The Alexanderson Transmitter: Very-low Frequency Radio Rides Again!

Is your ham radio rig made of iron and steel? Is it mechanically driven? Classified as a World Heritage Site? We didn’t think so. But if you’d like to tune in one that is, or if you’re just a ham radio geek in need of a bizarre challenge, don’t miss Alexanderson Day 2015 tomorrow, Sunday, June 28th

The Alexanderson Transmitter design dates back to around 1910, before any of the newfangled tube technology had been invented. Weighing in at around 50 tons, the monster powering the Varberg Radio Station is essentially a high-speed alternator — a generator that puts out 17.2 kHz instead of the 50-60 Hz  that the electric companies give us today.

Most of the challenge in receiving the Alexanderson transmitter broadcasts are due to this very low broadcast frequency; your antenna is not long enough. If you’re in Europe, it’s a lot easier because the station, SAQ, is located in Sweden. But given that the original purpose of these behemoths was transcontinental Morse code transmission, it only seems sporting to try to pick it up in the USA. East Coasters are well situated to give it a shot.

And of course, there’s an app for that. The original SAQrx VLF Receiver and the extended version both use your computer’s sound card and FFTs to extract the probably weak signal from the noise.

We scouted around the net for an antenna design and didn’t come up with anything more concrete than “few hundred turns of wire in a coil” plugged into the mic input.  If anyone has an optimized antenna design for this frequency, post up in the comments?

Thanks [Martin] for the tip!

Building your own SDR-based Passive Radar on a Shoestring

Let’s start off with proof. Below is an animation of a measurement of airplanes and meteors I made using a radar system that I built with a few simple easily available pieces of hardware: two $8 RTL software defined radio dongles that I bought on eBay, and two log-periodic antennas. And get this, the radar system you’re going to build works by listening for existing transmissions that bounce off the targets being measured!

I wrote about this in a very brief blog posting a few years ago. It was mainly intended as a zany little side story for our radio telescope blog, but it ended up raising a lot of interest. Because this has been a topic that keeps attracting inquiries, I’m going to explain how I did the experiment in more detail.

It will take a few posts to show how to build a radar capable of performing these types of measurements. This first part is the overview. In later postings I will go through more detailed block diagrams of the different parts of a passive radar system, provide example data, and give some Python scripts that can be used to perform passive radar signal processing. I’ll also go through strategies to determine that everything is working as expected. All of this may sound like a lot of effort, but don’t worry, making a passive radar isn’t too complicated.

Let’s get started!

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The Biggest Day At Hamvention

We capped off day-2 of the Hamvention with an unexpected rain shower, and some arcing back in the hotel room.  Historically, Saturday is the best attended day of the show.  As normally, we spent most of the day outside in the flea market.  One of our friends allowed us to use his AN/GRC-9 army surplus radio to check into one of the nets.  The radio was powered by hand-crank.  Later, we attended a forum on the construction of HF antennas for camping trips, and obtained parts for our project back in the room. More about that later?  Overall, a great day.

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Hamvention Just Getting Started

For one weekend in May, the landscape of Dayton, Ohio is dominated by ham radio operators. The Dayton Hamvention (“ham-convention”), sponsored by the Dayton Amateur Radio Association, is the preeminent gathering of hams from around the world. This is where industry rolls out new products, friends gather to catch-up, and old equipment is “re-distributed” amongst willing parties in the sprawling swap meet which subsumes the entire Hara Arena parking lot where you can find almost anything and meet some of the most interesting people.

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Inside The Amazon Dash Button

The Amazon Dash Button is a tiny WiFi-enabled device that’s a simple button with a logo on the front. If you get the Tide-branded version, simply press the button and a bottle of laundry detergent will show up at your door in a few days. Get the Huggies-branded version, and a box of diapers will show up. Get the sugar-free Haribo gummi bear-branded version, and horrible evil will be at your doorstep shortly.

[Matt] picked up one of these Dash Buttons for 99 cents, and since a button completely dedicated to buying detergent wasn’t a priority, he decided to tear it apart.

The FCC ID reveals the Amazon Dash Button is a WiFi device, despite rumors of it having a Bluetooth radio. It’s powered by a single AA battery, and [Matt] posted pictures of the entire board.

