time domain reflectometer

2 Articles

Ask Hackaday: Is Our Power Grid Smart Enough To Know When There’s No Power?

June 23, 2020 by 63 Comments

Just to intensify the feeling of impending zombie apocalypse of the COVID-19 lockdown in the British countryside where I live, we had a power cut. It’s not an uncommon occurrence here at the end of a long rural power distribution network, and being prepared for a power outage is something I wrote about a few years ago. But this one was a bit larger than normal and took out much more than just our village. I feel very sorry for whichever farmer in another village managed to collide with an 11kV distribution pole.

What pops to mind for today’s article is the topic of outage monitoring. When plunged into darkness we all wonder if the power company knows about it. The most common reaction must be: “of course the power company knows the power is out, they’re the ones making it!”. But this can’t be the case as for decades, public service announcements have urge us to report power cuts right away.

In our very modern age, will the grid become smart enough to know when, and perhaps more importantly where, there are power cuts? Let’s check some background before throwing the question to you in the comments below.

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Fast Video Covers Coax Velocity Factor

October 11, 2019 by 14 Comments

We once saw an interview test for C programmers that showed a structure with a few integer, floating point, and pointer fields. The question: How big is this structure? The correct answer was either “It depends,” or “sizeof(struct x).” The same could be said of the question “What is the speed of light?” The flip answer is 186,282 miles per second, or 299,792,458 metres per second. However, a better answer is “It depends on what it is traveling in.” [KB9VBR] discusses how different transmission lines have different velocity factors and what that means when making RF measurements. A cable with a 0.6 velocity factor sees radio signals move at 60% of that 186,282 number.

This might seem like pedantry, but the velocity factor makes a difference because it changes the actual measurements of such things as dipole legs and coax stubs. The guys make a makeshift time domain reflectometer using a signal generator and an oscilloscope.

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