Shoot The Moon With This Homebrew Hardline RF Divider

March 31, 2020 by Dan Maloney 21 Comments

You can say one thing for [Derek]’s amateur radio ambitions — he certainly jumps in with both feet. While most hams never even attempt to “shoot the Moon”, he’s building out an Earth-Moon-Earth, or EME, setup which requires this little beauty: a homebrew quarter-wave hardline RF divider, and he’s sharing the build with us.

For background, EME is a propagation technique using our natural satellite as a passive communications satellite. Powerful, directional signals can bounce off the Moon and back down to Earth, potentially putting your signal in range of anyone who has a view of the Moon at that moment. The loss over the approximately 770,000-km path length is substantial, enough so that receiving stations generally use arrays of high-gain Yagi antennas.

That’s where [Derek]’s hardline build comes in. The divider acts as an impedance transformer and matches two 50-ohm antennas in parallel with the 50-ohm load expected by the transceiver. He built his from extruded aluminum tubing as the outer shield, with a center conductor of brass tubing and air dielectric. He walks through all the calculations; stock size tubing was good enough to get into the ballpark for the correct impedance over a quarter-wavelength section of hardline at the desired 432-MHz, which is in the middle of the 70-cm amateur band. Sadly, though, a scan of the finished product with a NanoVNA revealed that the divider is resonant much further up the band, for reasons unknown.

[Derek] is still diagnosing, and we’ll be keen to see what he comes up with, but for now, at least we’ve learned a bit about homebrew hardlines and EME. Want a bit more information on Moon bounce? We’ve got you covered.

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Two Perspectives On James Clerk Maxwell And His Equations

March 26, 2020 by Al Williams 30 Comments

We are unabashed fans of [The History Guy’s] YouTube channel, although his history videos aren’t always about technology, and even when they are, they don’t always dig into the depths that we’d like to see. That’s understandable since the channel is a general interest channel. However, for this piece on James Clerk Maxwell, he brought in [Arvin Ash] to handle the science side. While [The History Guy] talked about Maxwell’s life and contributions, [Arvin] has a complementary video covering the math behind the equations. You can see both videos below.

Of course, if you’ve done electronics for long, you probably know at least something about Maxwell’s equations. They unified electricity and magnetism and Einstein credited them with spurring one of his most famous theories.

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Quantum Sensor Receives From 0 Hz To 1000 GHz

March 24, 2020 by Al Williams 29 Comments

Although it isn’t that uncommon to have broadband radio coverage in a single device, going from 0 Hz to 1000 GHz with one antenna and receiver is a bit much. But not for the US Army it seems, because they’ve developed a quantum sensor that can cover that range.

The technology uses Rydberg atoms, which are atoms with a highly excited valence electron. They’ve been used for a variety of sensing applications before, such as reading the cosmic microwave background radiation. However, until the Army’s work there has been no quantitative analysis of using them for wide-spectrum communications.

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High Power LoRa And Tropospheric Reflection Experiments

March 21, 2020 by Jenny List 29 Comments

We’re used to LoRa as a free-to-use digital radio protocol allowing not-very-high data rate communications over distances of a few miles. It’s made all kinds of distributed sensor systems a breeze, and some experimenters have made an art of achieving communication over hundreds of miles. But what would happen if you took a brute-force approach to LoRa and simply wound up the power?

In a bid to test its efficiency at bouncing off the troposphere in normal conditions, [Inductive Twig] hooked up a HamShield 70cm LoRa shield to an 80W power amplifier and a high-gain Yagi antenna pointing directly upwards mounted with ingenuity on a spade, and drove around looking at the received result. With an effective radiated power of 1500W this wasn’t your normal LoRa, instead being operated with LoRa as an amateur radio mode.

For those not familiar with radio propagation, radio waves bounce off some surprising things. In this case the aim was to bounce them off the troposphere, but while radio amateurs and LoRa distance chasers wait until weather conditions deliver a so-called “lift” in which the troposphere is especially reflective, here the experiment was performed under normal flat conditions. The result characterizes LoRa’s possibilities for everyday extreme-range mode rather than chasing records, and in that there were some interesting results. The reflected signal was receivable in bursts with low but consistent signal strength, with the limiting factor during the test as that they ran out of land upon which to drive in the southernmost peninsula of New Jersey. We’ve heard of War-Driving for open WiFi… does this car dashboard setup count as LoRa-Driving?

