Coverage Area of your Part 15 Radio Station

One of the first things any one with any sort of radio station wants to know is their coverage area. How far are you reaching out?  Who can hear you?  How's that transmitter working?  Indeed valuable information to have.

  The first thing a Part 15 operator tends to do is head out in the car, drive around, and see how far from the station he can get and still hear it.  This is a great idea to give you quick verification that you are indeed transmitting and that someone may hear you. But in reality driving around in your vehicle is one of the worst ways to check your coverage.  The radio in a typical vehicle is equipped with all sorts of circuits to “even out” reception – that's why they work so well as car radios when you're driving around. To some extent they automatically adjust for varying signal strength, otherwise you'd go nuts trying to listen to ANY radio station, much less a low power one.  Additionally a car radio is MUCH more sensitive than nearly any other radio a person is likely to use in their homes, what with an external antenna, the ground plane effects of the car itself, and the fact that it's outside.  So you get an exaggerated perspective of how listenable your signal is.  I'm going to give you a few tips and ideas as to how to get a better grasp on your signal and coverage area.

  Clearly the best way to do this is to use an actual Field Strength Meter, or more accurately, a Field Intensity Meter.  The type used by actual radio station engineers to make sure they are meeting the FCC limitations for their stations.  A new modern device for this purpose is very expensive – even for a broadcast engineer, coming in somewhere around $15,000 for a base model.  The good news is there are vintage units that are still quite serviceable and in fact are used regularly by broadcast stations engineers with accurate, calibrated results.  Clearly for a Part 15 station you're not necessarily trying to meet FCC rules by using a FIM, but they can still give you good information.  If you were to seek out such a unit some models to watch for are: The Nems-Clarke 120-E.  I presently use one of these in maintenance of a directional array AM station that requires specific limits at various monitor points at headings around the transmitter site.  Accuracy passed our recent “Mock FCC Inspection” where an actual inspector did a full FCC inspection of our stations to insure our compliance.  This model uses “D” batteries, two 67 ½ volt batteries and a nice selection of tubes.  Yes.  Tubes.  A new set of tubes lasts nearly forever, “D” batteries are readily available, and if you don't like using the expensive and hard to get vintage 67 ½ volt carbon batteries (there are no alkalines for this application) you can make up battery packs to replace them with standard 9V batteries that again, nearly last forever.  These machines are vintage, analog, somewhat mechanical, but completely serviceable.  As far as I know no one is calibrating them any more, but since the FCC deregulated so much years ago all a commercial station has to do is monitor accurately enough and often enough to know they're in compliance. Most stations have more than enough leeway to allow for some pretty inaccurate meters. As long as they come in under the limit, they're OK. I calibrate this one against one of those $15,000 units owned by our contract engineer.  If you don't bang them around they stay pretty darn accurate.   But, like any vintage, limited use equipment, there never were a LOT of them, and they're getting harder to find.  They only tune to 1600 Khz according to their specs, but I have no trouble tuning to my Part 15 station at 1620 even though this is beyond what the unit is designed for.  

If you can't find a Nems-Clarke 120-E watch for an RCA WX-2C.  Similar to the N-C unit – in fact more than similar but I don't have the time to go into the corporate relationship – These often turn up for sale by people who have no idea what they are. Again, tubes, “D” and 67 ½ volt batteries.  But very serviceable.  And usually cheaper than the Nems-Clarke units. The Holy Grail of vintage FIM's is the Potomac FIM-41.  Far more capable with many other features, tuning 540 Khz to 5 MHz.  But this capability doesn't come without a price. Solid state and requiring only “D” batteries these often sell for as much as $2,000, because they're in demand by radio engineers. But you never know when you might find a deal.  Ask at radio stations, check with engineers. Put the word out.  I know a contract engineer who was GIVEN one after the station was bought by a big group and their fancy group engineer had the big dollar unit so the old stuff was no longer needed.  With one of these units listed about you can head out and take actual readings of just how much signal you have in the air at any given location.  Read the instructions and practice using and calibrating with the built in oscillator before you head out.  It seems complicated until you get used to it.

If you can find, can't afford, or simply don't want to invest in a FIM you can STILL get a good idea of your station coverage, and here are some tips.

