Wednesday, June 30, 2010

Monday at the TX, and installing new coax

Note: "TX" is shorthand for "transmitter". Also Texas.

Monday I took the readings at the KCUR transmitter. I remembered where almost everything was except for the bits about the T1 (the studio is connected to the TX via a dedicated T1. I didn't ask if it was fractional or full.) Robin had turned off the HD transmitter hardware (I'll be doing a post about terminology shortly, don't worry) the night before because it had overheated. This was due to the air conditioning being unable to cope with the level of heat in the transmitter building, as it had been beastly hot that weekend. Turning off the HD transmitter hardware allowed the building to cool down to normal levels.

It's worth noting here that the transmitter building is not properly air conditioned, according to Robin. It's not that the AC is underpowered. Rather, the AC is apparently placed wrong - it's set up to keep the building cool in the areas where humans would move around (i.e. it's blowing from the top), rather than where the air intakes for all the equipment are (i.e. near the floor). I always thought that cold air sank, though, so I'm not sure why blowing from the top is such a big problem. Maybe it's that the AC is blowing across the top of the equipment, effectively cooling the warm air that the equipment exhausts, rather than cooling the air going into the equipment.

Robin told me that there are a few big names in transmitter hardware. Broadcast Electronics is what we use, and they're apparently pretty popular, as I was easily able to find a video about their FM30B transmitter (by which I mean the big metal box that you connect to the broadcast antenna). There's also Harris, who apparently do all sorts of things besides make transmitters. Now, Robin tells me that both BE and Harris are out of Quincy, IL, but Harris' web site says they're based in Florida. A bit of Googling suggests that maybe their broadcast equipment division is in Quincy, or at least the broadcast equipment factory.

BE and Harris, along with Nautel (based in Nova Scotia), Continental Electronics (out of Dallas), and several Italian manufacturers make tetrode-based transmitters. A couple of other manufacturers, QEI, and CCA (now defunct, link goes to a guy who sells CCA replacement parts) use triodes. Both of these are ginormous vacuum tubes (yes, vacuum tubes aren't just for audiophiles and people restoring antique radios - they're apparently very popular for all sorts of RF applications) used to amplify the signal coming from the studio.

After getting the readings done, I had to run to the airport to pick up a friend, so Robin and I didn't do much more talking that day. The next day, however, or rather the next evening, I got to help Robin replace the feed line from our BE FM30B to the broadcast antenna. And this requires a description of this feed line. The transmitter is connected to the antenna via a coaxial line. But this isn't the regular RG59 that connects your TV to the wall. This is some high-grade stuff here.

Regular coax has a center conductor, an inner insulator, a braided wire shield, and an outer insulator. Wikipedia explains it better than I can, but basically the wire shield keeps the radio signal inside the cable, and the outer insulator keeps anything metal from touching the wire braid and screwing up its electrical characteristics somehow. The coax we use runs along the same principle, with a couple of minor differences.

First of all, the stuff we use is big. Regular coax has a diameter of maybe half an inch. The stuff we use is something like 4 1/2 inches in diameter. The inner conductor is hollow and about an inch and a half in diameter. There's no dielectric (the stuff between the inner conductor and the outer conductor). Instead, every so often there are Kevlar spacers to keep the inner conductor in the center of the outer conductor. The inner conductor is hollow, too - skin effect. The outer conductor is just a hollow copper cylinder, rather than braided wire. Both the inner conductor and outer conductor are rigid - to bend them, you've got to connect them to an elbow like you would a water pipe. And, like a water pipe (ok, some water pipes), there's no insulation on the outside. It doesn't seem to be a problem for the outer conductor to touch other conductive things, though - the feed line from the transmitter to the antenna relay (which in turn connects to the antenna) is supported by metal straps hanging from the ceiling. Maybe it's OK 'cos the straps aren't grounded.

I'm sure you can order this stuff cut to any length you want. That gets expensive pretty quick, though, I'll bet. At KCUR, we just have these huge (~ 15 ft) lengths of the stuff that you have to cut yourself with a hacksaw or a cutting disc. Precision is not required, though - using the elbows and such you've got a little bit of wiggle room.

So when I got to the transmitter last night, the first thing to do was to switch over to the backup transmitter, which is an Armstrong unit. It's about 1/3 the size of our main transmitter setup, and doesn't do HD at all. I think it's lower power, too. Anyway, to do that, Robin took the power down on the BE FM30B, warmed up the Armstrong, then switched from the BE to the Armstrong. I think we may have been briefly off-air, but I'm not sure.

Once the BE was down, and we were sure that the Armstrong was operating properly, we measured the distance from the harmonic filter on top of the BE to the elbow coming out of the antenna relay. 18.5", give or take. (Remember what I said about wiggle room). Anyway, after we had the run measured, Robin and I took the ginormous length of coax outside and set it on top of one of the AC units. We figured that would make a good solid base for sawing on. I measured 18.5 inches, then Robin said we needed to take an inch and a half off of that. I misunderstood him to mean that the length of both inner and outer conductor (the inner is inside the outer) needed to be 17". In fact, the inner conductor needed to be 17". IIRC, the inner conductor in the elbow sticks out past the outer. Anyway, because I misunderstood him, I cut both pieces to 17", which was too short. Because I am not good with a hacksaw, and because the hacksaw we have at the transmitter is kinda crap, I cut the copper at too much of an angle. So we had to cut a new piece, but we should clean the piece we did cut up first. No sense in letting it go to waste, after all.

Cleaning up a piece of coax basically means filing down all the sharp edges left over from the hacksaw. If you don't do this, you end up with places where the current collects, and that could heat up the copper and potentially burn a hole in one or both of your conductors. Bad times. But this piece not only had burrs (sharp bits) from being cut with a hacksaw; it was cut at an angle too far off from the vertical to be within that "wiggle room" I mentioned earlier. Getting that piece down closer to square with just a file sounded like a long and arduous task to me, so I suggested we take a bench grinder to it. Robin just happened to have one at his house, so off we went. An hour and various assaults with file, bench grinder, and cutting disc later, we had a reasonably square piece about 16.5 inches long. :) We still needed that 18.5" piece, though, so back to the transmitter we went with Robin's considerably nicer hacksaw.

Once the piece of correct length had been cut and filed, it was time to install it. This is not exactly easy. Getting the elbow or the flange on to the outer conductor isn't bad. It's just a press-fit type affair that you tighten up with a hose clamp. And connecting the flanges isn't bad - you just bolt them together. No, the real trick of it is getting the inner conductors to connect. The inner conductors are joined by these things called "bullets" - metal dinguses slightly smaller than the inner conductor with a stopper ring in the middle. But these bullets aren't solid - they're hollow, and they have "fingers". The last thing you want is for those fingers to split, so you put a thingy in the middle of all the fingers (I wish I could find pictures, 'cos I can't explain this properly) to keep that from happening. These things always fall out, requiring you to stick them back in before you try to get your inner conductor over the fingers, which also never works. But eventually, with much manhandling and bumping of your head (because the building is cramped), things get connected and bolted together.

Once everything was back together, Robin shut down the Armstrong (or possibly shifted it into the dummy load and then shut it down, I'm not sure) and brought the FM30B back up. Everything came back up properly, and we went to our respective homes. It was 0100 or so. :)

Oh, I also got to see Robin's ham (radio) shack and meet his cat, which was cool. :)

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