Everything You Didn’t Know You Were Missing About Bias Tees

Do you need a bias tee? If you want to put a DC voltage on top of an RF signal, chances are that you do. But what exactly are bias tees, and how do they work?

If that’s your question, [W2AEW] has an answer for you with this informative video on the basics of bias tees. A bias tee allows a DC bias to be laid over an RF signal, and while that sounds like a simple job, theory and practice often deviate in the RF world. The simplest bias tee would have a capacitor in series with the RF input and output to pass AC but block DC from getting out the input, and a DC input with a series inductance to prevent RF from getting into the DC circuit. Practical circuits are slightly more complicated, and [W2AEW] covers all you need to know about how real-world bias tees are engineered. He also gives some use cases for bias tees, from sending DC signals up a feed line to control an antenna tuner or rotator to adding a DC bias to a high-speed serial line.

It’s an interesting circuit, and we learned a lot, which is par for the course with [W2AEW]’s videos. Check out some of his other offerings, like a practical guide to the mysteries of Smith charts, or his visualization of how standing waves work.

Continue reading “Everything You Didn’t Know You Were Missing About Bias Tees”

Fallen Radiosonde Reborn as Active L-band Antenna

If your hobby is chasing radiosondes across vast stretches of open country, and if you get good enough at it, you’ll eventually end up with a collection of the telemetry packages that once went up on weather balloons to record the conditions aloft. Once you’ve torn one or two down though, the novelty must wear off, which is where this radiosonde conversion to an active L-band antenna comes from.

As it happens, we recently discussed the details of radiosondes, so if you need a primer on these devices, check that out. But as Australian ham [Mark (VK5QI)] explains, radiosondes are a suite of weather instruments crammed into a lightweight package with a GPS receiver and a small transmitter. Lofted beneath a weather balloon into the stratosphere, a radiosonde transmits a wealth of data back to the ground before returning on a parachute after the balloon bursts. [Mark] had his eyes on the nice quadrifilar helical antenna used by the Vaisla R92 radiosonde’s GPS receiver, with the aim of repurposing them. He had a lot of components to remove while still retaining the low-noise amplifier (LNA), but in the end managed to get a working antenna with 40 dB gain in the L-band, and with the help of an RTL-SDR dongle he picked up solid signals from Iridium satellites.

Want to score your own radiosonde to play with? First, you have to know how to listen in so you can find them. Or, you know – there’s always eBay.

[via RTL-SDR.com]

Cascade LNAs and Filters for Radioastronomy with an SDR

It may not be the radio station with all the hits and the best afternoon drive show, but 1420.4058 MHz is the most popular frequency in the universe. That’s the electromagnetic spectral line of hydrogen, and it’s the always on the air. But studying the H-line is a non-trivial task unless you know how to cascade low-noise amplifiers and filters to use an SDR for radio astronomy.

Because the universe is mostly made of hydrogen, H-line emissions are abundant, and their distribution can tell us a lot about the structure of galaxies. The 21-cm emission line is so characteristic and so prevalent that we used it as a unit of measurement on the plaques aboard the Pioneer probes as well as in the instructions for playing back the Voyager recordings. But listening in on 21-cm here on Earth requires a special setup, which [Adam (9A4QV)] describes in a detailed paper on the subject (PDF). [Adam] analyzes multiple configurations of LNAs and filters, both of which he sells, to determine the optimum front-end for 21-cm work. His analysis is a good primer on LNAs and explains why the front-end gear needs to be as close to the antenna as possible. Using his LNAs and filters and an SDR dongle, a reasonable 21-cm rig can be had for about $200 or so, less the antenna. He promises a follow-up paper on homebrew 21-cm antennas; we’ll be looking forward to that.

Not keen on the music of the spheres and prefer to listen to our own spacecraft instead? Then read up on the Deep Space Network and how you can snoop in.

Portable Classroom Upgrade: Smaller, Cheaper, Faster

[Eric] at MkMe Lab has a dream: to build a cheap, portable system that provides the electronic infrastructure needed to educate kids anywhere in the world. He’s been working on the system for quite a while, and has recently managed to shrink the suitcase-sized system down to a cheaper, smaller form-factor.

