Radio Hacks

1388 Articles

A Walk-In Broadcast Transmitter

April 5, 2017 by 20 Comments

[Mr. Carlson] likes electronics gear. Mostly old gear. The grayer the case, the greener the phosphors, and the more hammertone, the better. That’s why we’re not surprised to see him with a mammoth AM radio station transmitter in his shop. That it’s a transmitter that you can walk into while it’s energized was a bit of a surprise, though.

As radio station transmitters go, [Mr. Carlson]’s Gates BC-250-GY broadcast transmitter is actually pretty small, especially for 1940s-vintage gear. It has a 250 watt output and was used as a nighttime transmitter; AM stations are typically required to operate at reduced power when the ionosphere is favorable for skip on the medium frequency bands. Stations often use separate day and night transmitters rather than just dialing back the daytime flamethrower; this allows plenty of time for maintenance with no interruptions to programming.

If you enjoy old broadcast gear, the tour of this transmitter, which has been rebuilt for use in the ham bands, will be a real treat. Feast your eyes on those lovely old bakelite knobs and the Simpson and Westinghouse meters, and picture a broadcast engineer in white short sleeves and skinny tie making notations on a clipboard. The transmitter is just as lovely on the inside — once the plate power supply is shut down, of course, lest [Mr. Carlson] quickly become [the former late Mr. Carlson] upon stepping inside. Honestly, there aren’t that many components inside, but what’s there is big – huge transformer, giant potato slicer variable caps, wirewound resistors the size of paper towel tubes, and five enormous, glowing vacuum tubes.

It’s a pretty neat bit of broadcasting history, and it’s a treat to see it so lovingly restored. [Mr. Carlson] teases us with other, yet larger daytime transmitters he has yet to restore, and we can’t wait for that tour. Until then, perhaps we can just review [Mr. Crosley]’s giant Cincinnati transmitter from the 1920s and wait patiently.

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The Right Circuit Turns Doppler Module Into A Sensor

March 31, 2017 by 22 Comments

Can you buy a working radar module for $12? As it turns out, you can. But can you make it output useful information? According to [Mathieu], the answer is also yes, but only if you ignore the datasheet circuit and build this amplification circuit for your dirt cheap Doppler module.

The module in question is a CDM324 24-GHz board that’s currently listing for $12 on Amazon. It’s the K-band cousin of the X-band HB100 used by [Mathieu] in a project we covered a few years back, but thanks to the shorter wavelength the module is much smaller — just an inch square. [Mathieu] discovered that the new module suffered from the same misleading amplifier circuit in the datasheet. After making some adjustments, a two-stage amp was designed and executed on a board that piggybacks on the module with a 3D-printed bracket.

Frequency output is proportional to the velocity of the detected object; the maximum speed for the sensor is only 14.5 mph (22.7 km/h), so don’t expect to be tracking anything too fast. Nevertheless, this could be a handy sensor, and it’s definitely a solid lesson in design. Still, if your tastes run more toward using this module on the 1.25-cm ham band, have a look at this HB100-based 3-cm band radio.

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Real World RF Filter Design And Construction

March 30, 2017 by 13 Comments

We bet when [devttyS0] made his latest video about RF filter design (YouTube, embedded below), he had the old saying in mind: in theory, there’s no difference between theory and practice, but in practice, there is. He starts out pointing how now modern tools will make designing and simulating any kind of filter easy, but the trick is to actually build it in real life and get the same performance. You can see the video below.

One of the culprits, of course, is we tend to design and simulate with perfect components. Wires have zero resistance, capacitance, and inductance. Inductors and capacitance have no parasitic elements in our rosy design world. Even the values of components will vary from their ideal values and may change over time.

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Vintage Transistor Powers QRP Transmitter

March 28, 2017 by 27 Comments

[Pete Juliano, N6QW] built a 20 M QRP CW transmitter using just a handful of parts. That in itself will not raise any eyebrows, until you find that he built it using one of the very first RF transistors manufactured all the way back in 1955. That’s from before the time most of us were born and not many years after the invention of the transistor in late 1947.

QRP in HAM-speak technically stands for a request to “reduce power” or an offer of “should I reduce power” when appended with a question mark. A QRP transmitter is designed to transmit at really low powers. The accepted upper power limit for QRP transmitters is 5 W, at least for modes like CW using FM or AM modulation. [Pete]’s interest was piqued when he read about a 10 mW 10 M QRP transmitter design in a vintage Radio magazine from the late ’50’s and decided to replicate it. We aren’t sure, but it appears he had a Philco SB-100 RF transistor lying around in his parts bin. The SB-100 was one of the first surface-barrier transistors and could output 10 mW at frequencies up to 30MHz.

