Imagine that you’re coordinating a large scale search-and-rescue mission in a cave. You need to know where all your groups are, and whether or not they’ve found anything. But how do they all communicate to the command center?
You’d guess radio, but you’d guess wrong. Radio doesn’t propagate well at all in a maze of twisty passages, all alike; rocks absorb radio waves, especially in the VHF/UHF range that’s best suited for most small radios. In the past, you’d run wire and transmit along it. This article runs through the options in detail. But adding miles of wire to your already heavy caving and climbing gear is a nuisance or worse.
Since the discovery that some USB TV tuner dongles could be used to monitor radio waves across a huge amount of spectrum, the software-defined radio world has exploded with interest. The one limiting factor, though, has been that the dongles can only receive signals; they can’t transmit them. [Evariste Okcestbon, F5OEO] (if that is his real name! Ok c’est bon = Ok this is good) has written some software that will get you transmitting using SDR with only a Raspberry Pi and a wire.
There have been projects in the past that use a Pi to broadcast radio (PiFM), but this new software (RPiTX) takes it a couple steps further. Using just an appropriately-sized wire connected to one of the GPIO pins, the Raspberry Pi is capable of broadcasting using FM, AM, SSB, SSTV, or FSQ signals. This greatly increases the potential of this simple computer-turned-transmitter and anyone should be able to get a lot of use out of it. In the video demo below the break, [Evariste] records a wireless doorbell signal and then re-transmits it using just the Rasbperry Pi.
The name of the game in rocketry or ballooning is weight. The amount of mass that can be removed from one of these high-altitude devices directly impacts how high and how far it can go. Even NASA, which estimates about $10,000 per pound for low-earth orbit, has huge incentives to make lightweight components. And, while the Santa Barbara Hackerspace won’t be getting quite that much altitude, their APRS-enabled balloon/rocket tracker certainly helps cut down on weight.
Tracksoar is a 2″ x .75″ x .5″ board which weighs in at 45 grams with a pair of AA batteries and boasts an ATmega 328P microcontroller with plenty of processing power for its array of on-board sensors. Not to mention everything else you would need like digital I/O, a GPS module, and, of course, the APRS radio which allows it to send data over amateur radio frequencies. The key to all of this is that the APRS module is integrated with the board itself, which saves weight over the conventional method of having a separate APRS module in addition to the microcontroller and sensors.
As far as we can see, this is one of the smallest APRS modules we’ve ever seen. It could certainly be useful for anyone trying to save weight in any high-altitude project. There are a few other APRS projects out there as well but remember: an amateur radio license will almost certainly be required to use any of these.
Ever been out in a big field and need to tell something to Joe at the other end? If you’re lucky Sally is in between and you can shout to Sally to tell Joe your message. Maybe Joe shouts back to Sally in reply.
The higher radio frequencies, say 50 MHz and above, typically only propagate within line of sight. Add in limited power and antennas from a hand-held, typically under 5 watts and the ubiquitous ‘rubber ducky’ antenna, and you cannot talk very far. Mobile rigs in vehicles with 50 watts and larger antennas do better but in reality they don’t help all that much.
What really makes an improvement is height to improve range. Height provides a longer line of sight with fewer obstructions. Hams created repeaters and put them on towers, buildings or hill tops to expand the radio horizon. The ultimate repeaters are space satellites. Can’t get much higher than that. A close second are balloons going to near space altitudes with repeaters which will provide multi-state coverage.
Besides providing height, a repeater will also have higher output power and much better antennas, especially important for receiving weak signals from distant handhelds. A signal comes in and is repeated back out on a slightly different frequency. All modern ham gear on these frequencies is setup to handle this offsetfrequency operation.
Whether hams came up with the idea is arguable, but they were certainly there during the early days.
The Rigol DS1000 series of oscilloscopes are popular with hobbyists for good reason: they provide decent specs at a low price. However, their spectrum analysis abilities are lacking. While these scopes do have a Fast Fourier Transform (FFT) function, it’s limited and nearly useless for RF.
[Rich] wanted a spectrum analyzer for amateur radio purposes, but didn’t want to build his own sampling hardware for it. Instead, he wrote PyDSA, a software spectrum analyzer for Rigol DS1000 oscilloscopes. This tool uses the USB connection on the scope to fetch samples, and does the number crunching on a far more powerful PC. It’s able to plot a 16,000 point FFT at two sweeps per second when run on a decent computer.
PyDSA is a Python script that makes use of the Virtual Instrument Software Architecture (VISA) interface to control the scope and fetch the sample data. Fortunately there’s some Python libraries that take care of the protocol.
[Rich] is now able to use his scope to measure amateur radio signals, which makes a nice companion to his existing Teensy based SDR project. If you have a Rigol, you can grab the source on Github and try it out.
If you are a hacker, you might consider ham radio operators as innovative. Most people, however, just see them as cheap. So it is no surprise that hams like [jmharvey] will build an antenna analyzer from a DDS module and an Arduino instead of dropping a few hundred dollars on a commercial unit. As he points out, you probably only need an analyzer for a day or two while you set up an antenna. Unless you are a big time antenna builder, the unit will then sit idle on the shelf (or will wind up on loan to hams even cheaper than you are).
The design is rooted in another proven design, but changed to take advantage of parts he happened to have on hand. Although the build is on a universal circuit board, [jmharvey] used Eagle to lay out the circuit as though it were a PCB. Since placement can be important with an RF circuit, this isn’t a bad idea. It’s always easier to move stuff around on the screen than on the perf board.
Since this is a no frills, unit, you are expected to grab the output from the Arduino and manually put it in a spreadsheet to plot the results. There is another version of the Arduino code that drives an OLED screen, although you still need a PC to kick the process off. One interesting feature of the Arduino code is how it deals with the nonlinear nature of the diodes used in the circuit. After plotting the values with known loads, [jmharvey] broke the diode operation into three regions and used different equations for each region. Even so, he warns that readings higher than 1:1 VSWR are only accurate to 10% or 20% – still good enough for ham shack use.
If you want an antenna analyzer for $40 (or less, if you have a good stock of parts) this looks like a worthwhile project. If, however, you want to repurpose it to Rickroll your neighbor’s AM radio, you might want to go with the commercial unit.
Click past the break to see the analyzer in action.
A few days ago, [Ben Caudill] of Rhino Security was scheduled to give a talk at DEFCON. His project, ProxyHam, is designed for those seeking complete anonymity online. Because IP addresses can be tied to physical locations, any online activities can be tracked by oppressive regimes and three letter government agencies. Sometimes, this means doors are breached, and “seditious” journalists and activists are taken into custody.
With the ProxyHam, the link between IP addresses and physical locations is severed. ProxyHam uses a 900MHz radio link to bridge a WiFi network over miles. By hiding a ProxyHam base station in a space with public WiFi, anyone can have complete anonymity online; if the government comes to take you down, they’ll first have to stop at the local library, Starbucks, or wherever else has free WiFi.
[Ben Caudill] will not be giving a talk at DEFCON. It wasn’t the choice of DEFCON organizers to cancel the talk, and it wasn’t his employers – [Ben] founded and is principal consultant at Rhino Security. The talk has been killed, and no one knows why. Speculation ranges from National Security Letters to government gag orders to a far more pedestrian explanations like, “it doesn’t work as well as intended.” Nevertheless, the details of why the ProxyHam talk was cancelled will never be known. That doesn’t mean this knowledge is lost – you can build a ProxyHam with equipment purchased from Amazon, Newegg, or any one of a number of online retailers.