Cold War Listening Post Antennas

With a UHF antenna, it is easy to rotate a directional antenna to find the bearing to a transmitter. But at HF, it is more common to use an array of antennas that you can electrically switch as well as analyze the phase information between the elements. [Ringway Manchester] has a look at the “elephant cage” antenna used by the US Iron Horse listening network from the 1950s. You can see a video about the giant antenna system, the AN/FLR-9.

Technically, the ring of concentric antenna elements forms a Wullenweber antenna. The whole thing consists of three rings built on a ground screen nearly 1,500 feet across. The outer ring covers from 1.5 to 6 MHz or band A. The band B ring in the center covers 6 to 18 MHz. The inner ring covers band C which was from 18 to 30 MHz.  Band A was made up of 48 monopoles while band B used 96 elements. The much smaller band C elements were 48 pairs of horizontally polarized dipoles. Continue reading “Cold War Listening Post Antennas”

Cat Feeder Depends On RFID To Keep The Peace At Dinnertime

Anyone with more than one cat can tell you that the joy mischief they bring into your life is much more than twice that of a single cat. And if those felines have different dietary needs, you can end up where [Benjamin Krejci] found himself, which resulted in this fancy RFID cat feeder.

For a little backstory, [Ben]’s furry friends [Luna] and [Fermi] have vastly different eating styles, with the former being a grazer and the latter more of a “disordered eater,” to put it politely. [Fermi] tends to eat until she vomits, which is fun, and muscles her pickier sister away from the bowl if there’s anything left in it. [Ben]’s idea was to leverage [Luna]’s existing RFID chip, which he figured would be a breeze. But the vet-inserted chip is designed to be read by a high-power reader directly in contact with the cat’s skin, which made reliably reading the chip a challenge.

Several round of design iteration resulted in the current configuration, with a large antenna coil poised above and behind the food dispenser. [Luna] has no choice but to put the back of her neck and shoulder blades almost directly in contact with the coil, which makes it easier to read the 134.2-kHz chip with a long-distance RFID module. If [Luna]’s chip is found, the lid on the food bowl opens gently and quietly, so as not to spook the mild-mannered cat. The lid stays open as long as [Luna] is in place thanks to some IR sensors, but as soon as she backs out, the lid comes down to keep [Fermi] from gorging herself.

Hats off to [Ben] for working through the problem and coming up with what looks like a fine solution. We suppose he could have tried something easier like weighing the two cats to distinguish between them, but this seems like a cleaner solution to us.

Antenna Hidden In Holiday Lights Skirts HOA Rules

For all their supposed benefits, homeowner’s associations (HOAs) have a reputation of quickly turning otherwise quaint neighborhoods into a sort of Stanford prison experiment, as those who get even the slightest amount of power often abuse it. Arbitrary rules and enforcement abound about house color, landscaping, parking, and if you’ve ever operated a radio, antennas. While the FCC (at least as far as the US is concerned) does say that HOAs aren’t permitted to restrict the use of antennas, if you don’t want to get on anyone’s bad side you’ll want to put up an antenna like this one which is disguised as a set of HOA-friendly holiday lights.

For this build, a long wire is hidden along with a strand of otherwise plain-looking lights. While this might seem straightforward at first, there are a few things that need to be changed on the lighting string in order to make both the antenna and the disguise work. First, the leads on each bulb were removed to to prevent any coupling from the antenna into the lighting string. Clipping the leads turns what is essentially a long wire that might resonate with the antenna’s frequency into many short sections of wire which won’t have this problem. This also solves the problem of accidentally illuminating any bulbs when transmitting, as the RF energy from the antenna could otherwise transfer into the lighting string and draw attention from the aforementioned HOA.

Tests of this antenna seemed to show surprising promise while it was on the ground, but when the string and antenna was attached to the roof fascia the performance dropped slightly, presumably because of either the metal drip edge or the gutters. Still, the antenna’s creator [Bob] aka [HOA Ham] had excellent success with this, making clear contacts with other ham radio operators hundreds of miles away. We’ve shared another of [Bob]’s HOA-friendly builds below as well which hides the HF antenna in the roof’s ridge vent, and if you’re looking for other interesting antenna builds take a look at this one which uses a unique transformer to get wide-band performance out of an otherwise short HF antenna.

Continue reading “Antenna Hidden In Holiday Lights Skirts HOA Rules”

Hunting For Space Pirates

Ever since the first artificial satellite was launched into orbit, radio operators around the world have been tuning in to their space-based transmissions. Sputnik 1 only sent back pulses of radio waves, but in the decades to follow ever more advanced radio satellites were put into service that could support two-way communications from Earth to space and back again.

