Retrotechtacular: 934 MHz CB Radio

November 21, 2019 by Jenny List 49 Comments

The radio spectrum is carefully regulated and divided up by Governments worldwide. Some of it is shared across jurisdictions under the terms of international treaties, while other allocations exist only in individual countries. Often these can contain some surprising oddities, and one of these is our subject today. Did you know that the UK’s first legal CB radio channels included a set in the UHF range, at 934 MHz? Don’t worry, neither did most Brits. Behind it lies a tale of bureaucracy, and of a bungled attempt to create an industry around a barely usable product.

Hey, 2019, Got Your Ears On?

Did this car make you want a CB radio? Stuurm [CC BY-SA 3.0]

Mention CB radio in 2019 it’s likely that the image conjured in the mind of the listener will be one from a previous decade. Burt Reynolds and Jerry Reed in Smokey and the Bandit perhaps, or C. W. McCall’s Convoy. It may not be very cool in the age of WhatsApp, but in the 1970s a CB rig was the last word in fashionable auto accessories and a serious object of desire into which otherwise sane adults yearned to speak the slang of the long-haul trucker.

If you weren’t American though, CB could be a risky business. Much of the rest of the world didn’t have a legal CB allocation, and correspondingly didn’t have access to legal CB rigs. The bombardment of CB references in exported American culture created a huge demand for CB though, and for British would-be CBers that was satisfied by illegally imported American equipment. A vibrant community erupted around UK illegal 27 MHz AM CB in the late 1970s, and Government anger was met with campaigning for a legal allocation. Brits eventually got a legal 27 MHz allocation in November 1981, but the years leading up to that produced a few surprises.

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Just How Simple Can A Transceiver Be?

November 9, 2019 by Jenny List 20 Comments

We’ve frequently talked about amateur radio on these pages, both in terms of the breadth of the hobby and the surprisingly low barrier to entry. It’s certainly the case that amateur radio does not have to mean endlessly calling CQ on SSB with an eye-wateringly expensive rig, and [Bill Meara N2CQR] is on hand with a description of a transceiver that’s so simple it only uses one transistor.

It’s a 40 meter (7 MHz) QRP or low power transceiver in which the transmitter is a simple crystal oscillator and the receiver is an equally simple regenerative design. What makes it so simple is the addition of a three-way switch to transfer the single transistor — a J310 FET — between the two halves of the circuit. It’s no slouch as QRP radios go, having clocked up real-world contacts.

This circuit shows us how a little can go a long way in the world of amateur radio, and we can’t help liking it for that. It’s worth saying though that it’s not without flaws, as a key click filter and another transistor would make for a much higher quality transmitted signal. But then it would no longer be a single-transistor rig, and thus would miss the point, wouldn’t it.

This Radio Control Sailboat Uses 2X4s

November 8, 2019 by Al Williams 5 Comments

When [PeterSripol] was a kid, he made a simple sailboat from a scrap piece of 2×4 and some napkin sails. He’s not 8 years old anymore, but he decided he wanted to make another 2X4 sailboat using the skills he’s learned since he was a kid.

You’ll have to get past storytime and mice, but the build skill is evident. There’s a RC rudder, a keel with lead shot and overall it is a good looking boat for such a simple build.

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Keep An Eye On The Neighborhood With This Passive Radar

November 8, 2019 by Dan Maloney 25 Comments

If your neighborhood is anything like ours, walking across the street is like taking your life in your own hands. Drivers are increasingly unconcerned by such trivialities as speed limits or staying under control, and anything goes when they need to connect Point A to Point B in the least amount of time possible. Monitoring traffic with this passive radar will not do a thing to slow drivers down, but it’s a pretty cool hack that will at least yield some insights into traffic patterns.

The principle behind active radar – the kind police use to catch speeders in every neighborhood but yours – is simple: send a microwave signal towards a moving object, measure the frequency shift in the reflected signal, and do a little math to calculate the relative velocity. A passive radar like the one described in the RTL-SDR.com article linked above is quite different. Rather than painting a target with an RF signal, it relies on signals from other transmitters, such as terrestrial TV or radio outlets in the area. Two different receivers are used, both with directional antennas. One points to the area to be monitored, while the other points directly to the transmitter. By comparing signals reflected off moving objects received by the former against the reference signal from the latter, information about the distance and velocity of objects in the target area can be obtained.

