Beat Your Coat Hangers Into Antennas, Not Plowshares

April 28, 2020 by Al Williams 37 Comments

If you are looking for a fun project while you are cooped up and you have some spare coathangers, why not try this 4-element Yagi antenna (PDF)? [Pete N8PR] showed it off at his local ham radio club and it looked like something good for a lazy afternoon. If you aren’t a ham, you could adjust it all for a different VHF or UHF frequency.

For the boom, [Pete] mentions you can use wood, but it isn’t weather resistant. He chose half-inch PVC pipe. He also offers you a choice of material for the elements: #8 wire, welding rod, or — our favorite — coat hangers.

This is a big upgrade from a simple dipole or a vertical made from coax. The yagi should have about 8 dBi gain in the direction it is pointing. The center of the boom doesn’t have any elements, so that simplifies mounting. The insulating boom also makes mounting the driven element a breeze.

If you use the coat hangers, we’ve heard an easy way to get them very straight is to put one end on a vise and the other end in a drill chuck (see the video below). The method will weaken the wire, but the elements won’t have much stress. If it worries you, just go slow on the drill and you might consider annealing the wire with a torch afterward.

It would be easy to make this portable like some other designs we’ve seen. If you want the history and theory behind the venerable yagi antenna, you’ll want to revisit this post.

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Odd Crosley Radios From The 1920s

April 24, 2020 by Al Williams 37 Comments

You may sometimes see the Crosley name today on cheap record players, but from what we can tell that company isn’t connected with the Crosley Radio company that was a powerhouse in the field from 1921 to 1956. [Uniservo] looks at two of the very early entries from Crosley: the model VIII and the XJ. You can see the video of both radios, below.

The company started by making car parts but grew rapidly and entered the radio business very successfully in 1921. We can only imagine what a non-technical person thought of these radios with all the knobs and switches, for some it must have been very intimidating.

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So. You Bought A VNA. Now What?

April 23, 2020 by Jenny List 33 Comments

It’s never too late in life for new experiences, but there’s a new experience I had a few weeks ago that I wasn’t expecting. I probably received my first piece of test equipment – a multimeter – in the early 1980s, and since then every time I’ve received a new one, whether an oscilloscope, logic analyser, spectrum analyser or signal generator, I’ve been able to figure out how to use it. I have a good idea what it does, and I can figure out whatever its interface may be to make it do what I want it to. My new experience came when I bought a piece of test equipment, and for the first time in my life didn’t have a clue how to use it.

That instrument is a Vector Network Analyser, or VNA, and it’s worth spending a while going through the basics in case anyone else is in the same position. My VNA is not a superlative piece of high-end instrumentation that cost the GDP of a small country, it’s the popular $50 NanoVNA that has a fairly modest frequency range and performance, but is still a functional VNA that can take useful measurements. But I’m a VNA newbie, what does a VNA do? Continue reading “So. You Bought A VNA. Now What?” →

Lock Your Keys In The Car On Purpose With Aluminum Foil

April 22, 2020 by Bryan Cockfield 74 Comments

[TJ] is a surfer, and drives his car to get to the beach. But when he gets there he’s faced with a dilemma that most surfers have: either put his key in your baggies (shorts) or wetsuit and hope it doesn’t get lost during a wipeout, or stash it on the rear wheel of his car. Hiding the keyfob by the car isn’t an option because it can open the car doors just by being in proximity to the car. He didn’t want to risk losing it to the ocean either, so he built a waveguide of sorts for his key out of aluminum foil that lets him lock the key in the car without locking himself out.

Over a series of trials, [TJ] found out that his car, a 2017 Chevy Cruze, has a series of sensors in it which can determine the location of the keyfob based on triangulation. If it thinks the keyfob is outside of the car, it allows the door to be locked or unlocked with a button on the door handle. If the keyfob is inside the car, though, it prevents the car from locking via the door handles so you don’t accidentally lock yourself out. He found out that he could “focus” the signals of the specific sensors that make the car think the keyfob is outside by building an open Faraday cage.

