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DO YOU WANT TO LISTEN TO THE AIR TRAFFIC CONTROLLERS WITH YOUR OLD FM RADIO???

 

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Listen to the air traffic controllers is possible without buying a radio scanner worth 120 $. The first thing we have to know is that the communications between air traffic controllers and pilots are developed among the 118 and 136 MHz frequency (more or less), modulated in frequency, I mean FM.

If you look at a FM radio, you will see that the commercial radio band works from the 88 to the 107 MHz. In order to tune in the frequency of the control tower of an airport, which is around the 118 MHz, we just need to increase our frequency capture in about 11 MHz, what is achievable.

So, it is worth to learn a new formula that has something to do with the frequency and it says that: the frequency is equal to the inverse of the square root of the capacity multiplied by the inductancy.

Well, keep cool we wonīt have to calculate anything. This formula is useful because this way we certainly know that if we decrease the capacity or the inductancy we will be able to increase the frequency till 20 MHz, which is the maximum rise of frequency that I have been able to achieve.

 

1.- The first step is seek for an old FM or FM/AM radio, what do I mean with old???? At first sight the radio has to tune in the radio stations in a manual way (nothing digital), i mean, with the common cogwheel that when you turn it the radio station you were listening to change. Finally i mean the tuning.

2.- The second step is open the radio in order to see the inside. The first clue that will tell us if this radio is suitable or not for our purpose is the capacitor that is used to tune in the radio stations. To find it out what you have to do is to search for the gear that moves the tuningīs cogwheel. If this capacitor looks like this, we are doing well, this is a capacitor that has four small capacitors like these.

  

 

 But you can also find something like what is showed in the following photo. In this photo you can see four screws that are at the other side of the circuit where the main tuning capacitor is placed. This four screws change the capacity of the small tuning capacitors.

 

  

3.- The third step is test which of the four small tuning capacitors works in order to tune FM. To find that out, we just have to listen to a FM radio station and then turn the small tuning capacitors one by one. If when we turn one of them, the radio station we were listening to changes is because we have found our small tuning capacitor. So from now on is very important not to forget which of them is the useful FM capacitor.

  4.- The fourth step is find the FM coil in order to decrease inductancy (L). The coils you can come across in a circuit can be like this one that is showed in the second upper photo, with the sign COIL. These coils are kind of a plastic tube with a wire wound around the tube and in the center part there is a screw that is called nucleus. As you can see in the photo, the coil have no nucleus (screw) so as to decrease inductancy as we aim.

Another kind of coils you can come across in a circuit are these that are showed in the photo below. They are "bare coils" and the way we reduce inductancy in these coils is by separating its turns. Simply keep the turns away from each other.

 

 

Well, in the circuit of a radio there are lots of coils, so the question is: How do I know which one is the FM coil??? To find out lets listen to a FM radio station and mess around a bit.

If our radio have the first kind of coils I have just explained, we just have to unscrew the nucleus. Otherwise if our coils are "bare coils" we just have to put the screwdriver through the turns of the coil. In both cases once we hear any kind of estrange sound or interference, that means that this is our FM coil. So lets reduce its inductancy now.

5.- The fifth step is turn the main capacitor to minimum capacity, just turn the tuning wheel till the MHz indicator shows 107 MHz.

At this right moment we have already achieved the inductancy reduction (L) and the capacity reduction (C), what means that we can tune in higher frequencies with our old FM radio.

6.- The sixth step is make an antenna, something so simple like and old TV antenna with a wire attached to its edge. Something like that:

 

 

 In case you cannot build this rustic antenna, an electric wire is suitable. But the length of the wire must be 55cm (22 inches). For those who really care about the reason of this length, here is the explanation:

The electromagnetic waves that carry the sound, after all are waves, and they can be described through a range of magnitudes. Is like saying that a person has blue eyes and brown hair, so we can also talk about the wave length (lambda), its amplitude or its frequency.

In this case we care about the calculation of the wave length, that is showed with the greek letter l (lambda), what is the distance between crest and crest or valley and valley. The crest is the highest point that the wave reaches and the valley is the lowest.

The formula that enable us to obtain l is: l= c / f , where c is the speed of light (speed reached by an electromagnetic wave 300.000.000 m/s or 909,090,909 feet/s) and f the frequency of the wave, for this case we will take 136 MHz (136.000.000 Hz).

l= 300.000.000 / 136.000.000 l= 2.21 meters or 88.4 inches or 670 feet

 

But we havenīt already finished, because the theory says that the length of an antenna should be a quarter of the wave length l, so dividing 2.21 meters by 4 is equal to 55 cm (22 inches).

 

 

Now we should be patient and live next to an airport. We have to take into account that the frequencies we can reach with this radio will not exceed the 127 MHz, approximately. We can listen to the Control Tower, Ground, Clearance, Approach, Atis and a bit of Rute, which is in higher frequencies than 127 MHz. So, if we do not have and airport near, we will not be able to listen to the air traffic controllers.

If you take a look at the first photo, it shows one of the four antennas that are at each corner of the main capacitor. These antennas are very important if you want to listen to the air traffic controllers, if we are 20 or 30 kilometers (12.5-18.8 miles) away from an airport they are too small to capture waves, that is why we have built the rustic antenna in the sixth step. The problem is that just one of these four antennas is the one that increase the gain in FM, improving the capture.

That is why we have to be patient, bring the wireīs edge of the rustic antenna together with one of the four small antennas of our main capacitor (make sure they connect together) and turn the small tuning capacitor we discovered in the third step. If after turning the small capacitor "several times" we donīt hear any controller or pilot, letīs try the same procedure with another of the four small antennas located at the corners of the main capacitor.

Something that really bothers me is that the air traffic controllers or pilots are not talking all the time. I mean that moments in which the air traffic controller has already given an instruction and has nothing more to say or just there are few planes, what means less work. And what happens when we sweep that frequency with our radio when the air traffic controller is not transmitting is that we hear nothing, although our radio works properly.

 So having this drawback into account, there is a small trick. This is the ATIS what stands for "Airport Terminal Information Service" is an automatic message that is repeated again and again by a machine. So the ATIS is an aid that will help us to find out which is the useful small antenna that we are looking for.

I encourage you to go to an airport and try it without the rustic antenna. You will not need it because when you are close to the emitting antennas of the air traffic controllers the small antennas of the main capacitor are big enough to capture the waves.

To give you more clues here is showed the location of the useful small tuning capacitor, small antenna and FM coil that I use to listen to the air traffic controllers.

  

  

 

Specially dedicated to my teacher of electronics...

 

If you have any questions email me at: atcradio@hotmail.com