Categorie: Hackaday

Hundreds of years from now, the story of humanity’s inevitable spread across the solar system will be a collection of engineering problems solved, some probably in heroic fashion. We’ve already tackled a lot of these problems in our first furtive steps into the wider galaxy. Our engineering solutions have taken humans to the Moon and back, but that’s as far as we’ve been able to send our fragile and precious selves.

While we figure out how to solve the problems keeping us trapped in the Earth-Moon system, we’ve sent fleets of robotic emissaries to do our exploration by proxy, to make the observations we need to frame the next set of engineering problems to be solved. But as we reach further out into the solar system and beyond, our exploration capabilities are increasingly suffering from communications bottlenecks that restrict how much data we can ship back to Earth.

We need to find a way to send vast amounts of data back as quickly as possible using as few resources as possible on both ends of the communications link. Doing so may mean turning away from traditional radio communications and going way, way up the dial and developing practical means for communicating with X-rays.

The Tyranny of Physics

The essential problems with deep space communications come from two sources – the inverse-square law and information theory. The inverse-square law states that the amount of energy at the receiving end of a radio communications link is inversely proportional to the square of the distance to the transmitter. Basically, radio waves spread out from the source and at very great distances tend to diminish into the background noise. That’s why deep-space communications networks tend to have large antennas on both ends of the link, to gather and focus as much of the weak signal as possible, as well as to be able to transmit a powerful and narrowly focused beam.

Information theory tells us that more data can be packed into higher frequency signals than lower frequencies. Early satellites didn’t need much bandwidth to do their jobs, so VHF and UHF radios were generally sufficient. But as spacecraft became more sophisticated and the amount of data they needed to send back increased, their communications links began shifting gradually up the electromagnetic spectrum into the microwave region. The Voyager probes, currently in interstellar space, have an uplink using 2.1 GHz for the relatively low-bandwidth tasks of vehicle control, with a downlink at 8.1 GHz, reflecting the increased bandwidth needed to send scientific data back to Earth.

For as stunning an engineering achievement as Voyager has been, and notwithstanding the fact that it’s still working more than 40 years after launch, its radio gear only barely supports its interstellar mission. To be fair, Voyager was never meant to last this long, and every bit of data that makes it back to Earth is just icing on the cake. But for future missions specifically designed for interstellar space, sending back enough data to make such missions feasible will require more bandwidth.

Small, Bright, and Fast

In late April, NASA is sending a pallet of gear up to the ISS, and one of the experiments stashed in the cargo is meant to explore the potential for X-ray communications, or XCOM, for deep space. The Modulated X-Ray Source (MXS) is a compact X-ray transmitter that will be mounted outside the space station. The receiver for this experiment is already installed; the Neutron Star Interior Composition Explorer (NICER) has been gathering X-ray spectra from neutron stars since 2017, while also gathering data about the potential for using X-ray pulsars as navigational beacons in a sort of “Galactic Positioning System”.

MXS is an interesting instrument. When one thinks of making X-rays, the natural tendency is to assume a traditional hot-cathode vacuum tube, where electrons are boiled off a filament and accelerated by an electric field in the range of 100 kilovolts to slam into a tungsten anode, would be used. But vacuum tubes like those found in a hospital X-ray suite aren’t the best space travelers, and even when ruggedized they’re too bulky and heavy to send upstairs.

So NASA researchers developed a more spaceflight-friendly X-ray generator. Rather than heating a filament to generate electrons, the X-ray source in MXS uses creates photoelectrons by bombarding a magnesium photocathode with UV light from LEDs. The few photoelectrons produced then enter an electron amplifier, an off-the-shelf component found in mass spectrometers that uses specially shaped chambers coated with a thin layer of semiconducting material. Each incident electron liberates a few secondary photoelectrons, which bounce off the other wall of the multiplier to create more electrons, greatly amplifying the signal. The huge stream of electrons is then accelerated by a 10 kV field to collide with the target anode and produce X-rays.

While the MXS source sounds similar to a hot-cathode tube, there are important differences. First, the source can be made cheaply from off-the-shelf components and a 3D-printed metal enclosure. The whole assembly weighs only about 160 grams, fits in the palm of a hand, and has no unusual power or temperature control requirements. The big difference, though, is with how fast the X-rays can be turned on and off. A glowing filament can only heat up and cool down so quickly, meaning that effective modulation of X-ray from hot-cathode sources is difficult. In the MXS, X-rays are produced only when the UV LEDs are on, and those can be switching very quickly, in the sub-nanosecond range. The ability to modulate an X-ray beam lead to data rates in the gigabits per second range, greatly enhancing our ability to move data around in space.

