MIKE LANGER PROVIDES A NON-TECHNICAL EXAMINATION OF YOUR OPERATING POWER
You want the best possible, most powerful signal you can put on the air to attract and hold an audience, right?
In this two-part series we’ll examine in non-technical terms how to know of your station is operating at all the power it legally can and if your station is as loud as it can legally be. Additionally, we’ll explore “audience ear fatigue” (yes, it’s real) that could be causing listeners to change stations away from yours.
You’re perfectly comfortable reading a speedometer, a thermometer, or checking your weight on a scale, right? Great. That, and a tiny bit of 4th grade arithmetic is all you need!
FM Station
Let’s start with how to determine the power of an FM station. There are two power indications on the transmitter – it’s good practice to compare them. If they don’t agree, call your engineer!
The first thing to look at is easy – it’s a meter that says “Power Output” or something similar. Just look at it. It should read between 90% and 105% assuming it was set up properly. Easy peasy!
The next one requires the tiny bit of 4th grade arithmetic. Look for two meters. For older transmitters they’ll be labeled “Plate Volts” and “Plate Current.” Make a note of what these two read. Now look to see if your engineer has written the transmitter efficiency on the front of the transmitter. If not, take a peek in the transmitter’s manual for the manufacturer’s test data – you know, the measurements that the manufacturer made to prove that your transmitter works properly before it leaves the factory. Can’t find the manual? Can’t find the test data? Call your engineer! Hint: if your engineer can’t find it either, or questions what’s in the manual, have your engineer look up your latest FCC Form 302-FM on the Internet. That’ll have it!
The number you want is called “Efficiency.” Essentially in this case it means how much power is being used in the part of the transmitter that makes the power for your antenna. Nothing in life is 100% efficient, so the number should be somewhere between 70% and 90%. Here’s where the 4th grade math comes into play.
Write down the Plate Volts, the Plate Current, and the efficiency. Let’s say the efficiency is 78.5%. The 4th grade math is Plate Volts reading times Plate Current reading times the efficiency expressed as a decimal – that is, 78.5% becomes .785 for your math. Let’s say the Plate Volts is 9000, the plate current is 2.8 amperes, and the efficiency is 78.5%. Here’s our math: 9000 times 2.8 times 78.5 equals 19, 782 watts. If your transmitter’s licensed transmitter power output is 20,000 watts, for example, you’re close enough. That’s 99% of your licensed power. You’re in good shape!
What if you have a newer FM transmitter that doesn’t have any tubes in it? The part of the transmitter that makes the power for the antenna will have its meters labeled “Collector Volts” and “Collector Current” or something like that. Ask your engineer – he or she will be thrilled that you care!
The whole point is to run as close to exactly 100% of your license power as is reasonably practical.
But why do the two methods? Why bother with the 4th grade math part? Because that “easy to read” Power Output meter can be set to read anything by anyone with access to it. It can be set to read 100% power when your station is realistically only making 50% of full power. How? There’s a little screwdriver adjustment inside the transmitter. It’s truly worthwhile to check the FM transmitter by the 4th grade arithmetic method!!
AM Station
Measuring the power of an AM station is likely to require that you walk out to the tower and look at a meter out there. The meter you’ll be reading is called the “antenna base current meter.” This makes sense, since the meter is at the base (the bottom) of your antenna tower. Don’t touch the tower – the way most AM towers are hooked up means if you reach out to touch it or anything connected to it you’ll get a nasty burn. There are some towers that are safe to touch, but even they often have wires hanging from them that can give you a nasty burn.
At the bottom of the tower will be a small structure or perhaps a metal box. Inside it is the meter we’re looking for, the antenna base current meter. In older installations here will be a “plunger” type switch to turn it on – we never leave them on because one lightning strike can wipe it out in less than a second! These older type meters are usually in a black plastic case. Newer installations which typically are in a grey case have a safer, less vulnerable kind of meter which typically has a very small switch, if it has any switch at all.
In any case, see what it reads when there’s a pause in the modulation – that is, when some announcer pauses to breathe, or in the short quiet time between a commercial and the program. You might have to wait a few minutes since the meter will be wiggling around to the sound of your station. Write down what it reads. Now turn the meter switch back off and go back into the transmitter building.
Your license will have a number on it called “Antenna Resistance.” Write that one down, too. Now here comes the AM station version of 4th grade math! Take the number you got at the tower, let’s say it’s 3.7. Multiply the reading from the tower by itself – that is, in our example, 3.7 times 3.7. That will give us 13.69. Write that down, we’ll need it in a minute. Now find that “Antenna Resistance” number your wrote down from your license – let’s say it’s 73 ohms. Here’s the 4th grade math part! Multiply the 13.67 you got by this number from your license – that is, 13.67 times 73 which gets you 997.9. In our example, we’re looking at a 1,000 watt AM station. 997.9 watts is close enough to our licensed 1,000 watts! If you’re one of those rare stations that uses several towers at night (directional antenna) you’ll have another meter to check – for your “after dark” hours. Since you’re using several towers, reading any one tower won’t tell you how much power your station is running. In this case, inside the transmitter building will be a meter labeled “Common Point.” To figure your power after dark, read this meter, and compare it with what your license says it should read at night.
Note: there area a very few AM stations in NM that use more than one tower during the day – for these stations, you’ll want to read the “Common Point” meter, not any meter at the towers. Save yourself a trip out to any of the towers!!
And that’s all there is to it! Given the number of stations I regularly find in my ABIP visits that are running more than their licensed power or less than their licensed power, checking up on this part of your operation from time to time is a really good idea!
Remember – the FCC limits on your station’s power are 90% to 105% of what your license says. More or less power than this, should an FCC inspector come by, could result in a Notice of Violation and a fine!
Bonus! Your engineer will be really happy that you’ve taken the time to look at what he or she works with every time your engineer visits your transmitter!!
Next time we’ll look at how your station can be loud, but sound really good! It’s easier than you think!
