December 2018
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Brief Update

All is well in Engineering Radio Land, just very busy with typical projects. Nothing really note worthy, but there are some interesting things in the pipe line, which will be posted once those projects begin. Regarding myself, Hockey Season is in full swing.

Ruined shot on goal

My son (in blue) messing up somebody’s shot on goal. Good stuff.

I received an interesting question from occasional reader Gary over the weekend:

Have you ever heard of, or looked into, the “Max Headroom incident”?

Yes, I have heard of it, but never really thought about it that much. At that time, I was in California getting ready to ship out to Guam. I remember some brief news reports on it when it happened. After some reflection, I sent along this reply:

Good question. I have done a little bit of work at TV stations from time to time. In the mid to late 80’s, most television stations used 6 GHz analog microwave links between their studios and transmitters. These where unencrypted. Most often, these links used Microwave Associates gear, which had transmitter power output of about 3 watts into a rectangular wave guide. That was coupled, via elliptical wave guide into a 24 or 36 inch parabolic antenna. As you know, parabolic antennas have a main lobe and side lobes in their radiation pattern. The smaller the dish size, the broader the main lobe is and the further out the side lobes are. Since the Chicago studio to transmitter path was fairly short, I’d hazard a guess that they where using 24 inch antennas. In major markets, there was often a backup STL system on a different frequency (as the Wikipedia article indicates). Later on, most stations had a third backup via the cable company using either directly fed coax or fiber optic cable.

Since cutting into the transmission line of the microwave link at either side of the system would almost certainly fail, what I think happened is the perpetrators discovered (easy to do) the frequency of the 6 GHz link and over powered the normal signal at the receive antenna. This is possible if they used a much larger antenna and where located in either the main lobe or a side lobe of the receive antenna. If you recall, in the late 80’s C band satellite dishes where very popular. It would not be too difficult to repurpose one of these dishes for a 6 GHz antenna. Most C band dishes where 5-6 feet in diameter, which would give them much more gain than a 2 foot dish. They simply would have had to figure out a way to feed the dish with elliptical wave guide and adjust the focal length for 6 GHz. I’d bet there where dozens of C band dishes on Chicago roof tops.

Anyway, that is my theory.

I watched a few youtube clips of the event. The fact the video was noisy indicates some type of co-channel interference. I think there was no audio because they guessed the microwave audio subcarrier frequency wrong the first time. The second incident, there was audio. Whoever did to this had to have pretty good knowledge of television STL system. Wikipedia article here:

What do you guys think?

In other news, I have begun messing around with a few ideas on single ended tube audio amps and contemplating a DIY tapered transmission line speaker build. Actually, there is some pretty interesting software out there for speaker design which would be fun to play with.

I have also been messing about with room EQ Wizard, which I think I will do a separate post on.

That is all from here, hope that all of you are well.

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