The RTL-SDR test used a pair of cheap Yagi antennas for a nearby DVB-T channel to feed their KerberosSDR four-channel coherent SDR, a device we last looked at when it was still in beta. Essentially four SDR dongles on a common board, it’s available now for $149. Using it to build a passive radar might not save the neighborhood, but it could be a lot of fun to try.

Roofing Radio Telescope Sees The Galaxy

November 7, 2019 by Al Williams 6 Comments

[David Schneider] asked himself, “How big a radio antenna would you need to observe anything interesting?” The answer turns out to be a $150 build of a half meter antenna. He uses it to detect the motions of the spiral arms of the Milky Way. The first attempt was a satellite TV dish and a cantenna feed, which didn’t work as the can wasn’t big enough to pick up signals at the 21cm wavelength of hydrogen emissions. Interstellar gas clouds are known to emit radio energy at this frequency.

Looking online, [David] tried aluminized foam board insulation, but was worried that the material didn’t seem to actually be conductive. A quick thrown-together Faraday cage with a cell phone didn’t seem to block any calls. Abandoning that approach, he settled on aluminum flashing used for roofing.

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Well-Engineered RF Amplifier Powers Ham Radio Contacts

November 7, 2019 by Dan Maloney 35 Comments

Typically, amateur radio operators use the minimum power needed to accomplish a contact. That’s just part of being a good spectrum citizen, and well-earned bragging rights go to those who make transcontinental contacts on the power coming from a coin cell. But sometimes quantity has a quality all its own, and getting more power into the ether is what the contact requires. That’s where builds such as this well-engineered 600W broadband RF amplifier come into play.

We’re really impressed with the work that [Razvan] put into this power amp. One of the great joys of being a ham is being able to build your own gear, and to incorporate the latest technology long before the Big Three manufacturers start using it. While LDMOS transistors aren’t exactly new – laterally-diffused MOSFETs have been appearing in RF power applications for decades – the particular parts used for the amp, NXP’s MRF300 power transistors, are pretty new to the market. A pair of the LDMOS devices form the heart of the push-pull amp, as do an array of custom-wound toroids and transformers including a transmission line transformer wound with 17-ohm coax cable. [Razvan] paid a lot of attention to thermal engineering, too, with the LDMOS transistors living in cutouts in the custom PCB so they can mate with a hefty heatsink. Even the heatsink compound is special; rather than the typical silicone grease, he chose a liquid metal alloy called Gallinstan. The video below gives a tour of the amp and shows some tests with impressive results.

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Intercontinental Radio Communications With The Help Of Fly Fishing Reels

November 3, 2019 by Danie Conradie 3 Comments

All of us have experience in trying to explain to a confused store assistant exactly what type of kitchen implement you’re looking for, and why it is a perfectly suitable part for your autonomous flying lawn mower. Or in the case of [MM0OPX] trying to find fly fishing reels that are suitable for his Adjustiwave multi-band VHF-HF ham radio antenna.

HF radios allow intercontinental communication but require very large antennas which can be tricky to tune properly, and this antenna helps ease both these problems. The basic configuration is quarter wave, linear loaded (folded), vertical antenna. A quarter wave length radiator wire runs up a fibreglass pole, folds over the top, and comes back down, to form a shorter, more practical antenna while remaining the required length. Ground plane radial wires are usually added to improve performance by helping to reflect signals into antenna.

[MM0OPX] expanded this concept by using two pairs of fly fishing reels to quickly adjust the length of the radiators and radials. One reel holds the actual antenna wire while the second holds fishing braid, which is tied to the end of the wire to provided tension. The radials wire is exactly the same, it just runs across the ground.

The four reels are mounted to a plastic junction box, which houses the feed line connector and matching transformer, which is attached to the base of a fibreglass pole with hydraulic pipe clamps. Each wire is marked with heat shrink at defined points to allow quick tuning for the different frequencies. [MM0OPX] tried a couple of wire types and found that 1 mm stainless steel cable worked best.

This being Hackaday, we are big fans of repurposing things, especially when the end product is greater than the sum of its parts, as is the case here. Check out the walk around and build discussion videos after the break. Continue reading “Intercontinental Radio Communications With The Help Of Fly Fishing Reels” →