A handheld device to measure electromagnetic fields

Measuring Electromagnetic Fields With Just An Arduino And A Piece Of Wire

Electromagnetic interference problems can be a real headache to debug. If you need to prove what causes your WiFi to slow down or your digital TV signal to drop, then the ability to measure electromagnetic fields (EMF) can be a big help. Professional equipment is often very expensive, but building an EMF detector yourself is not even that difficult: just take a look at Arduino expert [Mirko Pavleski]’s convenient hand-held electromagnetic field detector.

The basic idea is quite simple: connect an antenna directly to an Arduino’s analog input and visualize the signal that it measures. Because the input of an ADC is high impedance, it is very sensitive to any stray currents that are picked up by the antenna. So sensitive in fact, that a resistor of a few mega-Ohms to ground is required to keep the sensor from triggering on any random kind of noise. [Mirko] made that resistance adjustable with a few knobs and switches so that the detector can be used in both quiet and noisy environments.

Making the whole device work reliably was an interesting exercise in electromagnetic engineering: in the first few iterations, the detector would trigger off its own LEDs and buzzer, trapping itself in a never-ending loop. [Mirko] solved this by encasing the Arduino inside a closed, grounded metal box with only the required wires sticking out. The antenna’s design was largely based on trial-and-error; the current setup with a 7 cm x 3 cm piece of aluminium sheet seemed to work well.

While this is not a calibrated professional-grade instrument, it should come in handy to find sources of interference, or even simply to locate hidden power cables. You can view this as a more advanced version of [Mirko]’s Junk Box EMF Detector; if you have a second Arduino lying around, you can use that one to generate interference instead. Continue reading “Measuring Electromagnetic Fields With Just An Arduino And A Piece Of Wire”

Wind-Up Tape Measure Transformed Into Portable Ham Antenna

If there’s one thing that amateur radio operators are good at, it’s turning just about anything into an antenna. And hams have a long history of portable operations, too, where they drag a (sometimes) minimalist setup of gear into the woods and set up shop to bag some contacts. Getting the two together, as with this field-portable antenna made from a tape measure, is a double win in any ham’s book.

For [Paul (OM0ET)], this build seems motivated mainly by the portability aspect, and less by the “will it antenna?” challenge. In keeping with that, he chose a 50-meter steel tape measure as the basis of the build. This isn’t one of those retractable tape measures, mind you — just a long strip of flexible metal on a wind-up spool in a plastic case. His idea was to use the tape as the radiator for an end-fed halfwave, or EFHW, antenna, a multiband design that’s a popular option for hams operating from the 80-m band down to the 10-m band. EFHW antennas require an impedance-matching transformer, a miniature version of which [Paul] built and tucked within the tape measure case, along with a BNC connector to connect to the radio and a flying lead to connect to the tape.

Since a half-wave antenna is half the length of the target wavelength, [Paul] cut off the last ten meters of the tape to save a little weight. He also scratched off the coating on the tape at about the 40-meter mark, to make good contact with the alligator clip on the flying lead. The first video below details the build, while the second video shows the antenna under test in the field, where it met all of the initial criteria of portability and ease of deployment.

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Retrotechtacular: Measuring TV Audiences With The “Poll-O-Meter”

It may come as a shock to some, but TV used to be a big deal — a very big deal. Sitting down in front of the glowing tube for an evening’s entertainment was pretty much all one had to do after work, and while taking in this content was perhaps not that great for us, it was a goldmine for anyone with the ability to monetize it. And monetize it they did, “they” being the advertisers and marketers who saw the potential of the new medium as it ramped up in early 1950s America.

They faced a bit of a problem, though: proving to their customers exactly how many people they were reaching with their ads. The 1956 film below shows one attempt to answer that question with technology, rather than guesswork. The film features the “Poll-O-Meter System,” a mobile electronic tuning recorder built by the Calbest Electronics Company. Not a lot of technical detail is offered in the film, which appears aimed more at the advertising types, but from a shot of the Poll-O-Meter front panel (at 4:12) and a look at its comically outsized rooftop antenna (12:27), it seems safe to assume that it worked by receiving emissions from the TV set’s local oscillator, which would leak a signal from the TV antenna — perhaps similar to the approach used by the UK’s TV locator vans.

