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. :)

Sunday, June 27, 2010

Friday at the KCUR Transmitter

Friday was largely uneventful at the transmitter. Robin and I talked about a few things, including how he could reduce the actual power going into the transmitter if he could get it up a bit higher. He also told me that he had a site survey done to see if it was feasible for us to set up a wireless data link between the studio and the transmitter. The result of the survey? We would need to put the studio antenna at a height of 75 or 100 feet for it to work, which would require putting up a tower on the roof of the studio. As the studio is a University building, University regulations would require contractors, insurance, and a host of other things that would make the job much more expensive than if were being done at somebody's home. Which is a pity, as I was really looking forward to building such a beast. Maybe I'll spec it out anyway as an exercise. Robin also told me that he got the conductor he ordered in (it's replacing a line going from the transmitter hardware in the equipment building to the transmitter hardware on the tower), so maybe this coming week I'll be helping him install that. I hope so. :)

Later that day I went to help set up the ARRL Field Day activities being hosted by the Raytown Amateur Radio Club. I helped put up a big antenna mast with three horizontal antennas on it (Like the one on the right in this photo. I don't know what sort of antenna it was and didn't think to ask.) and someone's satellite antenna. I was surprised to find that the satellite antenna was this sort of V-shaped affair, rather than a dish.

Saturday I went to the field day proper, but it wasn't exactly clear what there was for me to do. People seemed to be intent upon their own business, and as I don't yet have a license, I didn't think I was allowed to ask, "Hey, can I try to transmit on this equipment?" As it turns out, there was a GOTA (Get On The Air) trailer set up, but it wasn't clear that was what it was. I realize that I could have asked, and I probably should have asked. But I let my timidity get the better of me. I also could have come back later, but by that point I was feeling ill and in no condition to go anywhere. Next year, perhaps.

So it occurs to me that all I've been talking about is KCUR and radio stuff. That's not all I'm interested in. I'm also interested in motorcycle maintenance, as I think I've stated previously on this blog. I'm at a bit of an impasse there, however. To learn more about motorcycle maintenance requires having a motorcycle in need of maintenance, and my motorcycle, thankfully, is not in need of the maintenance type things I want to learn more about - mostly engine stuff. I can do an oil change, replace turn signals, and replace a chain. I feel confident that I could do my fork seals if I needed to. I can replace my own brake pads, and rebuild my brake calipers. I can replace my own spark plugs and coil wires. But I've never dipped my carburetors. I've never adjusted my valves. I've never been inside the guts of an engine, and that's something I want to learn about. I asked if I could work some hours at Blue Star Motorcycles in exchange for wrenching lessons, but Don (my friend who's the GM of the shop) said he didn't have enough work coming in for that. Maybe I should see if I can just hang around and pester the mechanics with questions.

Wednesday, June 23, 2010

And we're back!

It's a new day here at KCD, and I've decided to take this blog in a new direction. I've decided to start pursuing other career options besides IT, and more broadly I'm going to start pursuing the things that interest me. It's my goal in life to be as competent as possible in as many areas as possible.

One of those areas is radio. I've long been fascinated by radio. I can remember as a child staring at microwave towers. There was also a radio transmitter (my parents called it a booster station, I'm not sure if that was accurate or not) by my house; I always wondered what was in the little attached shack.

Having recently become unemployed, I found myself with a lot of time on my hands. In an effort to fill up some of that time in pursuing interesting things, I decided to call up KCUR's chief engineer, Robin Cross, and ask him if I could help out around the station in exchange for learning about radio engineering. He said yes. :)

To start with, I've been meeting Robin on Mondays and Fridays at the KCUR transmitter. This past Monday, Robin let me take the readings for the maintenance log. He handed me the clipboard and told me to have at it, and to come to him with any questions. The maintenance log is laid out very straightforwardly, so it wasn't difficult, but it was still cool.

Like many radio stations, KCUR uses RDS to transmit information about what's currently playing. This is handled by a dedicated PC at the transmitter running software that interfaces with the RDS transmitter. If we had a dedicated data (i.e. TCP/IP) link from the station to the transmitter, we could let the on-air people edit the RDS via a computer in the studio. We do not have such a link at the moment, but I have an idea for one: set up a point-to-point 802.11g link between the station and the transmitter. This will require a specialized antenna, as the station and the transmitter are roughly 7 miles apart from one another as the crow flies, but I think we can do it. It will certainly be fun to try.