Radio Hacks

1317 Articles

Don’t Let The Baluns Float Over Your Head

April 29, 2023 by 5 Comments

Most ham radio operators will build an antenna of some sort when they first start listening or transmitting, whether it’s a simple dipole, a beam antenna like a Yagi, or even just a random wire vertical antenna. All of these will need to be connected feedline of some sort, and in the likely event you reach for some 50-ohm coax cable you’ll also need a balun to reduce noise or unwanted radiation. Don’t be afraid of extra expenses when getting into this hobby, though, as [W6NBC] demonstrates how to construct an “ugly balun” out of the coax wire itself (PDF).

The main purpose of a balun, a contraction of “balanced-unbalanced” is to convert an unbalanced transmission line to a balanced one. However, as [W6NBC] explains, this explanation obscures much of what baluns are actually doing. In reality, they take a three-wire system (the coax) and convert it to a two-wire system (the antenna), which keeps all of the electrical noise and current on the shield wire of the coax from interfering with the desirable RF on the interior of the coax.

This might seem somewhat confusing on the surface, as coax wires only have a center conductor and a shield wire, but thanks to the skin effect which drives currents to the outside of the conductor, the shield wire effectively becomes two conductors when taking into account its inner and outer surfaces. At these high frequencies the balun is acting as a choke which keeps these two high-frequency conductors separate from one another, and keeps all the noise on the outside of the shield wire and out of the transmitter or receiver.

Granted, the world of high-frequency radio circuits can get quite complex and counter-intuitive and, as we’ve shown before, can behave quite unexpectedly when compared to DC or even mains-frequency AC. But a proper understanding of baluns and other types of transformers and the ways they interact with RF can be a powerful tool to have. We’eve even seen other hams use specialty transformers like these to make antennas out of random lengths and shapes of wire.

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Building A Receiver With The ProgRock2 Programmable Crystal

April 28, 2023 by 11 Comments

Crystals are key to a lot of radio designs. They act as a stable frequency source and ensure you’re listening to (or transmitting on) exactly the right bit of the radio spectrum. [Q26] decided to use the ProgRock2 “programmable crystal” to build a receiver that could tune multiple frequencies without the usual traditional tuning circuitry. 

 The ProgRock2 is designed as a tiny PCB that can be dropped into a circuit to replace a traditional crystal. The oscillators onboard are programmable from 3.5KHz to 200 MHz, and can be GPS discliplined for accuracy. It’s programmable over a micro USB pot, and can be set to output 24 different frequencies, in eight banks of three. When a bank is selected, the three frequencies will be output on the Clock0, Clock1, and Clock2 pins.There was some confusion regarding the bank selection on the ProgRock2. It’s done by binary, with eight banks selected by grounding the BANK0, BANK1, and BANK2 pins. For example, grounding BANK2 and BANK0 would activate bank 5 (as 101 in binary equals 5). Once this was figured out, [Q26] was on top of things.

In his design, [Q26] hooked up the ProgRock2 into his receiver in place of the regular crystal. Frequency selection is performed by flipping three switches to select banks 0 to 7. It’s an easy way to flip between different frequencies accurately, and is of particular use for situations where you might only listen on a limited selection of amateur channels.

For precision use, we can definitely see the value of a “programmable crystal” oscillator like this. We’ve looked at the fate of some major crystal manufacturers before, too. Video after the break.

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The Cheap And Available Microwave Playground

April 28, 2023 by 11 Comments

There’s something of a mystique about RF construction at the higher frequencies, it’s seen as a Black Art only practiced by elite wizards. In fact, UHF and microwave RF circuitry is surprisingly simple and easy to understand, and given the ready availability of low-noise block downconverters (LNBs) for satellite TV reception there’s even a handy source of devices to experiment on. It’s a subject on which [Polprog] has brought together a handy guide.

A modern LNB has some logic for selecting one of a pair of local oscillators and to use vertical or horizontal polarization, but remains otherwise a very simple device. There’s an oscillator, a mixer, and an RF amplifier, each of which uses microwave transistors that can with a little care be repurposed. The page demonstrates a simple transmitter, but it’s possible to create more powerful  devices by using the amplifier stage “in reverse”.

Meanwhile the oscillator can be moved by loading the dielectric resonators with PVC sleeving, and the stripline filters can even be modified with a fine eye for soldering and some thin wire. Keep an eye out in thrift stores and yard sales for old satellite dishes, and you can give it a go yourself. It’s a modern equivalent of the UHF tuner hacking enjoyed by a previous generation.

Low-Cost RF Power Sensor Gets All The Details Right

April 25, 2023 by 25 Comments

Dirty little secret time: although amateur radio operators talk a good game about relishing the technical challenge of building their own radio equipment, what’s really behind all the DIY gear is the fact that the really good stuff is just too expensive to buy.

A case in point is this super-low-cost RF power sensor that [Tech Minds (M0DQW)] recently built. It’s based on a design by [DL5NEG] that uses a single Schottky diode and a handful of passive components. The design is simple, but as with all things RF, details count. Chief among these details is the physical layout of the PCB, which features a stripline of precise dimensions to keep the input impedance at the expected 50 ohms. Also important are the number and locations of the vias that stitch the ground planes together on the double-sided PCB.

While [Tech Minds]’ first pass at the sensor hewed closely to the original design and used a homebrew PCB, the sensor seemed like a great candidate for translating to a commercial PCB. This version proved to be just as effective as the original, with the voltage output lining up nicely with the original calibration curves generated by [DL5NEG]. The addition of a nice extruded aluminum case and an N-type RF input made for a very professional-looking tool, not to mention a useful one.

[Tech Minds] is lucky enough to live within view of QO-100, ham radio’s first geosynchronous satellite, so this sensor will be teamed up with an ADC and a Raspberry Pi to create a wattmeter with a graphical display for his 2.4-GHz satellite operations.

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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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