Forget The Coax, Wire Up Your Antennas With Cat 6 Cable

January 15, 2025 by Dan Maloney 65 Comments

These days, anything with copper in it is expensive. If you doubt that, a walk into any Home Depot electrical department, where the wire is locked up tighter than Fort Knox, will prove otherwise. Coaxial cable is a particularly expensive species, which is a pity for hams and other radio enthusiasts since it’s the only thing we can use for antenna feedlines.

Or is it? [Steve (VE6WZ)] has found a way to use ordinary Cat 6 Ethernet cable for antenna feed lines that seems pretty clever. As he points out, Ethernet cables are designed to handle frequencies that coincide nicely with most of the interesting amateur radio bands, and their insertion losses are acceptably low, especially for Cat 6 cable. The twisted pairs are also a balanced system that’s good at rejecting common mode noise. Cat 6 cable also has four pairs of conductors, allowing you to feed multiple antennas with one cable, or to distribute power to amplifiers and switches along with antenna feeds.

The downside? Cat6 conductor pairs have a characteristic impedance of around 100 ohms, which isn’t a match for the 50-ohm feedline impedance universally expected by ham radios. Also, the relatively small wires probably aren’t up to the job of carrying much current, limiting their use to feedlines for receive-only antennas. That works for [Steve] since he uses Cat 6 to support his massive Beverage antenna farm (Beverage antennas are non-resonant horizontal antennas that live close to the ground and point in the direction of the signal, rather than broadside to the signal as with a resonant antenna like a dipole.) Each antenna in his farm has a transimpedance amplifier that needs to be powered, plus switching relays so he can turn the correct antennas on for the signals he wants to receive. He describes the amps in detail in the video below, along with the custom impedance-matching transformers he uses and the combining gear.

Coax will probably still be the cable of choice for most feedline applications, but it’s nice to know there are alternatives. And who knows—if you stick to QRP work, maybe Cat 6 could even be used for transmitting.

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Making Sure Your Patch Cables Are Ready For RF Work

January 8, 2025 by Dan Maloney 3 Comments

How do you know that your patch cables are good? For simple jumper wires, a multimeter is about all you need to know for sure. But things can get weird in the RF world, in which case you might want to keep these coaxial patch cable testing tips in mind.

Of course, no matter how high the frequency, the basics still apply, and [FesZ] points out in the video below that you can still get a lot of mileage out of the Mark 1 eyeball and a simple DMM. Visual inspection of the cable and terminations can reveal a lot, as can continuity measurements on both the inner and outer conductors. Checking for shorts between conductors is important, too. But just because the cable reads good at DC doesn’t mean that problems aren’t still lurking. That’s when [FesZ] recommends breaking out a vector network analyzer like the NanoVNA. This tool will allow you to measure the cable’s attenuation and return loss parameters across the frequency range over which the cable will be used.

For stubborn problems, or just for funsies, there’s also time-domain reflectometry, which can be done with a pulse generator and an oscilloscope to characterize impedance discontinuities in the cable. We’ve covered simple TDR measurement techniques before, but [FesZ] showed a neat trick called time-domain transformation, which uses VNA data to visualize the impedance profile of the whole cable assembly, including its terminations.

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Coax Stub Filters Demystified

July 21, 2024 by Dan Maloney 9 Comments

Unless you hold a First Degree RF Wizard rating, chances are good that coax stubs seem a bit baffling to you. They look for all the world like short circuits or open circuits, and yet work their magic and act to match feedline impedances or even as bandpass filters. Pretty interesting behavior from a little piece of coaxial cable.

If you’ve ever wondered how stub filters do their thing, [Fesz] has you covered. His latest video concentrates on practical filters made from quarter-wavelength and half-wavelength stubs. Starting with LTspice simulations, he walks through the different behaviors of open-circuit and short-circuit stubs, as well as what happens when multiple stubs are added to the same feedline. He also covers a nifty online calculator that makes it easy to come up with stub lengths based on things like the velocity factor and characteristic impedance of the coax.

It’s never just about simulations with [Fesz], though, so he presents a real-world stub filter for FM broadcast signals on the 2-meter amateur radio band. The final design required multiple stubs to get 30 dB of attenuation from 88 MHz to 108 MHz, and the filter seemed fairly sensitive to the physical position of the stubs relative to each other. Also, the filter needed a little LC matching circuit to move the passband frequency to the center of the 2-meter band. All the details are in the video below.

It’s pretty cool to see what can be accomplished with just a couple of offcuts of coax. Plus, getting some of the theory behind those funny little features on PCBs that handle microwave frequencies is a nice bonus. This microwave frequency doubler is a nice example of what stubs can do.

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Bring Out The Fine Detail In Small Objects With This Coaxial Lighting Rig

December 3, 2022 by Dan Maloney 11 Comments

All things considered, modern photography is pretty easy. It’s really just a matter of pointing the camera at the thing you want to take a picture of and letting the camera do the rest. But that doesn’t mean good photographs are easy to make, especially when fine detail is required. And that’s the reason this 3D printed coaxial lighting setup was built — to make quality photographs of small objects a snap.

