Radio Hacks

1341 Articles

Indoor Antennas Worthy Of 007

December 26, 2020 by 21 Comments

Many ham radio operators now live where installing an outdoor antenna is all but impossible. It seems that homeowner’s associations are on the lookout for the non-conformity of the dreaded ham radio antenna. [Peter] can sympathize, and has a solution based on lessons of spycraft from the cold war.

[Peter] points out that spies like the [Krogers] needed to report British Navy secrets like the plans for a nuclear boomer sub to Russia but didn’t want to attract the attention of their neighbors. In this case, the transmitter itself was so well-hidden that it took MI5 nine days to find the first of them. Clearly, then, there wasn’t a giant antenna on the roof. If there had been, the authorities could simply follow the feedline to find the radio. A concealed spy antenna might be just the ticket for a deed-restricted ham radio station.

The antenna the [Kroger’s] used was a 22-meter wire in the attic of their home. Keep in mind, the old tube transmitters were less finicky about SWR and by adjusting the loading circuits, you could transmit into almost anything. Paradoxically, older houses work better with indoor antennas because they lack things like solar cell panels, radiant barriers, and metallic insulation.

Like many people, [Peter] likes loop antennas for indoor use. He also shows other types of indoor antennas. They probably won’t do as much good as a proper outdoor antenna, but you can make quite a few contacts with some skill, some luck, and good propagation. [Peter] has some period spy radios, which are always interesting to see. By today’s standards, they aren’t especially small, but for their day they are positively tiny. Video after the break.

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SpaceAusScope Team Listens To The Galaxy

December 23, 2020 by 2 Comments

Australia has always had a reputation for astronomy. It is a great site low in the Southern hemisphere and there are lots of sparsely inhabited areas free from light and radio interference. Some of the first video from the Apollo 11 landing, for example, came in from “the dish” — a very large radio telescope down under. Australian hobbyists have formed a group, SpaceAusScope, where teams across Australia are building radio telescopes with the plan — which has been delayed by the pandemic — of collecting data and providing it for public analysis.

A secondary goal of the group is to provide better documentation for amateur radio telescope builders. So even if you don’t live in Australia, you might want to check out their website. It looks as thoughthe documentation will arrive in the future, but there is a very informative blog post from one team member about the helical antenna design most of the teams are using to eavesdrop on the hydrogen line.

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Why Your Scanner Has A Hole In It

December 19, 2020 by 51 Comments

The SDR revolution has completely changed the way radio enthusiasts pursue their hobby, but there is still a space for the more traditional scanning receiver. If you are an American, have you ever noticed that it has a gap in its coverage between 800 and 900 MHz? The curious reason for this is explored by [J. B. Crawford], and it’s a tale of dusty laws relating to a long-gone technology, remaining on the books only because their removal requires significant political effort.

What we might today refer to as “1G” phones used an entirely analogue transmission scheme, with an easily-receivable FM carrier for the voice and extremely low-bandwidth bursts of serial data only for the purposes of managing the call. Listening to these calls was an illegal activity, but for those with the appropriate scanners it became a voyeuristic hobby within a hobby. It even made the world news via the pages of the gossip sheets, when (truthfully or not) it was credited for the leak of a revealing and controversial conversation involving Diana Princess of Wales.

This caused significant worry to the cellular phone companies who understandably didn’t want their product to become associated with insecurity. Thus they successfully petitioned the US Congress to include a clause restricting the capabilities of scanning receivers into another telecoms-related Act, and here we are three decades later with analogue phones a distant memory and the law still on the books. It may be ancient and unnecessary but there is neither the will nor the resources to remove it, so it seems destined to become one of those curious legal oddities that remains on the books for centuries. Whether an RTL-SDR breaks it is something we’ll leave for the lawyers, but the detail in the write-up makes it well worth a read.

Header image: krystof.k (Twitter) & nmuseum, CC BY-SA 3.0.

Tracking Satellites: The Nitty Gritty Details

December 16, 2020 by 18 Comments

If you want to listen to satellites, you have to be able to track them as they pass over the sky. When I first started tracking amateur satellites, computing the satellite’s location in the sky was a part of the challenge. Nowadays, that’s trivial. What’s left over are all the extremely important real-world details.  Let’s take a look at a typical ham satellite tracking setup and see how it all ties together.