First get a map.  Not a state highway map.  Go to the county office, city office, or wherever in your area they have actual maps.  Secret: You can also use Google maps, or better Google satellite view.  Get a map where you can make circles out to about 3 miles from your transmitter site.  If you're getting more than three miles something has gone devilishly wrong!  Put a dot where your transmitter is.  Measure out ¼ mile, ½ mile, ¾ mile, 1 mile, maybe 1 ¼ and 1 ½ mile too if you like. Draw the circles right on the map.  Goggle will facilitate you doing this on their satellite maps and it's really handy but I'm not the one to explain to you how to use Google maps!  Or just use your paper map from the County, City or wherever, use the distance scale and a compass. Once you have your circles you can take a look and see where they intersect with roads, parking lots, any place you can drive to and park for a few minutes without getting in anyone's way.  Try to choose spots around the circle, I suggest at least 8 spots as equally separated as possible on each circle.  Maybe you want to add a few spots of special interest like nearby beaches, business districts, stores, etc. Places where you would like to know if your signal reaches.

Once you have your spots you're ready to head out.  Clearly if you've purchased one of the FIM units above you want to bring it along and take actual readings.  But if you didn't, don't give up yet!  Your car probably has a radio.  There's a place to start.  Then, grab some portable radios.  Maybe a little transistor radio, a boom box, a big ol' portable from the 60's, whatever you have.  Be sure they have fresh batteries in them as weak batteries can greatly decrease reception ability.  If at all possible see if you can pick up any sort of AM radio that has a signal strength meter in it.  This means nothing as far as actual field strength but will give you a relative reading that can give you figures you can compare both in different directions and at different distances.  Go to each spot.  Listen to the car radio.  If all is well, shut OFF the car.  See if the radio sounds even better!  Running cars can generate some weird noises in the AM band, especially when listening to weak stations.  Then GET OUT OF THE CAR, walk 15-20 feet away from the car, and try out the gaggle of radios you brought with you.  Remember that portable AM radios have very directional antennas in them, so turn one on, rotate it for best signal and note how well it's receiving.  Shut off the radio you just used before trying the next one, as the radios can actually interfere with each other.  I forgot to mention way above – bring a big legal pad and several pens.  Note your location.  Write down how well each radio received your signal.  If you have a FIM write down your readings and double check them. Then move to the next spot and do it again.  Work your way around the circle then move to the next circle and start over.  By the time you're done you'll know if your signal is stronger in certain directions, you'll know what your vehicle can pick up that a portable radio can't.  If you have a radio with ANY sort of field strength meter on it, rotate for best reception, write the number of the reading down, and go to the next one. There is virtually NO portable radio that gives you any sort of real field strength readings but in this case you're looking for RELATIVE readings.  You can compare directions and distances.  You might be surprised how much your signal can vary by direction as well as distance.  When you're out doing readings if you choose spots near buildings, go inside and listen for what you can receive.  Indoor reception will be quite different than outdoor reception.

When all is said and done, you'll have several sheets of notes and a pretty good idea of where your signal goes, how far it goes, and how listenable it is on various radio sets.  For a real experiment use someone else's vehicle and visit some of the spots, especially on the fringe, you may find their car receives further away, or not nearly as far as your car can.  The ability of AM car radios to receive varies GREATLY from vehicle to vehicle, even among the same makes and models of cars.

Once you have all this data, save it.  Then 6 months later, or whenever, you can head out again and see if there is any variation.  Seasons, position of the sun, Earth conductivity, and many other factors can have an affect on your signal.

They key is the map, with points at a given distance from the transmitter.  Reading the car odometer tells you how far you drove, not how far you are from the transmitter, unless your station is located in the center of a set of wagon wheel spoke roads.  Even if a road goes straight for three miles right from your transmitter, that's going to give you data for one direction.  What if there's something blocking your signal?  What if there's something along the route reflecting your signal?  This will give you exaggerated increased or reduced signal results.

When I did the coverage area test for my station, I tested at 38 points in and around town, at the fringes of town, and out on the highway well past town, just to see where I wound up losing useable signal. I used a Nems-Clarke 120-E that I use in my daily work as engineer for an AM station, which is calibrated. I noted all my readings, then several months later when and did them all again, and they were all virtually identical.  This tells me no evil has befallen my transmitter and I'm presenting my listeners with a consistent signal.

This should give you some ideas to help you determine your useable coverage area.

Tim in Bovey

FCC General Radiotelephone Operator License
SBE Certified
31 Years Commercial Broadcast Engineer
Amateur Radio KC0JEZ