts. Everyone should make a regenerative radio receiver once in their life!

Tiny Three-Tube Receiver Completes Spy Radio Suite

April 22, 2023 by 6 Comments

In our surface-mount age, it’s easy to be jaded about miniaturization. We pretty much expect every circuit to be dimensionally optimized, something that’s easy to do when SMDs that rival grains of sand are available. But dial the calendar back half a century or so and miniaturization was a much more challenging proposition.

Challenging, perhaps, but by no means unachievable, as [Helge Fyske (LA6NCA)] demonstrates with this ultra-compact regenerative vacuum tube receiver. It’s a companion to his recent “spy transmitter,” a two-tube radio built in — or on, really — an Altoids tin. The transmitter was actually a pretty simple circuit, just a crystal-controlled oscillator and an RF amplifier really, but still managed about 1.5 Watts output on the 80-meter ham band.

The receiver circuit ended up being much more complicated, as receivers do, and therefore harder to cram into the allotted space. [Helge]’s used a three-tube regenerative design, with one tube each devoted to the RF amp, detector/mixer, and audio amplifier stages. As in the transmitter, the receiver tubes are mounted on the outside of the box, with the inside crammed full of components. [Helge] had to be quite careful about component positioning, to prevent interstage coupling and other undesirable side effects of building in such close quarters.

Was it worth it? Judging by the video below, absolutely! We’ve rarely heard performance like that from even a modern receiver with all the bells and whistles, let alone from a homebrew design under such constraints. It sounds fantastic, and hats off to [Helge] for completing his spy radio suite in style.

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NFC Antennas Have Other Uses

April 22, 2023 by 38 Comments

As NFC chips proliferate, so do the antennas they require for operation, and since many NFC-enabled items are single-use, this means there’s an opportunity to put them to other uses. It’s an avenue pursued by [Brother-live], as he strips the antennas from spent metro tickets and gets experimenting.

The antenna in an NFC-enabled card is a flexible PCB laminated between the plastic outer layers, with the tracks forming a coil round the outside of it. Using some solvent the cards can be easily separated and the antennas retrieved. Once the chip has been removed they can be cleaned up and soldered to, allowing wires to be connected.

What can you do with an NFC antenna? Not a huge amount as you can see in the (Russian language, English subtitles) video below the break, but he tries it as a not-very-good heating pad, a power harvesting antenna from NFC readers, and perhaps most promisingly, as the coil of a moving-coil speaker. We’re not sure how much effort would be worth making on that last front, but we think with a bit of care there might be room for audible improvement.

If you’ve ever been tempted to have a look at an NFC card, it’s a subject we’ve covered before.

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This One Simple Trick Rehabilitates Scratchy Sounding Speakers

April 20, 2023 by 13 Comments

We’ve all picked up a radio and switched it on, only to hear an awful scratchy noise emitting from the speaker. [Richard Langer] is no stranger to this problem, and has identified a cheap and unusual solution—using toilet paper!

The cause of the scratchy sound is that when the speaker’s paper cone warps, it can cause the voice coil to rub up against the magnet assembly. In time, this wears out insulation on the coil’s turns, damaging the speaker. [Richard] found that realigning the coil to its proper place would rectify the issue. This can be achieved by stuffing a small amount of toilet paper in the back of the speaker, between the cone and the metal housing.

To identify the right spot to put the paper, one simply presses on the back of the speaker with a pen while listening out for the scratchy sound to stop. The paper can then be stuffed into this area to complete the fix. This can realign the cone and voice coil and stop the scratchy sound for good.

[Richard] notes that this method can be quite long-lasting in some cases. Failing that, it should serve long enough for you to order a replacement speaker. Video after the break.

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Mag Loop Antenna Has A Brain

April 11, 2023 by 26 Comments

Magnetic loop antennas are great if you are limited on space since they are just a potentially small loop of wire. The problem is, they are sharply tuned. You normally have an adjustment capacitor to tune the antenna to different frequencies. [TekMakerUK] built one with a motor and an Arduino that he can tune from an Android phone. You can see more about the project in the video below.

If you want to transmit, the capacitor is often the weak part of the system. Luckily, some old gear yielded a capacitor with multiple sections and enough plate distance to handle the 5W desired. Of course, motor driving a capacitor isn’t a new idea, but this setup is nice since it uses a stepper motor and a rotary encoder.

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Russia’s New Mystery Shortwave Station

April 8, 2023 by 36 Comments

The Buzzer, also known as UVB-76 or UZB-76, has been a constant companion to anyone with a shortwave radio tuned to 4625 kHz. However, [Ringway Manchester] notes that there is now a second buzzer operating near in frequency to the original. Of course, like all mysterious stations, people try to track their origin. [Ringway] shows some older sites for the Buzzer and the current speculation on the current transmitter locations.

Of course, the real question is why? The buzzing isn’t quite nonstop. There are occasional voice messages. There are also jamming attempts, including one, apparently, by Pac Man.

Some people think the new buzzer is an image, but it doesn’t seem to be the same signal. The theory is that the buzzing is just to keep the frequency clear in case it is needed. However, we wonder if it isn’t something else. Compressed data would sound like noise.  Other theories are that the buzzing studies the ionosphere or that it is part of a doomsday system that would launch nuclear missiles. Given that the signal has broken down numerous times, this doesn’t seem likely.

What’s even stranger is that occasional background voices are audible on the signal. That implies that buzzing noise isn’t generated directly into the transmitter but is a device in front of a microphone.

We’ve speculated on the buzzer and the jamming efforts around it before. Not exactly a numbers station, but the same sort of appeal.

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Reactivating A Harris RF-130 URT-23 Transmitter

April 8, 2023 by 13 Comments

If you enjoy old military hardware, you probably know that Harris made quite a few heavy-duty pieces of radio gear. [K6YIC] picked up a nice example: the Harris RF-130 URT-23. These were frequently used in the Navy and some other service branches to communicate in a variety of modes on HF. The entire set included an exciter, an amplifier, an antenna tuner, and a power supply and, in its usual configuration, can output up to a kilowatt. The transmitter needs some work, and he’s done three videos on the transmitter already. He’s planning on several more, but there’s already a lot to see if you enjoy this older gear. You can see the first three below and you’ll probably want to watch them all, but if you want to jump right to the tear down, you can start with the second video.

You can find the Navy manual for the unit online, dated back to 1975. It’s hard to imagine how much things have changed in 50 years. These radios use light bulbs and weigh almost 500 pounds. [Daniel] had to get his shop wired for 220 V just to run the beast.

It is amusing that some of this old tube equipment had a counter to tell you how many hours the tubes inside had been operating so you could replace them before they were expected to fail. To keep things cool, there’s a very noisy 11,000 RPM fan. The two ceramic final amplifier tubes weigh over 1.5 pounds each!

The third video shows the initial power up. Like computers, if you remember when equipment was like this, today’s lightweight machines seem like toys. Of course, everything works better these days, so we won’t complain. But there’s something about having a big substantial piece of gear with all the requisite knobs, switches, meters, and everything else.

We can’t wait to see the rest of the restoration and to hear this noble radio on the air again. You can tell that [Daniel] loves this kind of gear and you can pick up a lot of tips and lingo watching the videos.

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