What’s more, X-rays can be more tightly collimated than radio waves or even light, which is also being experimented with for space communications. The tighter X-ray beam spreads out less, making transmission more power efficient and reception easier by virtue of the strong signal from relatively bright transmitters.

Although the distance between the MXS and NICER in these XCOM experiments is only about 50 meters, they stand to position us for much better bandwidth for deep space communications. The MXS source itself has a lot of potential applications beyond XCOM too, from cheap, lightweight, low-power medical imaging on Earth and in space, navigational beacons for spacecraft, and even advanced chemical analysis by X-ray spectroscopy

We’ve seen lots of hacks about capturing weather images from the satellites whizzing over our heads, but this nicely written how-to from [Eric Sorensen] takes a different approach. Rather than capturing images from polar satellites that pass overhead a few times a day, this article looks at capturing images from GOES-17, a geostationary satellite that looks down on the Pacific Ocean. The fact that it is a geostationary satellite means that it captures the same view all the time, so you can capture awesome time-lapse videos of the weather. 

The fact that GOES-17 is a geostationary satellite means that it is a bit more involved. While polar satellites that orbit at an altitude of 800km or so can be received with a random piece of wire, the 35,800 km altitude of geostationary satellites means that you need a better antenna. That doesn’t have to be that expensive, though: [Eric] used a $100 parabolic antenna and a $100 Airspy Mini SDR receiver connected to an Ubuntu laptop running some open source software to receive and decode the 1.7GHz signal of the satellite.

The other trick is to figure out where to point the dish. Because it is a geostationary satellite, this part has to be done carefully, as the parabolic antenna has only a small receiving angle. [Eric] designed a 3D-printed mount that fits onto a tripod for his antenna.

Capturing satellite weather images is a fascinating thing to do, and this adds another level of interest, as the images show the full disc of the earth. Capture a series over time, and you can see storms spin around and across the ocean, and see just how complicated they are.

If you are looking for a simpler way to get started in receiving weather satellite images, check out this guide to converting an old TV antenna and USB receiver to capture images from polar satellites.

This is an exciting day for me — we finally get to build some ham radio gear! To me, building gear is the big attraction of amateur radio as a hobby. Sure, it’s cool to buy a radio, even a cheap one, and be able to hit a repeater that you think is unreachable. Or on the other end of the money spectrum, using a Yaesu or Kenwood HF rig with a linear amp and big beam antenna to work someone in Antartica must be pretty cool, too. But neither of those feats require much in the way of electronics knowledge or skill, and at the end of the day, that’s why I got into amateur radio in the first place — to learn more about electronics.

To get my homebrewer’s feet wet, I chose perhaps the simplest of ham radio projects: dummy loads. Every ham eventually needs a dummy load, which is basically a circuit that looks like an antenna to a transmitter but dissipates the energy as heat instead of radiating it an appreciable distance. They allow operators to test gear and make adjustments while staying legal on emission. Al Williams covered the basics of dummy loads a few years back in case you need a little more background.

We’ll be building two dummy loads: a lower-power one specifically for my handy talkies (HTs) will be the subject of this article, while a bigger, oil-filled “cantenna” load for use with higher power transmitters will follow. Neither of my designs is original, of course; borrowing circuits from other hams is expected, after all. But I did put my own twist on each, and you should do the same thing. These builds are covered in depth on my Hackaday.io page, but join me below for the gist on a good one: the L’il Dummy.

L’il Dummy

As Al points out in the article linked above, a dummy load is just a resistive element that matches the characteristic impedance of the transmitter’s antenna connection. In almost every case, that’s going to be 50 ohms. The reason that the load needs to be as resistive as possible is that it needs to continue looking like a flat 50-ohm load no matter what frequency is applied to it. Any inductive or capacitive elements in the load will make it more reactive, changing the impedance as the input frequency changes. This could lead to RF power getting reflected back into the final amplifier transistors in the transmitter, possibly damaging them or destroying them altogether. Not what you’re looking for.

That means our resistive elements need to be as non-inductive as possible. But, they also need to be able to dissipate a lot of power. The HT dummy load, which I’ve dubbed L’il Dummy, needs to handle the 5 to perhaps 8 watts an HT can output. Trouble is, power resistors in that range are often wirewound, and a coil of wire will have too much inductance. We’ll need to be clever in sourcing components.

The circuit for L’il Dummy is hardly worth a schematic – it’s just an SMA jack with a 50-ohm resistor across the outer ground and the inner conductor. I chose to build the circuit on an RF Biscuit board. This is an open-source design that enables all kinds of handy little RF circuits — attenuators, filters, and as in this case, dummy loads. The resistive element I chose was a thick-film SMT device capable of dissipating 35 watts – way more than enough for this job. That and an edge-mount SMA jack should have been all I needed to make a working dummy load.

