Finding RF Cable Impedance

At DC and low frequency, we can pretend wires are perfect conductors. At radio frequencies, though, there are many effects that you need to take into account for wires and cables. One of these is characteristic impedance. If you have a marked cable, you can look it up on the Internet, of course. But what if you don’t know what kind of wire it is? With help from [The Offset Volt], you can measure it as he shows in the video below.

This is one of those things that used to take exotic test equipment like an LCR bridge, but these days meters that measure inductance and capacitance are commonplace. The trick is simple: measure the capacitance and then short one end of the cable and measure the inductance.

Continue reading “Finding RF Cable Impedance”

Oliver Heaviside: Rags To Recognition, To Madness

Like any complex topic, electromagnetic theory has its own vocabulary. When speaking about dielectrics we may refer to their permittivity, and discussions on magnetic circuits might find terms like reluctance and inductance bandied about. At a more practical level, a ham radio operator might discuss the impedance of the coaxial cable used to send signals to an antenna that will then be bounced off the ionosphere for long-range communications.

It’s everyday stuff to most of us, but none of this vocabulary would exist if it hadn’t been for Oliver Heaviside, the brilliant but challenging self-taught British electrical engineer and researcher. He coined all these terms and many more in his life-long quest to understand the mysteries of the electromagnetic world, and gave us much of the theoretical basis for telecommunications.

Continue reading “Oliver Heaviside: Rags To Recognition, To Madness”

Long PCB Shows Effects Of Ludicrous Speed

Transmission lines can seem like magic. When you make use of them it seems strange that a piece of wire can block or pass certain frequencies. It is less common to use transmission lines with pulses and typically your circuit’s transmission line behavior isn’t all that significant. That is, until you have to move a signal a relatively long distance. [Robert Baruch] has been using a long PCB to test pulse behavior on a bus he’s working on. He actually has a few videos in this series that are worth watching.

What makes it interesting is that [Robert] has enough distance on the board to where light-speed effects show up. By using a very nice DPO7104 oscilloscope and a signal generator, he shows how the signal reflects on the line at various points, adding and subtracting from it. The measurements matched theory fairly closely. You shouldn’t expect them to match exactly because of small effects that occur randomly throughout the system.

Continue reading “Long PCB Shows Effects Of Ludicrous Speed”

Model Of A Transmission Line

Transmission lines are the kind of thing that seems to confuse beginners. After all, the fact that short-circuits can have infinite impedance and open-circuits can behave like a short is not intuitive at all!. That’s why we like [Tinselkoala]’s latest video that shows a nice model of a transmission line. It helps to understand the line as inductors and capacitors in series-parallel connection.

Any pair of wires used to transmit electrical power have tiny amounts of inductance and capacitance. This is not a problem with DC or low-frequency AC, but when the frequency is sufficiently high, weird things start to happen. The energy tends to escape as radio waves, and current reflects from discontinuities such as connectors and cable joints.  For this reason, transmission lines for high frequency signals use specialized construction to minimize those effects and reduce power losses.

[Tinselkoala] has built a model of a transmission line using coils and capacitors to simulate the inductance and capacitance of the line, with LED’s placed between the coils. He feeds the system with the signal generator with frequencies from 10 kHz to 1 MHz. In his words, they act as simple “visual voltmeters” to show the peaks and nodes of the standing waves of voltage in the line.

It is relatively simple to build your own version if you want to experiment with this fascinating subject. You will only need some magnet wire, capacitors, resistors and LED’s. If the subject sounds interesting to you,  here you can find an excellent introduction to transmission lines.

Continue reading “Model Of A Transmission Line”

HF-Powered Drone Antenna

Amateur radio has a couple of sweet allocations in the VHF bands, but because the signals don’t reflect off the ionosphere like shortwave signals, the use is limited basically to line-of-sight. One workaround is to use a repeater with a tall antenna, but that requires a lot of infrastructure or a mountainside lair.


What if you could just fly your antenna up in a drone? Well, for starters, you’d run out of batteries pretty quickly unless you could power it remotely. And if you try to tether it, the supply wires end up being too heavy to lift. Or do they?!?!

Continue reading “HF-Powered Drone Antenna”

Retrotechtacular: Transmission Lines

This great old video (embedded below the break) from Tektronix in the mid-60s covers a topic that seems to confuse folks more than it should — transmission lines. We found it on Paul Carbone’s blog, a great site for aficionados of old analog scopes in its own right.

As with many of these older videos, the pacing is a bit slow by today’s standards, but the quality of the material eventually presented more than makes it worth the effort to reign in your ADHD. For a preview, you can skip to the end where they do a review of all the material.

They start off 5:31 with a pulse travelling down a wire pair, and take a very real-world approach to figuring out the characteristic impedance of the line: if the pulse was created by a battery of 9V, how much current is flowing? If the DC resistance of the wire is zero then there should be an infinite current by Ohm’s law, and that’s clearly not happening. This motivates the standard analysis where you break the wire down into distributed inductance and capacitance.

Of course they do the experiment where you inject a pulse into a long loop of coaxial cable and play around with the termination at the other end of the line. They also measure the velocity factor of the line. Our only gripe is that they don’t tap the line in different places to demonstrate standing waves. The good news is that we’ve got YouTube (and [w3aew]) for that.

If you’ve got 23 minutes to spare, and are curious about transmission lines or just enjoy the soothing voice of a trained radio announcer reading out values of various termination resistors, this old gem is just the ticket. Enjoy!

Continue reading “Retrotechtacular: Transmission Lines”

Hackaday Prize Entry: A Very Small Power Grid

If it hasn’t been made readily apparent to you by now, power grids are astonishing marvels of technology and quite possibly one of the greatest engineering feats of history. Learning how these systems work is easy in theory, but in practice you will be shot if you try to screw around with at a power station. [Tim] and [Marissa] figured there must be an easier way to learn about power grids so they made their own. It’s small, but it still has everything you’d find in high voltage power lines, minus a hundred kilovolts or so.

This mockup of a power grid simulates a power plant by taking a normal DC motor and connecting that to an alternator and transformer. This is two of the simulated generation points, with the third AC/AC power supply serving as a reference generator for synchronizing phase and frequency. It’s only 12V at 60Hz, but it gets the job done.

A power grid isn’t power plants – there’s also transmission line theory. For this, [Tim] and [Marissa] have a few boards packed with inductors to simulate power lines. There are boards for simulated loads, and synchronization systems built on the MSP430.

In the video below, [Marissa] goes over all the ins and out of the system. It’s very well made and excellent for teaching something that can’t be demonstrated without a practical example.

The 2015 Hackaday Prize is sponsored by:

Continue reading “Hackaday Prize Entry: A Very Small Power Grid”