RF

194 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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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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JFET Stands In For Triode In This Infinite Impedance Detector

February 10, 2023 by 6 Comments

An “Infinite Impedance Detector” might sound a little like something that [Zaphod Beeblebrox] would use to zip around the galaxy. It’s not, of course, but it is an interesting and useful demodulator for AM radio signals, as [Sebastian Westerhold] over at Baltic Labs explains in the brief but well-done video below.

If you’ve ever browsed through schematics of old vacuum tube radios, [Sebastian]’s JFET-based detector circuit might look strangely familiar. That’s because this demodulator is about as close to a direct translation between a vacuum tube circuit and a silicon circuit as possible. In fact, [Sebastian] even used literature from the triode version of this detector to figure out the values for some of the components. The only active component is a BF256B JFET; the rest are a small handful of resistors and caps. Construction is in the ever-popular ugly style.

The test setup is simple — a function generator set to 455 kHz and modulated with a 1,000 Hz sine wave. The detector demodulates the audio signal very cleanly, judging by the oscilloscope traces. Just for fun, [Sebastian] also tried a 10.7 MHz carrier with a 1,500 Hz audio modulation, and that worked fine too. He also tried a variation on the circuit with an IF transformer on the input. That circuit works just about the same as the transformerless version, although it does provide a little gain.

Earth-shattering stuff? Probably not. But it does show the fun you can have with a scrap of PCB and a few components, and seems like it could easily be the kind of project that would take you down the RF rabbit hole. Thanks to [Sebastian] for sharing this one with us.

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RF Spectrometer Sees Inside

January 30, 2023 by 7 Comments

Spectrometry is a well-known technique or, more correctly, a set of techniques. We usually think of it as the analysis of light to determine what chemicals are producing it. For example, you can tell what elements are in a star or an incandescent based on the spectrum of light they emit. But you can also do spectroscopy with other ranges of electromagnetic radiation. [Applied Science] shows how to make an RF spectroscope. You can see the video below.

An oscilloscope-resident function generator creates a signal that he feeds to an amplifier because you need a fair amount of power going out. However, you also need to sense a very tiny amount of power coming back, and that requires a special circuit that will block high-power signals while passing low-level signals.

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Q Meter Measures… Q, Of Course

January 19, 2023 by 20 Comments

If you’ve ever dealt with RF circuits, you probably have run into Q — a dimensionless number that indicates the ratio of reactance to resistance. If you ever wanted to measure Q, you could do worse than pick up a vintage Boonton 160A Q meter. [Mikrowave1] did just that and shows us how it works in the video below.

Most often, the Q is of interest in an inductor. A perfect inductor would have zero resistance and be all reactance. If you could find one of those, it would have an infinite Q because you divide the reactance by the resistance. Of course, those inductors don’t exist. You can also apply Q to any circuit with reactance and the video talks about how to interpret Q for tuned circuits. You can also think of the Q number as the ratio of frequency to bandwidth or the dampening in an oscillator. A versatile measurement, indeed.

It sounds as though you could just measure the resistance of a coil and use that to compute Q. But you really need to know the total loss, and that’s not all due to resistance. A meter like the 160A uses a signal generator and measures the loss through the circuit.

The best part of the video is the teardown, though. This old tube gear is oddly beautiful in a strange sort of way. A real contrast to the miniaturized circuits of today. The Q meter is one of those nearly forgotten pieces of gear, like a grid dip oscillator. If you need to wind your own coils, by the way, you could do worse than see how [JohnAudioTech] does it.

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Hackaday Links: December 11, 2022

December 11, 2022 by 13 Comments

“They paved paradise and put up a parking lot.” That might be stretching things a bit, especially when the “paradise” in question is in New Jersey, but there’s a move afoot to redevelop the site of the original “Big Bang Antenna” that has some people pretty upset. Known simply as “The Horn Antenna” since it was built by Bell Labs in 1959 atop a hill in Holmdel, New Jersey, the antenna was originally designed to study long-distance microwave communications. But in 1964, Bell Labs researchers Arno Penzias and Robert Wilson accidentally discovered the microwave remnants of the Big Bang, the cosmic background radiation, using the antenna, earning it a place in scientific history. So far, the only action taken by the township committee has been to authorize a study to look into whether the site should be redeveloped. But the fact that the site is one of the highest points in Monmouth County with sweeping views of Manhattan has some people wondering what’s really on tap for the site. A petition to save the antenna currently has about 3,400 signatures, so you might want to check that out — after all, you don’t know what you’ve got ’til it’s gone.

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Introducing FISSURE: A Toolbox For The RF Hacker

August 27, 2022 by 4 Comments

No matter what the job at hand is, if you’re going to tackle it, you’re going to need the right kit of tools. And if your job includes making sense out of any of the signals in the virtual soup of RF energy we all live in, then you’re going to need something like the FISSURE RF framework.

Exactly what FISSURE is is pretty clear from its acronym, which stands for Frequency Independent SDR-Based Signal Understanding and Reverse Engineering. This is all pretty new — it looks like [Chris Poore] presented a talk at DEFCON a few weeks back about using FISSURE to analyze powerline communications between semi-trucks and their trailers, and they’ve got a talk scheduled for next month’s GNU Radio Conference as well. We’ve been looking through all the material we can find on FISSURE, and it appears to be an RF hacker’s dream come true. They’ve got a few examples on Twitter, like brute-forcing an old garage door opener with a security code set by a ten-position DIP switch, and sending tire pressure monitoring system (TPMS) signals to a car. They also mention some of the framework’s capabilities on the GitHub README; we’re especially interested in packet crafting for various protocols. The video below has some more examples of what FISSURE can do.

It looks like FISSURE could be a lot of fun, and very handy for your RF analysis and reverse engineering work. If you’ve been using Universal Radio Hacker like we have, this looks similar, only more so. We’ll be downloading it soon and giving it a try, so be on the lookout for a hands-on report.

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