The last time we discussed [Eric]’s EduCase project was as part of his Hackaday Prize 2016 entry. There was a lot of skepticism from our readers on the goals of the project, but whatever you think of [Eric]’s motivation, the fact remains that the build is pretty cool. The previous version of the EduCase relied on a Ku-band downlink to receive content from Outernet, and as such needed to stuff a large antenna into the box. That dictated a case in the carry-on luggage size range. The current EduCase is a much slimmed-down affair that relies on an L-band link from the Inmarsat satellites, with a much smaller patch antenna. A low-noise amp and SDR receiver complete the downlink, and a Raspberry Pi provides the UI. [Eric]’s build is just a prototype at this point, but we’re looking forward to seeing everything stuffed into that small Pelican case.

Yes, Outernet is curated content, and so it’s not at all the same experience as the web. But for the right use case, this little package might just do the job. And with a BOM that rings up at $100, the price is right for experimenting.

Continue reading “Portable Classroom Upgrade: Smaller, Cheaper, Faster”

Listening to Jupiter on a DIY Radio

By Jove, he built a radio!

If you want to get started with radio astronomy, Jupiter is one of the easiest celestial objects to hear from Earth. [Vasily Ivanenko] wanted to listen, and decided to build a modular radio receiver for the task. So far he’s written up six of the eight planned blog posts.

The system uses an LNA, a direct conversion receiver block, and provides audio output to a speaker, output to a PC soundcard, and a processed connection for an analog to digital converter. The modules are well-documented and would be moderately challenging to reproduce.

Continue reading “Listening to Jupiter on a DIY Radio”

Hackaday Links: Summer, 2015

[Elia] was experimenting with LNAs and RTL-SDR dongles. If you’re receiving very weak signals with one of these software defined radio dongles, you generally need an LNA to boost the signal. You can power an LNA though one of these dongles. You’ll need to remove a few diodes, and that means no ESD protection, and you might push the current consumption above the 500mA a USB port provides. It does, however, work.

We’ve seen people open up ICs with nitric acid, and look inside them with x-rays. How about a simpler approach? [steelcityelectronics] opened up a big power transistor with nothing but a file. The die is actually very small – just 1.8×1.8mm, and the emitter bond wire doesn’t even look like it’ll handle 10A.

Gigantic Connect Four. That’s what the Lansing Makers Network built for a Ann Arbor Maker Faire this year. It’s your standard Connect Four game, scaled up to eight feet tall and eight feet wide. The disks are foam insulation with magnets; an extension rod (with a magnet at the end) allows anyone to push the disks down the slots.

[Richard Sloan] of esp8266.com fame has a buddy running a Kickstarter right now. It’s a lanyard with a phone charger cable inside.

Facebook is well-known for the scientific literacy of its members. Here’s a perpetual motion machine. Comment gold here, people.

Here’s some Hackaday Prize business: We’re giving away stuff to people who use Atmel, Freescale, Microchip, and TI parts in their projects. This means we need to know you’re using these parts in your projects. Here’s how you let us know. Also, participate in the community voting rounds. Here are the video instructions on how to do that.

Making a 20dB Low Noise Amplifier for a 400MHz radio

[Will] recently tipped us about a 400MHz Low Noise Amplifier (LNA) module he made. His detailed write-up starts by explaining the theory behind an amplifying chain. Assuming a 50 Ohm antenna system receives a -70dBm signal, the total peak to peak voltage would be less than 200uV (.0002 volts). If the first amplifying stage doesn’t consist of an LNA, then the added noise would later be amplified by the other elements of your system.

[Will] then detailed how he picked his LNA on Digikey, mainly by looking for one that had a less than 1dB Noise Figure. His final choice was the Sky65047: a small budget-priced 0.4-3.0GHz low noise amplifier with a theoretical gain of 20dB at 400MHz. He made the PCB you can see in the picture above, removing the soldermask on the signal path in order to lower the permitivity. Because of a few mistakes present in the application note, it took [Will] quite a while to get his platform up and running with a 20dB gain but a 4.5dB NF. He also measured the input return loss using a directional coupler, which ended up being quite close to the datasheet’s 14dB number.