Engineering FM – Part 5

A little break (strangely coincidentally, just as my penultimate term ended and my final year project got finished and handed in) later, and here we are again – another post in my series on engineering FM transmission at a community radio station.

This time I’m going to be talking about how to properly power a transmission system, and how best to deal with uninterruptable power supplies (UPSes) in terms of cabling, monitoring and long-term care.

FM systems are much easier to worry about power and grounding for than AM systems, at least on smaller installations. There’s fewer moving parts (so to speak), and so long as your equipment is properly earthed, there’s no real issue with grounding (as opposed to AM where you get to deal with grounding mats and so on at your mast). The main issue then is that of power distribution and redundancy.

Most community stations aren’t going to have the budget or resources to deal with a truly redundant system – that is, one which can run without any utility power for a period in excess of an hour. On the other hand, outages that are likely to take your utility power away for more than an hour are, at most stations, quite rare. Such a highly redundant system requires a generator, a means for switching to and from the utility power and the generator, and if this needs to run without human intervention (as it ideally should), you also need all of this to be automated (typically with an automatic transfer switch). This lot gets quite expensive rapidly, and there’s running costs for a generator (fuel doesn’t last forever, so even if you don’t use your generator for 12 months you need to empty and refill the tank, service it, and so on).

The cheaper option is to spring for an uninterruptable power supply, or UPS. There’s plenty of vendors out there but APC dominates the field, and with good reason – they’re reliable bits of kit and I’ve personally used them for everything I’ve needed a UPS for. They’re also very easy to pick up second-hand on eBay, which makes them very favourable from a cost perspective. I’m typing this on a computer which is plugged into my room’s power distribution system, which is fed from an APC Smart-UPS 1500VA, which I originally picked up because of a very unreliable 16A ring breaker in my first student digs. That cost the princely sum of £170 with new batteries fitted – well within any station’s budget.

 

There are a few types of UPS, but mostly at the sort of level community radio operates at you can break the considerations down to ‘standby’ UPSes, ‘line interactive’ UPSes and ‘online’ or ‘double conversion’ UPSes.

Standby UPSes are the dumbest sort of budget, entry-level UPS, and they simply check for voltage within a certain range on the utility feed. If that voltage falls outside a certain range, the batteries take over. They’re cheap, work fairly well, but don’t handle undervoltage/overvoltage situations very well as the system has to switch to batteries to continue running.

Line interactive UPSes are slightly cleverer, and contain a multitap transformer to let utility power keep powering systems even in under/overvolt situations in addition to having batteries for total outages or extreme under/overvolt scenarios. These are the most common sort of UPSes found in small to medium size deployments, and don’t cost a lot more than standby UPSes.

Finally we have the online or double conversion type of UPS – these are the best available type, as they use (as the name implies) online conversion to effectively constantly power the output from a rectifier, meaning that the resulting power feed is clean, free of noise and harmonics, and effectively perfectly stable. In the event of power under/over voltage they can use line interactive style autotransformers to correct until batteries are required.

For RF transmission installations the online UPS is absolutely the right thing to have, mostly because they effectively ensure your power will always be clean and free of anything nasty that could affect your gear, but also because they typically have longer battery lives (due to being able to avoid switching to batteries more regularly) and are more easily expandable with add-on battery packs because of their design.

 

The other major thing we have to be concerned about in a broadcast environment is monitoring. UPSes are totally useless if they cannot be monitored, as batteries have a limited lifespan, and power loss situations should always be reported and followed up on, or at the least logged for post-mortems. If you’ve got a PC in the rack with the UPS, then this is trivial with most systems – all modern APC UPSes feature a USB or serial port to connect them to a PC, and SNMP cards are available as an upgrade for most (but not all) models, allowing you to talk to the UPS via the network.

For Linux, the excellent program apcupsd is available, which can be used to connect to, query and control an APC UPS via serial or USB. This can then be queried from your monitoring system (we use Nagios, but anything with a plugin architecture should be able to make use of it, like Munin) to ensure proper operation.

Keeping logs of device behaviour and self-tests (which you should have set up to run every month) is important, and must be done locally to the machine – networks, particularly large ones, tend not to be fully power-maintained, and you shouldn’t rely on them even if they are to be able to work out what went wrong after the event. apcupsd can write a nice event log to disk for you, and can be queried via a friendly web interface, making management and post-mortems quite straightforward.

Finally, with computers at the TX site, shutting down those machines before they run out of power is a good last-ditch precaution. If you’ve got a minute of battery power left, abandon all hope and shut down – but make sure that on power restoration, the machine comes back up automatically, or you’ll end up not getting your TX back when you get power back. Which is bad, obviously. Again, apcupsd can handle automatic shutdown for you.

The only other thing that you need to think about in your system is ground loops and distribution. A solid approach is to have the UPS plumbed directly into utility power, but if you do have a power strip in your transmission rack, isolate it with some plastic washers to stop it from being in contact electrically with the rack itself. The UPS power distribution unit (PDU) can then be grounded to the rack along with the other equipment. Make sure that your PDUs have switch covers to prevent accidental shut-downs.

 

Once you’ve installed everything and tested it, a runtime calibration test will give you an idea of how long your batteries last under expected use. Don’t forget to label all your cables on both ends, and to document what cable is which device, particularly with concealed power cable routing (behind equipment). Once you’ve done this, do a complete power down test – remove utility power, and time how long it takes for the system to fail, ensuring that your PC shuts itself down just before failure. Then restore power and ensure everything comes back up. If this doesn’t happen, figure out why and resolve the issue until you can pass this test, and you’ll be well-powered forever.

Well, almost forever. Most batteries in UPSes should be replaced every 5 years, but defer to the manufacturer for exact replacement schedules. This is an important task and can often be forgotten until one day the UPS only runs for 5 minutes instead of 30 minutes. Monthly maintainence procedures should include a check of the last replacement date in the UPS firmware, and you should update this date in the firmware after doing a replacement so you can keep track. This method makes it very hard to forget, since you’re regularly checking it.

And there you have it – how to keep your station on-air through brownouts, surges, powercuts and more. Given how cheap UPSes are and how beneficial they can be to the lifespan of equipment behind them (particularly online UPSes), it’s a no-brainer to have one at your TX site.