Some of these early satellites were somewhat lacking in security, though, and have been re-purposed by various pirates around the world for their own ends. [Gabe] aka [saveitforparts] is here to show us how to hunt for those pirates and listen in on their radio traffic.

Pirates on these satellites have typically used them for illicit activities, and it is still illegal to use them for non-governmental or non-military purposes, so [Gabe] notes that he will only be receiving, not transmitting. The signals he is tuning in to are VHF transmissions, specifically around 220 MHz. That puts them easily within the reach of the RTL-SDR and common ham radio equipment, but since they are coming from space a more directional antenna is needed. [Gabe] quickly builds a Yagi antenna from scrap, tuned specifically to 255 MHz, and mounts it to an old remote-controlled security camera mount which allows him to point it exactly at the satellite and monitor transmissions.

From there he is able to pick up what looks like a few encrypted and/or digital transmissions, plus analog transmissions of likely pirates speaking a language he guesses to be Portuguese. He also hears what he thinks is a foreign TV broadcast, but oddly enough turns out to be NPR. These aren’t the only signals in space to tune to, either. There are plenty of purpose-built ham radio satellites available for any licensed person to use, and we’ve also seen this other RTL-SDR configured to snoop on Starlink signals.

Continue reading “Hunting For Space Pirates”

Electromagnetic Mechanism Makes Reconfigurable Antenna

Antennas are a key component to any RF gadget. But antennas often only perform well over a narrow band of frequencies. For some applications, this is acceptable, but often you would like to reconfigure an antenna for different bands. Researchers at Penn State say they’ve developed a tunable antenna using compliant mechanisms and electromagnets. The new scalable design could work in small areas to provide frequency agility or beamforming.

The prototype is a circular patch antenna made with 3D printing. If you want to read the actual paper, you can find it on Nature Communications.

A compliant mechanism is one that achieves force and motion through elastic body deformation. Think of a binder clip. There’s no hinge or bearing. Yet the part moves in a useful way, using its own deformation to open up or grip papers tightly. That’s an example of a compliant mechanism. This isn’t a new idea — the bow and arrow are another example. However, because 3D printing offers many opportunities to build and refine devices like this, interest in them have increased in recent years.

We couldn’t help but notice that the antenna is a variation of a “compliant iris” like the one in the video below. You can find designs for these online for 3D printing, so if you wanted to experiment,  you might think about starting there.

We’ve looked at compliant mechanisms before. Why would you want better chip-scale antennas? Why, indeed.

Continue reading “Electromagnetic Mechanism Makes Reconfigurable Antenna”

Antenna Mount Designed For On-The-Go SDR

Software-defined radio is all the rage these days, and for good reason. It eliminates or drastically reduces the amount of otherwise pricey equipment needed to transmit or even just receive, and can pack many more features than most affordable radio setups otherwise would have. It also makes it possible to go mobile much more easily. [Rostislav Persion] uses a laptop for on-the-go SDR activities, and designed this 3D printed antenna mount to make his radio adventures much easier.

The antenna mount is a small 3D printed enclosure for his NESDR Smart Dongle with a wide base to attach to the back of his laptop lid with Velcro so it can easily be removed or attached. This allows him to run a single USB cable to the dongle and have it oriented properly for maximum antenna effectiveness without something cumbersome like a dedicated antenna stand. [Rostislav] even modeled the entire assembly so that he could run a stress analysis on it, and from that data ended up filling it with epoxy to ensure maximum lifespan with minimal wear on the components.

We definitely appreciate the simple and clean build which allows easy access to HF and higher frequencies while mobile, especially since the 3D modeling takes it a step beyond simply printing a 3D accessory and hoping for the best. There’s even an improved version on his site here. To go even one step further, though, we’ve seen the antennas themselves get designed and then 3D printed directly.

AntRunner Is The Satellite Antenna Mount You Need To Take With You

It stands to reason, that should you wish to communicate with a satellite, whatever antenna you use should point at that satellite. Some of us have done this by hand, following the bright dot of the space station in the night sky. Still, for anything more serious than trying to catch a fleeting SSTV image, a more robust solution is called for. In other words, a motorized antenna rotator, and AntRunner from [Wuxx] is just the ticket. Better still, it’s portable for those /p operating sessions off the beaten track.

The rotator itself is an az-el design with a couple of geared stepper motors. The full mechanism design has been published, but it shouldn’t be too difficult to copy. The interesting part is the controller and software, which can work with Gpredict, Hamlib, and SDR for automated satellite tracking. The controller is as straightforward as an ESP32 running the ESP port of GRBL.

So here’s a portable antenna rotator that’s accessible and widely supported, what’s not to like? As you might expect though, it’s not the first we’ve seen. In fact, the 2014 Hackaday Prize was won by SatNOGs, which includes a 3D printed antenna positioner.

Thanks [Abe Tusk] for the tip!