The Poll-O-Meter seems to have supported seven channels; even though there were twelve channels back in the day, licenses were rarely granted for stations on adjacent channels in a given market, so getting a hit on the “2-3” channel would have to be considered in the context of the local market. The Poll-O-Meter had a charming, homebrew look to it, right down to the hand-painted logos and panel lettering. Each channel had an electromechanical totalizing counter, plus a patch panel that looks like it could be used to connect different counters to different channels. There even appears to be a way to subtract counts from a channel, although why that would be necessary is unclear. The whole thing lived in the back of a 1954 VW van, and was driven around neighborhoods turning heads and gathering data about what channels were being watched “without enlisting aid or cooperation of … users.” Or, you know, their consent.

It was a different time, though, which is abundantly clear from watching this film, as well as the bonus ad for Westinghouse TVs at the end. The Poll-O-Meter seems a little silly now, but don’t judge 1956 too hard — after all, our world is regularly prowled by equally intrusive and consent-free Google Street View cars. Still, it’s an interesting glimpse into how one outfit tried to hang a price tag on the eyeballs that were silently taking in the “Vast Wasteland.”

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Affordable HF Loop Antenna Reviewed

Modern ham radio operators often face restrictions on antennas. This has made small antennas more popular, despite some limitations. [Tech Minds] reviews the GA-450 indoor active HF loop antenna and finds it better than expected. You can see the video review below.

You can’t expect a little antenna to perform as well as giant skyhook. However, for such a small loop covering 3 to 30 MHz, the antenna seems to perform very well. We like that the active part of it has a rechargeable battery. Obviously, you will only want to use this antenna for receiving, but it would be a great pairing for an HF-capable software defined radio (SDR). Even just in the window sill with half gain, it was able to pick up quite a bit of signal on the 40 meter and 20 meter ham bands. According to the video, performance below 7 MHz was lackluster, but it worked nicely at higher frequencies.

The loop is directional and you can rotate the loop on the base to zero in on a particular signal. Of course, if the antenna were up in the air, it might be harder to rotate unless you work out something with a motor. If all you want to do is receive and you have a budget of under $100, this looks like it would be a nice portable option.

You can build your own loop and loop-like antennas, of course. Some of them can be quite portable.

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Ham Antenna Fits Almost Anywhere

[G3OJV] knows the pain of trying to operate a ham radio transmitter on a small lot. His recent video shows how to put up a workable basic HF antenna in a small backyard. The center of the system is a 49:1 unun. An unun is like a balun, but while a balun goes from balanced line to an unbalanced antenna, the unun has both sides unbalanced. You can see his explanation in the video below.

The tiny hand-size box costs well under $40 or $50 and covers the whole HF band at up to 200 W. The video shows the inside of the box which, as you’d expect, is a toroid with a few turns of wire.

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Frame Antenna Works The Low Bands

The lower the frequency of radio transmission, the more antenna that will be needed in general. [OM0ET] wanted to work the 80M to 20M ham bands and decided to turn to a frame antenna. You can see the project in the video below.

The antenna looks a lot like a magnetic loop antenna. The one in the video has seven loops forming a 520mm square. The loop is, of course, an inductor and by removing some insulation, the operator can clip a lead at different points to control the inductance. A variable capacitor resonates the antenna, so there is definitely tuning required.

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An NFC Antenna Ring With A Chip As Its Jewel

Contactless payment by means of NFC-enabled bank cards has made our everyday transactions far more convenient over the last decade, but there still remains the tedious task of finding the card and waving it over the reader. Maybe embedded chips are a step too far for many of us, but how about a bank card in a wearable such as a ring? [Jonathan Limén] shows us how, by taking the NFC chip module from a bank card and mounting it on a ring with a wire coil antenna embedded within it.

The chip in a bank card comes mounted on a small thin PCB with contacts on one side and a coil on the other that serves as its antenna. It’s not sensitive enough to work reliably with most card readers, so the card incorporates a separate printed circuit layer that forms a large-sized tuned circuit which couples to the chip antenna. After taking us through the removal of the chip from the card with some acetone, he proceeds to create a replacement for the card antenna by winding a wire coil round the ring. This becomes a trial-and-error process, but in the end, the result is a working NFC payment ring.

We quite like this idea, but would be tempted to both take away some of the trial and error with a vector network analyzer, and run a couple of turns of the wire as a closer coupling coil for the chip. This is a subject we’ve looked at before here at Hackaday, and we wouldn’t mind having another go at it.