To my surprise, once I soldered the resistor to the RF Biscuit board, the dummy load was almost as good an antenna as the stock rubber ducky on my Baofeng HT. I was able to hit a local repeater through the dummy load without any issues. Clearly not a good design. To correct it, I put the whole thing into an enclosure made from 1″ copper pipe. Not cheap stuff, but not too bad, and I like the look of polished copper. Soldering the whole case together was a challenge that my big Weller soldering gun wasn’t up to, and trying to get everything heated up enough with a propane torch without overdoing the heat was a fun time.

Testing on a Budget

Now for the $50 question: does it work? I tested the resistance with a DMM and it comes out to just about 49 ohms, which is close enough in my book. But that’s DC resistance; what about impedance? I don’t have an antenna analyzer, so I trolled around and found a simple method for measuring impedance with only a function generator and an oscilloscope. My scope has a 20-MHz function generator built in, so I whipped up a quick test jig from a BNC jack and an SMA jack, connected in series through a leftover 1000-ohm resistor.

Applying a sine wave into the dummy load, measuring peak-to-peak voltages on each side of the resistance, and doing a little math is all that’s needed to characterize the impedance from 2.5 MHz to 20 MHz. The math is simple:

with V1 being the voltage across the input, V2 being the voltage across the output, and Rref being the actual value of the series resistance, which I measured at 998 Ohms.

And the results are pretty close to 50 Ohms, and flat across the tested band

I wish I could measure it at VHF and UHF frequencies, but that will have to wait until I get a function generator that goes up to 400 MHz or so. I doubt very much that a $50 budget would cover that, though.

Next Time

I had intended to cover both L’il Dummy and its bigger, somewhat smarter brother in one article, but I still have some testing to do on Big Dummy. I’ll cover that next time, and after that we’ll move onto measuring the output of a cheap Chinese HT and perhaps building a filter to clean it up.

So far in this series, we’ve covered the absolute basics of getting on the air as a radio amateur – getting licensed, and getting a transceiver. Both have been very low-cost exercises, at least in terms of wallet impact. Passing the test is only a matter of spending the time to study and perhaps shelling out a nominal fee, and a handy-talkie transceiver for the 2-meter and 70-centimeter ham bands can be had for well under $50. If you’re playing along at home, you haven’t really invested much yet.

The total won’t go up much this week, if at all. This time we’re going to talk about what to actually do with your new privileges. The first step for most Technician-class amateur radio operators is checking out the local repeaters, most of which are set up exactly for the bands that Techs have access to. We’ll cover what exactly repeaters are, what they’re used for, and how to go about keying up for the first time to talk to your fellow hams.

Could You Repeat That?

Time to face some cold, hard facts about amateur radio: that spiffy new Baofeng radio I recommended last time as a great starter radio is actually pretty lame. That fact has little to do with the mere $25 you spent on it, or $40 if you opted to upgrade the antenna. It’s a simple consequence of physics: a radio that transmits at 5 watts will only have so much range on the VHF band, and even less on UHF. Even if you buy a more powerful HT, or invest in a mobile or base-station rig running 50 or 100 watts, the plain fact is that direct radio-to-radio contacts on the same frequency, or simplex contacts, are difficult on VHF and UHF because those bands are really best for line of sight (LOS) use.

That’s not to say that hams don’t use their VHF and UHF rigs for simplex communications, of course. Many hams like to see just how far they can push their signals on these bands, building big Yagi antennas and finding mountain peaks to operate from. But for general use around town, most hams rely on repeaters to extend the area they can communicate over. Repeaters are simply transceivers set up to receive signals on one frequency and transmit them on another at the same time, with the help of a device called a duplexer. This simultaneous reception and transmission gives rise to the term duplex communications, the general term for operating on a repeater.

Repeater usually transmit at a much higher power than an HT or even a mobile rig can manage, and they usually have the advantage of being located on a mountaintop or some other elevated place to gain the furthest possible radio horizon as possible. This arrangement vastly increases the area that you can cover with your tiny HT. Depending on how the repeater is sited and what sort of antenna it has, you may be able to cover hundreds of square miles, as opposed to perhaps a few miles radius under ideal conditions, or a few blocks in the typical urban or suburban setting with lots of clutter from buildings and trees. What’s more, some repeaters are linked to other repeaters either through backhaul connections, often via the Internet but also sometimes through powerful LOS microwave links. In these systems it’s possible to use a puny HT to talk to another ham over hundreds or even thousands of miles. It’s actually pretty cool.

