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Calculating FM transmission distance

  1. Apr 14, 2013 #1
    Hi -- I am trying to calculate the signal distance for a 2kw FM radio station broadcasting with a 157' antenna over average terrain. The FCC calculator requires field strength, and I don't have a clue what that would be. Can I roughly calculate broadcast distance using the known variables above?

  2. jcsd
  3. Apr 14, 2013 #2


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    Hi there,
    welcome to PF
    No, you need a lot more info :)

    You said a 157 ft antenna .... I assume you mean that's the mast height ?
    You also need to know the cable type and length from transmitter to antenna
    you also need to know the gain of the antenna
    You also need to state the specific frequency, as the losses of the coax and the gain of the antenna is proportional to the freq used

    So TX power - coax loss + antenna gain will give you the Effective Radiated Power, ERP. Some times the EIRP with be the one used = Effective Isotropic Radiated Power.

    You also said ... signal distance ... just saying that is pretty meaningless
    What you really need to know is how far or what coverage you can get and still have a xxx SNR ( Signal to Noise Ratio). There's no point in saying I have a 100km coverage distance, if at 100km the field strength, in uVolts (microVolts), is below the sensitivity of most receivers

    There are pathloss and coverage programs available that will calculate received power levels at a give distance over a given terrain when all of the above info is inputted eg Radio Mobile

  4. Apr 14, 2013 #3
    davenn is right but if you don't need a precise calculation, some of the parameters he mentioned can be estimated. What is needed is receiver sensitivity - the minimum signal level you consider acceptable.
    The frequency is needed as FM is used for most transmissions over 30 MHz.
    How much gain does the antenna have. If you don't have that info, can you describe the antenna?
  5. Apr 15, 2013 #4


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    The answer to this question can be as long or as short as you want.
    The position and type the receiving antenna is equally important, of course.
    For many FM broadcast transmissions, the service is determined by the topography and stations are often sited with local hills. mountains etc providing protection for adjacent service areas. If the query refers to an existing transmitter then you could find out the service area from the spectrum planning authority. If you are planning a broadcast station then you would be best to contact the authority for advice because in many countries (UK in particular) the broadcast network is centrally planned and frequencies are allocated so that the maximum use is made of the available spectrum (co-channel interference is often more significant than just the signal level.)
    So, to get a worthwhile answer to this, you'd need to specify much more than your bare facts.
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