LoRa is designed as a protocol tolerant of low signal levels and some packet loss, so this experiment is an interesting demonstration of its possibilities when used at higher powers under a licensed transmission. It shouldn’t be possible to use the 70cm band for reliable tropospheric propagation under non-lift conditions, but this shows that it can be done. Meanwhile, take a look at a previous attempt to push LoRa using a balloon.

Link Coupling Antenna Tuner Wordless Workshop

March 15, 2020 by Al Williams 13 Comments

Remember “Wordless Workshop” in Popular Science? [Roy Doty] illustrated a household problem and the solution for it cobbled up in the main character’s garage workshop. We wonder what [Roy] would have done with YouTube? Maybe something like the video from [VE2TAE] and [VE2AEV] showing their link coupling antenna tuning build. You can watch the video after the break, and if you aren’t a fan of Jazz, you can mute the volume.

Like [Doty’s] cartoons, the video presumes you are going to have your own idea about dimensions and component values to fit your needs. But the construction is beautiful in its own right. The tubing wound into giant coils is impressive and brings back memories of the old days. However, the construction of the variable capacitors really got us excited. Big air variable caps may be hard to find, but the video makes them look easy to make.

A couple of nice looking knobs and panel meters make for a great looking tuner. With that spacing, we imagine it would handle full legal power without any difficulty at all. If you want to learn more about this type of tuner, [VK1OD] had a great page about it which seems to be defunct now. But the Internet Archive comes to our rescue, as usual.

The design is quite old, so even a 1934 copy of “Radio” can explain it (look on page 6). If you want to see a more wordy example of making variable capacitors — although they are smaller, the same principles apply — [N4DFP] has a good write up for that.

Of course, these days, most people expect their antenna tuning to be automatic. With some Lego, though, you could refit your manual one, if you like.

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A Practical Look At Chokes For EMI Control

March 15, 2020 by Dan Maloney 6 Comments

Radio frequency electronics can seem like a black art even to those who intentionally delve into the field. But woe betide the poor soul who only incidentally has to deal with it, such as when seeking to minimize electromagnetic interference. This primer on how RF chokes work to reduce EMI is a great way to get explain the theory from a practical, results-oriented standpoint.

As a hobby machinist and builder of machine tools, [James Clough] has come across plenty of cases where EMI has reared its ugly head. Variable frequency drives are one place where EMI can cause problems, and chokes on the motor phase outputs are generally prescribed. He used an expensive choke marketed as specific for VFD applications on one of his machines, but wondered if a cheap ferrite core would do the job just as well, and set to find out.

A sweep of some ferrite cores with a borrowed vector network analyzer proved unsatisfying, so [James] set up a simple experiment with a function generator and an oscilloscope. His demo shows how the impedance of a choke increases with the frequency of the test signal, which is exactly the behavior that you’d want in a VFD – pass the relatively low-frequency phase signals while blocking the high-frequency EMI. For good measure, he throws a capacitor in parallel to the choke and shows how much better a low-pass filter that makes.

We love demos like this that don’t just scratch an intellectual itch but also have a practical goal. [James] not only showed that (at least in some cases) a $13 ferrite can do the same job as a $130 VFD choke, but he showed how they work. It’s basic stuff, but it’s what you need to know to move on to more advanced RF filter designs.

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Get Your Weather Images Straight From The Satellite

March 14, 2020 by Al Williams 15 Comments

[Josh] has a series called Ham Radio Crash Course and a recent installment covers how you can grab satellite images directly from weather satellites. This used to be more of a production than it is now thanks to software defined radio (SDR). Josh also has another project using a 3D printer to make an antenna suitable for the job. You can see the video below.

The software is the venerable WXtoImg program. This is abandonware, but the community has kept the software available. The program works on Linux, Windows, and Mac. The satellites in question operate around 137 MHz, but that’s easily in the range of even the cheap SDR dongles. [Josh] shows how to use a virtual audio cable on Windows to connect the output of the radio to the input of the WXtoImg program. Under Linux, you can do this with Pulse or Jack very easily without any extra hardware.

There’s some setup and calibration necessary for the software. You’ll also need the current orbital data and the program will tell you when you can find the next satellite passing overhead. Generally speaking you’ll want your antenna outside, which [Josh] solved by taking everything outdoors and having some lunch during the pass. It also takes some time to post-process the data into images and audio.

We know this isn’t new. But we did like [Josh’s] clear and up-to-date guide. We remember watching NOAA 15 as it started to lose its electronic mind.

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