Welcome to the Machine

So where are these repeaters, and how do you start working them? The first question is easy to answer: they’re everywhere. Look at any tall building, mountaintop antenna farm, or municipal water tank, and chances are pretty good there’s a ham repeater there. But being able to work them means you need to know exactly where they are, to be sure you’re in range of the repeater, or “the machine” as hams often refer to it, as well as the frequencies it operates on.

Luckily, there are online guides to help with that chore. RepeaterBook.com is usually the first place hams go to find machines in the area. There you can search by state, county, or city, or even via a map, and find what repeaters are available. They’ve even got a handy road search, so you can get all the repeaters listed as within range of a particular highway; that’s really handy for road-trip planning. Here’s what comes up for VHF and UHF repeaters when I search within 25 miles of my location, or QTH:

The first thing you’ll notice is that several machines at different sites have the same callsign. For example, K7ID runs a UHF repeater on Canfield Mountain and a VHF machine on Mica Peak. Both are LOS to me, and I can easily hit them with an HT. The frequency listed in the first column is the transmit frequency of the repeater. Your HT will need to be set to this frequency to hear what’s being said. Your radio will also need to be programmed for the correct tone, listed in the third column. That tone is an audio frequency signal known by a number of different trade names, but generically as continuous tone-coded squelch system (CTCSS). Your radio is capable of adding this sub-audible tone to your transmission; the repeater will only “open up” to transmissions that are correctly coded. Some repeaters have no tone coding, others have different tones for receive and transmit. When doubt, try to find out who runs the machine – most, but not all, are run by a ham radio club – and see if you can look up instructions on the web.

The offset shown in the second column is perhaps the most important bit of information. Recall that repeaters transmit and receive on different frequencies, and that they’re listed by their transmit frequency. The offset tells you what the repeater’s input frequency is, which is the frequency your radio will need to be set to transmit on. For example, the machine I most often used is the K7ID machine on Mica Peak. It’s at 146.980 and shows an offset of -0.6 MHz. That means that my radio has to be set to 146.380 MHz transmission frequency. VHF repeaters are usually 0.6 MHz, but could be plus or minus depending on which part of the VHF band they’re in. UHF repeaters usually have +5 MHz offsets.

Note: I’m not going to cover programming your radio, because there are plenty of guides online that do a better job than I can. DuckDuckGo is your friend.

Casting the Net

Once you’ve found your local repeaters and programmed your radio, it’s time to get on the air. My advice is to spend the first few days just listening to one or more repeaters. Activity levels vary – some machines are hopping all day, and some are barely used except during the typical commuting hours. When you hear a conversation, try to get a feeling for the culture of the repeater. Every group of hams has a culture, and as we discussed in the first installment of the series, it’s not always a healthy culture. My local repeater belongs to the Kootenai Amateur Radio Society, as friendly and as inviting a group of people as I’ve ever heard on the air. After listening to them chat for a few weeks, I was more than ready to reach out to them.

But first, a word about kerchunking. If you want to know if you’re in range of a repeater, you can test it out. Most repeaters have a “squelch tail” that keeps the repeater on the air for several seconds before going back to sleep, and this can be used to check if you’re in range. Some repeaters even identify themselves, either with a synthesized voice or Morse code when they “wake up”.  So you might want to ping the repeater.

Kerchunking, or transmitting into a repeater without identifying yourself, is one of those bad habits that everyone seems to have. But FCC part 97 rules, which cover the amateur radio service, require operators to transmit their call sign when they start a transmission. So don’t kerchunk; a simple identification like “This is KC1DJT testing and clear” will suffice. Nobody is likely to take that as an invitation to chat, but they might give you a reception report.

Once you’re feeling confident enough, try making a contact. I highly recommend checking out the local traffic networks. Hams pride themselves on having the skills and equipment to communicate in an emergency, but that means little without practice to keep everything sharp. Nets allow hams to practice message passing skills and to test their gear on a regular basis. My local group has a network check-in every night that follows a standard script and usually attracts about 30 check-ins. Here’s a sample from a recent check-in:

I’ve become a regular on this net and a few others, mainly because I want to practice, but also to get over my mic shyness. There’s another reason too – I want people to recognize my voice and callsign. If there ever is an emergency in my area, I feel like it’ll be easier to pitch in or to get help if I need it if people hear a familiar voice.

Next Time

Over the next few installments, we’re finally going to get to what I think ham radio is all about at its core: homebrewing. We’ll be building a few simple projects to make that cheap HT a little better, and also build a few tools to help run the shack a little more efficiently.