Rotators for Steering

The popularity of robotics, 3D printing, and CNC machines has resulted in a deluge of affordable electric motors and drivers. It’s hard to imagine that an electric motor for rotating an antenna would be anything special, but in fact, antenna rotators are non-trivial engineering designs. Most of the challenges are mechanical, not electrical — the antennas that they drive can be huge, have significant wind loading and rotational inertial, and just downright weigh a lot. A rotator design has to consider bearings, weather exposure, all kinds of loads, not just rotational. And usually a brake is required to keep the antenna pointed in windy conditions.

There’s been a 70-some year history of these mechanisms from back in the 1950s when Cornell Dubilier Electronics, the company you know as a capcacitor manufacturer, began making these rotators for television antennas in the 1950s. I was a little surprised to see that the rotator systems you can buy today are not very different from the ones we used in the 1980s, other than improved electronic controls. Continue reading “Tracking Satellites: The Nitty Gritty Details” →

FM Radio From Scratch Using An Arduino

December 4, 2020 by 50 Comments

Building radio receivers from scratch is still a popular project since it can be done largely with off-the-shelf discrete components and a wire long enough for the bands that the radio will receive. That’s good enough for AM radio, anyway, but you’ll need to try this DIY FM receiver if you want to listen to something more culturally relevant.

Receiving frequency-modulated radio waves is typically more difficult than their amplitude-modulated cousins because the circuitry necessary to demodulate an FM signal needs a frequency-to-voltage conversion that isn’t necessary with AM. For this build, [hesam.moshiri] uses a TEA5767 FM chip because of its ability to communicate over I2C. He also integrated a 3W amplifier into this build, and everything is controlled by an Arduino including a small LCD screen which displays the current tuned frequency. With the addition of a small 5V power supply, it’s a tidy and compact build as well.

While the FM receiver in this project wasn’t built from scratch like some AM receivers we’ve seen, it’s still an interesting build because of the small size, I2C capability, and also because all of the circuit schematics are available for all of the components in the build. For those reasons, it could be a great gateway project into more complex FM builds.

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Cheap DIY High Impedance Earphones

December 3, 2020 by 18 Comments

Crystal radios can feel magical, given their ability to tune in audio from distant stations with nothing but the energy from the radio signal itself. However, to achieve this feat, they typically rely on a high-impedance earphone to produce an audible sound with very little current. These earphones are hard to find, and thus can be expensive. However, [Billy] figured out a way to build them on the cheap. 

The build starts with a common piezoelectric buzzer. It’s torn down and the extraneous circuitry inside is removed. The piezo element itself is then directly hooked up to a mono audio jack for use with one of [Billy’s] crystal radios. To make it into a usable earpiece, the tip of a pen is cut off and glued to the buzzer’s plastic housing. Then, a rubber in-ear cup from regular modern earbuds is used to ensure a tight, comfortable fit in the ear.

It’s a great way to build something that’s now hard to source, and we bet that [Billy’s] design is more comfortable than the hard plastic models that shipped in Radio Shack kits in the 90s. Of course, there’s other ways to build high-impedance drivers, as we’ve featured before. And, if you’re looking to build a crystal radio, it’s hard to go past [Billy’s] credit card chip build. Video after the break.

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Gathering Eclipse Data Via Ham Radio

November 30, 2020 by 12 Comments

A solar eclipse is coming up in just a few weeks, and although with its path of totality near the southern tip of South America means that not many people will be able to see it first-hand, there is an opportunity to get involved with it even at an extreme distance. PhD candidate [Kristina] and the organization HamSCI are trying to learn a little bit more about the effects of an eclipse on radio communications, and all that is required to help is a receiver capable of listening in the 10 MHz range during the time of the eclipse.

It’s well-known that certain radio waves can propagate further depending on the time of day due to changes in many factors such as the state of the ionosphere and the amount of solar activity. What is not known is specifically how the paths can vary over the course of the day. During the eclipse the sun’s interference is minimized, and its impact can be more directly measured in a more controlled experiment. By tuning into particular time stations and recording data during the eclipse, it’s possible to see how exactly the eclipse impacts propagation of these signals. [Kristina] hopes to take all of the data gathered during the event to observe the doppler effect that is expected to occur.

The project requires a large amount of volunteers to listen in to the time stations during the eclipse (even if it is not visible to them) and there are only a few more days before this eclipse happens. If you have the required hardware, which is essentially just a receiver capable of receiving upper-sideband signals in 10 MHz range, it may be worthwhile to give this a shot. If not, there may be some time to cobble together an SDR that can listen in (even an RTL-SDR set up for 10 MHz will work) provided you can use it to record the required samples. It’s definitely a time that ham radio could embrace the hacker community.