The objects of [Peter Lin]’s photographic desire are coins, no doubt of the collectible variety. Since the condition of a coin is essential to determining its value, numismatic photographers really need to be meticulous about the quality of their work. The idea here is to keep the incoming light parallel to the optical axis of the camera, for which purpose ring lights around the camera lens are often used. But they can result in lighting artifacts, and can be awkward to use for such smaller subjects.

So for this setup, [Peter] essentially built a beam-splitter. The body is a printed block that’s painted matte black to keep reflections down; a little self-adhesive flocking paper helps with that too. The round aperture on the top is for the camera lens, with the square window on the side admitting light. The secret is a slot oriented at 45 degrees to both of those openings, into which the glass element from a cheap UV filter is inserted. The filter acts like a beam splitter which reflects light down onto the coin on the bottom of the block and lets it pass up into the camera lens directly above the coin, parallel to the optical axis. Genius!

The video below shows it in use with both DSLR and smartphone cameras, and the image quality is amazing. While most of us probably aren’t photographing coins, we do enough high-resolution photography of small objects that this seems applicable. In a way, it reminds us of [Big Clive]’s “TupperCam” method of high-res PCB photography (final item).

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MoCA Networking Is A Niche Solution For Coax Lovers

November 3, 2022 by Lewin Day 63 Comments

When it comes to networking these days, the vast majority of our devices are connected wirelessly. Beyond that, we’re all familiar with the Cat 5 and Cat 6 cables that form the high-capacity Ethernet networks in our homes, schools, and offices.

It’s only if you go back to the very dawn of Ethernet that coaxial cables are relevant… right? Wrong! MoCA networking is all about coaxial cables, designed to hook up devices over cable TV infrastructure!

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Omnirotor flies over obstacles with its gimballed, caged, coaxial rotors.

Gimballed Omnirotor Goes Over Great Obstacles

November 13, 2021 by Ryan Flowers 6 Comments

What can drive on the ground, hop in the air, and continuously move its coaxial rotor assembly without ever having to reset its position? The answer is [New Dexterity]’s Omnirotor All-Terrain Platform.

Although still very much a prototype, the video below the break shows that the dexterity claimed by Omnirotor isn’t just a lot of hype. Weaving through, around, and over obstacles is accomplished with relative ease by way of a coaxial rotor configuration that’s sure to turn some heads.

While not novel in every aspect, the Omnirotor’s strength comes from a combination of features that are fairly unique. The coaxial rotors are fully gimballed, and as such can be moved to and from any direction from any other direction. In other words, it can rotate in any axis infinitely without needing to return to a home position. Part of this magic comes from a very clever use of resources: The battery, speed controllers, and motors are all gimballed as one. This clever hack avoids the need for large, heavy slip rings that would otherwise be needed to transmit power.

Adding to the Omnirotor’s agility is a set of wheels that allow the craft to push itself along a surface, presumably to decrease power consumption. What if an obstacle is too difficult to drive around or past? The Omnirotor takes to the air and flies over it. The coaxial rotors are caged, protecting them from the typical rotor-snagging dangers you’d expect in close quarters.

[New Dexterity] has Open Sourced the entire project, with the Omirotor design, Firmware, and even the benchmarking platform available on Github so that others can share in the fun and iterate the design forward even further.

You might also enjoy this tetrahedron based omnirotor, or another omnirotor that knows how to play fetch. Really.

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Coaxcopter To Carry Man

September 3, 2021 by Danie Conradie 14 Comments

One of the major perks of all the affordable flight controllers and motors available from the hobby market is that you can really experiment with some crazy aircraft designs. [amazingdiyprojects] is experimenting with a coaxial helicopter design, with the goal off possibly using for a manned version in the future. (Video link, embedded below.)

The aircraft uses a pair of coaxial counter-rotating motors with large propellers, with several redundant control surfaces below the propellers. One of the theoretical advantages of this arrangement, compared to the more conventional quadcopter type designs, is redundancy. While a quadcopter will start tumbling when a single motor fails, this design will still be able to descend safely with just one motor.

It is also not dependent on the main motors for yaw, pitch and roll control. In multirotors, the motors need to keep a significant amount of the motor’s available power in reserve to increase torque at a moment’s notice for attitude control. This craft can use all the available thrust from the motors for lift, since control is provided by the control surfaces. There are five sets of redundant control surfaces below the propellers, each set connected to a separate flight controller.

Another advantage of this design is efficient for a given footprint, since one large propeller will always be more efficient than multiple smaller propellers. One of the goals for [amazingdiyprojects] is to fit the full size craft in a shipping container or on a trailer for transport without dissasembly.

[amazingdiyprojects] has built manned drones before, using both electric motors and internal combustion engines. And don’t miss the most gonzo wind tunnel ever at 7:00 in the video below. Continue reading “Coaxcopter To Carry Man” →