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Sunday, March 21, 2010

How to receive Distant FM Signals

share something which might be unknown to some people. Receiving far away FM stations is called FM Dxing. I am from Ranchi(Jharkhand) and unfortunately here we dont have private FM stations except Vivid Bharti  . But i do listen to FM stations of Kolkata(Calcutta).

I want to make it clear to everybody that high frequency signals travel in the line of sight and these high frequency signals can either be FM or AM.

So, saying that FM signals travel in the line of sight is partially correct.

Low frequency signals like MW and SW also travel in the line of sight but they also get reflected by the one of the layers of atmosphere. So we can listen to faraway stations in these bands.
High frequency signals do not get reflected rather they pierce the layers of atmosphere and are lost in space leaving only one option: line of sight transmission.

But there is a special layer in the atmosphere called ionosphere which helps in reflecting the high frequency signals. Although not always present, this layer is responsible for FM Dxing.

To receive distant signals you need:
1) A good FM receiver. (I have got my Sony home theatre which works very well)
2) An FM Booster(Not those TV boosters)
3) A home made antenna.

You can get a plenty of circuits diagrams in the internet for FM booster.
Print out the diagram and either make it yourself or give it to the technicians who repair TV and radio.
The antenna can be a simple one like a whip antenna if you want to listen to nearer stations or it can be a complicated one like Yagi antenna. I made a quadrangular antenna.

With the help of above things, i receive FM stations of Calcutta. In a straight line it is more than 350 Kms from Ranchi. The signals become clearer in the night and they become stereo from mono.
Thus my setup receives whatever signals it gets from line of sight and from ionosphere reflections.

Some other facts:-
Aircraft communications also use high frequencies (118-136Mhz) and they talk in line of sight BUT THEY ARE NOT FM, they are AM (amplitude modulated) signals like those of Medium wave and Short wave.
One interesting thing:- One day when i switched on my system i wasnt able to receive Kolkata FM stations rather i was receiving radio stations of Burma(Myanmar) . I took out the map and measured the distance of Burma. It was more than 1500 Kms. Isnt that interestingMay be due to changes in the ionosphere i was able to receive them.Go for it.

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