The only problem now is that while the doors can be locked, they could also can be unlocked. To solve that problem he rigged up an ESP32 to a servo to open and close the opening in the Faraday cage. This still means there’s a hidden device used to activate the ESP32, but odds are that it’s a cheaper device to replace than a modern car key and improves security “through obscurity“. If you have any ideas for improving [TJ]’s build, though, leave them in the comments below. Surfers across the world from [TJ] to the author would be appreciative.

An FPGA And A Few Components Can Make A Radio

April 18, 2020 by Jenny List 18 Comments

There was a time when making a radio receiver involved significant work, much winding of coils, and tricky alignment of circuitry. The advent of Software Defined Radio (SDR) has moved a lot of this into the domain of software, but there is of course another field in which a radio can be created via code. [Alberto Garlassi] has created a radio receiver for the AM and HF bands with a Lattice MachXO2 FPGA and minimal external components.

He describes it as an SDR, which given that it’s created from Verilog, is a term that could be applied to it. But instead of using an SDR topology of ADC and digital signal processing, it implements a surprisingly traditional direct conversion receiver.

It has a quadrature AM demodulator which has a passing similarity to an SDR with I and Q phased signals, but that’s where the similarity ends. Frequency selection is via an oscillator controlled from a serial port, and there is even a PWM amplifier on board that can drive a speaker. The result can be seen in the video below, and as you can hear the direct conversion with quadrature demodulator approach makes for a very effective AM receiver.

If this is a little much but you still fancy a radio with minimal components, you should have a look at the Silicon Labs range of receiver chips.

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As A Matter Of Fact, It’s All Dark

April 16, 2020 by Bryan Cockfield 41 Comments

While the dark side of the moon wasn’t seen by humans until the middle of the 20th century, that side of the moon isn’t always dark, just hidden from view of Earth by a quirk of gravity. The more appropriate name for the other half of the moon is the “far side”, but while it gets just as much sunshine as the near side does it is dark to one thing in particular: man-made radio waves. That, along with the lack of an atmosphere and ionosphere on the moon, makes it a perfect place for a new telescope.

This telescope isn’t like something you’d set up in your back yard, either. It’s more similar to the Aricebo Observatory in Puerto Rico which uses natural topography to help form the telescope. The proposed telescope on the far side of the moon would use a robot to deploy a net along a fairly large crater to act as a parabolic dish, while another robot would suspend the receiver above the crater. The large size is necessary for viewing deep into space, but is also because of the low-frequency radio signals they hope to capture.

Building a dish like this on the moon is sure to be no easy task, especially since remote control on the far side of the moon is difficult for precisely the reasons that make this a good location for a telescope. But with an appropriate amount of funding and some sufficiently autonomous robots it should be possible. Plus, you never know what you’ll find when looking deep into space.

A Dangerous Demonstration Of The Power Of Radio

April 16, 2020 by Dan Maloney 56 Comments

Terrestrial radio may be a dying medium, but there are still plenty of listeners out there. What would a commute to or from work be without a check of “Traffic on the Eights” to see if you need to alter your route, or an update of the scores from yesterday’s games? Getting that signal out to as many listeners as possible takes a lot of power, and this dangerous yet fascinating demo shows just how much power there is on some radio towers.

Coming to us by way of a reddit post, the short video clips show a crew working on a 15,000-Watt AM radio tower. They appear to be preparing to do tower maintenance, which means de-energizing the antenna. As the engineer explains, antennas for AM radio stations in the medium-wave band are generally the entire tower structure, as opposed to the towers for FM and TV stations, which generally just loft the antenna as high as possible above the landscape. The fun starts when the crew disconnects a jumper and an arc forms across the clamp and the antenna feed. The resulting ball of plasma acts like a speaker, letting us clearly hear the programming on the station. It’s like one of the plasma speakers we’ve seen before, albeit exceptionally more dangerous.

It’s an impressive display of the power coursing through broadcast towers, and a vivid reminder to not mess with them. Such warnings often go unheeded, sadly, with the young and foolish paying the price. There’s a reason they put fences up around radio towers, after all.

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