Since this piece of Amazon electronics is being sold for 99 cents, whatever WiFi radio chip is inside the Dash Button could be used for some very interesting applications. If you have an idea of what chips are being used in [Matt]’s pictures, leave a note in the comments.

FCC Creates Innovation Radio, The Future Of Wireless Broadband

Thirty years ago there was a lot of unused spectrum in the 900MHz,  2.4GHz, and 5.2GHz bands. They were licensed for industrial, scientific, and medical uses since their establishment in 1947. But by the 1980s, these bands were identified as being underused. Spectrum is a valuable resource, and in 1985, the FCC first allowed unlicensed, spread spectrum use of these bands. Anyone who has ever configured a router will know the importance of this slice of spectrum: they’re the backbone of WiFi and 4G. If you’re not connected to the Internet through an Ethernet cable, you have the FCC Commissioners and chairpersons in 1985 to thank for that.

Last week, the FCC unanimously voted to allow the use of spectrum in the 3.5GHz band with the Citizens Broadband Radio Service. This opens up 150 MHz of spectrum from 3550 – 3700MHz for new wireless broadband services. If history repeats itself, you will be connecting to the Internet with the Citizens Broadband Radio Service (CBRS) in a few years.

While the April 17th FCC meeting was the formal creation of the CBRS, this is something that has been in the works for a very long time. The band was originally proposed back in 2012 when portions of spectrum were, like the ISM bands back in the 80s, identified as being underused. Right now, the 3.5GHz band is being used for US military radars and aeronautical navigation, but new advances in frequency management as outlined by commissioner [Clyburn] will allow these to coexist with the CBRS. In the words of Chairman [Wheeler], “computer systems can act like spectrum traffic cops.”

Access to the 3.5GHz spectrum will be divided into three levels. The highest tier, incumbent access, will be reserved for the institutions already using it – military radars and aeronautical radio. The second tier, priority access, will be auctioned and licensed by the FCC for broadband providers via Priority Access Licenses (PALs). The final tier, general authorized access, will be available for you and me, provided the spectrum isn’t already allocated to higher tiers. This is an unprecedented development in spectrum allocation and an experiment to see if this type of spectrum allocation leads to more utilization.

There are, however, unanswered questions. Commissioner [O’Rielly] has said the three-year license with no renewable expectancy could limit commercial uptake of PALs. Some commentors have claimed the protocols necessary for the CBRS to coexist with WiFi devices does not exist.

Still, the drumbeat demanding more and more spectrum marches on, and 2/3rds of the 150MHz made available under this order was previously locked up for the exclusive use of the Defense Department. Sharing spectrum between various users is the future, and in this case has the nice bonus of creating a free citizens band radio service.

You can read the full order here, or watch the stream of the April 17th meeting.

TV Broadcasts from Outer Space

According to ARISS (Amateur Radio on the International Space Station), the ISS will be sending us images using slow-scan TV on April 11th in honor of Russian cosmonaut Yuri Gagarin’s birthday. Tune in and you’ll get to see 12 different commemorative images from space, and of course bragging rights that you directly received them with your radio setup.

For those who aren’t Ham radio types, slow-scan TV (SSTV) is a radio mode where the pixels in an image are sent by encoding the brightness and/or color as a tone, a lot like a modem, fax machine, or the data cassette tapes of yore.

The ISS uses PD-180 which is a color mode where each pixel’s red, green, and blue values are encoded in a pitch between 1500 and 2300 Hz. Each image takes just over three minutes to transmit, meaning you’ll have to track the ISS pretty well as it travels across the sky. But don’t fret, they send each message for around an hour, so you have a good chance to receive it. (We’ll be the first to admit that a frame rate of one frame in 187 seconds isn’t really “TV”, but that’s what they call it.)

SSTV’s use in the space program goes back even before the moon landing, but with modern software-defined radio setups, it all becomes a lot more convenient to receive. The ISS folks do this periodically as a service to the amateur radio community, so it’s a good time to try out your chops.

We’ve covered ARISS before, but Yuri’s birthday is always a good reason to celebrate the folks out there. And if you need a reminder of when to look up, this hack right here has you covered.

If you do receive some images, you can upload them to the ARISS Gallery.  Or you can just hit refresh to see them as others post them up.