[Pete]’s rig was originally putting out 0.4 mW with a 3 V supply, and oscillating at 14.060 MHz in the 20 M band. The design appears to be a simple Colpitts oscillator with just a few parts assembled in dead-bug style on a piece of copper clad laminate. After adding an output transformer, he managed to increase the power output to about 25 mW. Check out [Pete N6QW] sending out a CQ shout out from his QRP transmitter in the video after the break.

If this gets you interested in Amateur Radio, but you are mic-shy, then [Dan Maloney] has some options for you in Shut Up and Say Something: Amateur Radio Digital Modes.

SBF image via Historianbuff CC-BY-SA 3.0, Public Domain
[via Dangerous Prototypes]

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Shut Up And Say Something: Amateur Radio Digital Modes

March 24, 2017 by 66 Comments

In a recent article, I lamented my distaste for carrying on the classic amateur radio conversation — calling CQ, having someone from far away or around the block call back, exchange call signs and signal reports and perhaps a few pleasantries. I think the idle chit-chat is a big turn-off to a lot of folks who would otherwise be interested in the World’s Greatest Hobby™, but thankfully there are plenty of ways for the mic-shy to get on the air. So as a public service I’d like to go over some of the many digital modes amateur radio offers as a way to avoid talking while still communicating.

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See Satellites With A Simple Radio Telescope

March 23, 2017 by 17 Comments

Have you got a spare Dish Network antenna lying about? They’re not too hard to come by, either curbside on bulk waste day or perhaps even on Freecycle. If you can lay hands on one, you might want to try this fun radio telescope build.

Now, don’t expect much from [Justin]’s minimalist build. After all, you’ll be starting with a rather small dish and an LNB for the Ku band, so you won’t be doing serious radio astronomy. In fact, the BOM doesn’t include a fancy receiver  – just a hacked satellite finder. The idea is to just get a reading of the relative “brightness” of a radio source without trying to demodulate the signal. To that end, the signal driving the piezo buzzer in the sat finder is fed into an Arduino through a preamp. The Arduino also controls stepper motors for the dish’s azimuth and elevation control, which lets it sweep the sky and build up a map of signal intensity. The result is a clear band of bright spots representing the geosynchronous satellites visible from [Justin]’s location in Brazil.

Modifications are definitely on the docket for [Justin], including better equipment that will allow him to image the galactic center. There may be some pointers for him in our coverage of a tiny SDR-based radio telescope, or from this custom receiver that can listen to Jupiter.

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[Ashhar Farhan]’s Done It Again!

March 14, 2017 by 15 Comments

If you are a regular follower of these pages as well as a radio amateur, you may well have heard of [Ashhar Farhan, VU2ESE]. He is the designer of the BitX, a simple single-sideband transceiver that could be built for a very small outlay taking many of its components from a well-stocked junk box.

In the years since the BitX’s debut there have been many enhancements and refinements to the original, and it has become something of a standard. But it’s always been a single-band rig, never competing with expensive commercial boxes that cover the whole of the available allocations.

With his latest design, he’s changed all that. The uBITX (Micro-BITX when spoken aloud), is an SSB and CW transceiver that covers all of the HF amateur bands, and like the original is designed for the home constructor on a budget. It shows its heritage in the use of bi-directional amplifiers, but diverges from the original with a 45 MHz first IF and an Arduino/SI5351 clock generator in the place of a VFO. It looks to be an excellent design in the spirit of the original, and we can’t wait to see them in the wild.

He’s put up a YouTube video which we’ve placed below the break. His write-up is extensive and fascinating, but it is his closing remarks which sum up the project and the reason why you should build one. We don’t often reproduce entire blocks of text, but this one says it so well:

As a fresh radio amateur in the 80s, one looked at the complex multiband radios of the day with awe. I remember seeing the Atlas 210x, the Icom 720 and Signal One radios in various friends’ shacks. It was entirely out of one’s realm to imagine building such a general coverage transceiver in the home lab.

Devices are now available readily across the globe through online stores, manufacturers are more forthcoming with their data. Most importantly, online communities like the EMRFD’s Yahoo group, the BITX20’s groups.io community etc have placed the tribal knowledge within the grasp of far flung builders like I am.

One knows that it was just a matter of breaking down everything into amplifiers, filters, mixers and oscillators, but that is just theory. The practice of bringing a radio to life is a perpetual ambition. The first signal that the sputters through ether, past your mess of wires into your ears and the first signal that leaps out into the space from your hand is stuff of subliminal beauty that is the rare preserve of the homebrewer alone.

At a recent eyeball meet, our friend [Dev(VU2DEV)] the famous homebrewer said “Now is the best time to be a homebrewer”. I couldn’t disagree.

If you build a uBITX, please share it with us!

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