antenna

248 Articles

Beat Your Coat Hangers Into Antennas, Not Plowshares

April 28, 2020 by 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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The Rusty Nail Award For Worst WiFi Antenna

April 23, 2020 by 34 Comments

In general, you get what you pay for, and if what you pay for is a dollar-store WiFi antenna that claims to provide 12 dBi of signal gain, you shouldn’t be surprised when a rusty nail performs better than it.

The panel antenna that caught [Andrew McNeil]’s eye in a shop in Rome is a marvel of marketing genius. He says what caught his eye was the Windows Vista compatibility label, a ploy that really dates this gem. So too does the utterly irrelevant indication that it’s USB compatible when it’s designed to plug into an SMA jack on a WiFi adapter. [Andrew]’s teardown was uninspiring, revealing just a PCB with some apparently random traces to serve as the elements of a dipole. We found it amusing that the PCB silkscreen labels the thru-holes as H1 to H6, which is a great way to make an uncrowded board seem a bit more important.

The test results were no more impressive than the teardown. A network analyzer scan revealed that the antenna isn’t tuned for the 2.4-GHz WiFi band at all, and practical tests with the antenna connected to an adapter were unable to sniff out any local hotspots. And just to hammer home the point of how bad this antenna is, [Andrew] cobbled together a simple antenna from an SMA connector and a rusty nail, which handily outperformed the panel antenna.

We’ve seen plenty of [Andrew McNeil]’s WiFi antenna videos before, like his umbrella and tin can dish. We like the sanity he brings to the often wild claims of WiFi enthusiasts and detractors alike, especially when he showed that WiFi doesn’t kill houseplants. We can’t help but wonder what he thinks about the current 5G silliness.

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Lock Your Keys In The Car On Purpose With Aluminum Foil

April 22, 2020 by 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.

A Dangerous Demonstration Of The Power Of Radio

April 16, 2020 by 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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A Patch Antenna Is Just A Rectangle, It Should Be Easy To Design, Right?

March 1, 2020 by 20 Comments

If a grizzled RF engineer who bears the soldering-iron scars of a thousand projects could offer any advice, it would be that microwave antennas are not a field to be entered into lightly. Much heartache is to be saved by using an off-the-shelf design, and only the foolhardy venture willingly down the stripline into the underworld of complex microwave resonances.

But every would-be microwave designer has to start somewhere, and for [Adam Gulyas] that start came with a 2.4 GHz patch antenna. His write-up is a fascinating tale of the challenges and pitfalls of creating something which is deceptively simple at first sight but which becomes significantly more complex as he characterizes his design made real as a PCB.

The process started with a set of calculations to derive the patch dimensions and a bit of PCB work adding a stripline feed. This was produced on a PCB, a normal 1.6mm thick FR4 fiberglass board. When hooked up to a VNA its impedance was all wrong. Further, it had a resonance at the required frequency but also unexpected ones at 3.7 and 4.6 GHz. Simulation of the design also yielded a different resonance from the one calculated, and discussing it with others yielded the conclusion that the feed might be at fault. He ended up using an inset feed, with a co-axial cable emerging away from the edge of the patch, and was able to achieve a far better result.

We can all learn something from [Adam]’s write-up, and we salute him for staying the course to get the design to a usable point. It would be interesting to see the same antenna produced from a more consistent dielectric material than generic FR4. Meanwhile, if you are interested in microwave RF design, take a look at Michael Ossmann’s primer on the subject.

DIY Radio Telescopes Hack Chat

February 10, 2020 by 8 Comments

Join us on Wednesday, February 12 at noon Pacific for the DIY Radio Telescopes Hack Chat with James Aguirre!

For most of history, astronomers were privy to the goings-on in the universe only in a very narrow slice of the electromagnetic spectrum. We had no idea that a vibrant and wondrous picture was being painted up and down the wavelengths, a portrait in radio waves of everything from nearly the moment of creation to the movement of galaxies. And all it took to listen in was an antenna and a radio receiver.

Over the years, radio telescopes have gotten more and more sophisticated and sensitive, and consequently bigger and bigger. We’re even to the point where one radio telescope often won’t cut it, and astronomers build arrays of telescopes spread over miles and miles, some with antennas that move around on rails. In the search for signals, radio astronomy has become the very definition of “Big Science.”

But radio astronomy doesn’t have to be big to be useful. James Aguirre, an astronomer at the University of Pennsylvania, spends his days (and nights) studying the radio universe with those big instruments. But he’s also passionate about down-scaling things and teaching everyone that small radio telescopes can be built on the cheap. His Mini Radio Telescope project uses a cast-off satellite TV dish and a couple of hundred bucks worth of readily available gear to scan the skies for all sorts of interesting phenomena.

Dr. Aguirre will join us on the Hack Chat to discuss all things radio astronomy, and how you can get in on the radio action on the cheap. Chances are good your junk pile — or your neighbor’s roof — has everything you need, and you might be surprised how approachable and engaging DIY radio astronomy can be.

join-hack-chatOur Hack Chats are live community events in the Hackaday.io Hack Chat group messaging. This week we’ll be sitting down on Wednesday, February 12 at 12:00 PM Pacific time. If time zones have got you down, we have a handy time zone converter.

Click that speech bubble to the right, and you’ll be taken directly to the Hack Chat group on Hackaday.io. You don’t have to wait until Wednesday; join whenever you want and you can see what the community is talking about. Continue reading “DIY Radio Telescopes Hack Chat” →

Ham Antennas From MIT

February 4, 2020 by 16 Comments

Dealing with an antenna is one of those topics we never feel like we know enough about. MIT had a live stream of [Dr. Kiersten Kerby-Patel] discussing antennas in a talk, sponsored by the ham radio club on campus. You can see the recording below.

The main assertion of the presentation is that everything is a dipole unless it is a loop. Although the professor probably deals with antennas at an extremely high theoretical level, she did a great job of keeping it aimed at ham radio operators.

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