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B How are radio waves boosted?

  1. Jul 6, 2016 #1
    So I have basic knowledge of radio waves and how they work but how does an amplifier boost the signal? Does it just make the waves stronger (if so how) or does it effect another aspect of the radio wave?

    Thanks in advance for the assistance.
     
  2. jcsd
  3. Jul 6, 2016 #2

    sophiecentaur

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    An amplifier increases the signal that is received on an aerial. It's the same principle as the action of an audio amplifier. The received radio (/TV etc) signal consists of high frequency (Radio Frequency) variations of the Volts on the feeder cable. This voltage can be extremely low: fractions of a microVolt. All receivers have an amplification stage at their input, before the demodulation stage but sometimes it is worth while boosting the signal, perhaps right next to the TV aerial, on the chimney, which will compensate for the loss in a long feeder cable (down-lead) and that can increase the level of the signal going into the set - or provide usable signals for several sets from one aerial.

    Any amplifier will leave its mark on an analog signal and add to the degradation. That can produce distortion of the signal or degrade the frequency response. If a 'good quality' amplifier is used then the fidelity of the signal will not be affected but multiple received signals can cause crosstalk from one channel onto another.
     
  4. Jul 6, 2016 #3
    Thank you for the magnificent explaination. You added all the details other write-ups did not. This is exactly what I was looking for. ^^
     
  5. Jul 7, 2016 #4
    Just to add something, any kind of signal has noise together with the pure signal that carries the information. When we amplify the signal we also amplify the noise that is already in the signal (but of course not the noise that will be later added to the signal as the signal propagates through air or wires). In telecommunication a very important parameter is the Signal to Noise Ratio known as SNR. In order to improve the SNR we have to amplify the signal as much as we can, BEFORE we transmit it through a wire or air, where it will pickup additional noise (if we amplify it after the transmission, we amplify the noise as well, so we don't improve the SNR). That's why amplifier boxes are put near the antennas, where the signal is strong and hasn't pickup additional noise (though already has pick some noise in the path from the source antenna to the receiver antenna, but we cant do much about that noise).
     
  6. Jul 7, 2016 #5
    Noise is not so much a problem with FM.
     
  7. Jul 7, 2016 #6
    Yes that's true. However I am not sure if the OP means FM radio waves or when he says radio waves he means any sort of EM-waves (in the frequency range 1Mhz-1Ghz) that are used in the transmission of FM/AM Radio or TV signals.

    And now that I look again at the OP, I see that none of us has directly answer the question he poses.

    The amplifier increases a property of the wave that is called the amplitude of the wave, while leaving the frequency of wave unchanged. Mathematically a signal is a sum of sine and cosine terms. In its simplest form we can take a signal S(t) to be:

    ## S(t)=S_0sin(2\pi ft)##

    where f is the frequency and ##S_0## is the amplitude. If an amplifier takes S(t) in its input, then at its output it puts a signal S'(t) that is

    ##S'(t)=S'_0sin(2\pi ft)##

    so we can see that the frequency (and the phase) of S' is the same as that of S, but the amplitude has changed, the amplifier increases the amplitude to an ##S'_0## value which will be ##S'_0=bS_0## where ##b>1## is the amplification factor.
     
  8. Jul 7, 2016 #7

    sophiecentaur

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    That's too wide a statement. The noise behaviour of FM channels depends on whether it's wide or narrow band FM. Broadcast FM is wide band and uses wide deviation. That gives the 'FM Improvement' of about 20dB, or an increase in 'quality' of ten times (loosely). BUT it involves using a lot more spectrum space and there is a sharp threshold for the quality of signal. Once the signal gets weaker than that level, it will drop out completely. Great when you are in the official service area but not if you are near the edge. When FM is used for Comms, they use narrow band FM and the noise performance is pretty much the same as for AM. Look at the frequency allocation tables and you will find a mix of AM and FM over the comms bands. Using a narrow band system means that the channel will 'die gracefully' as the signal level goes down which means you can still communicate in conditions where just listening to music on the radio would not be pleasant.
    True. But there is no simple answer to his question because he needs to know more before he can answer a well defined question. The topic is a bit of a blur to him - and to most people, too. Hopefully, he will be able to trawl through this typical overkill PF response and get some useful bits out of it for his needs. Hopefully, your introduction of some Maths will help him. Otoh, he may run away screaming :wink:
     
  9. Jul 7, 2016 #8
    You don't get spikes in FM from lightning etc or channels on top of each other.
     
  10. Jul 7, 2016 #9

    sophiecentaur

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    Wide band AM with dynamic limiting doesn't suffer from impulsive interference either. Wide band AM was considered when the higher frequency bands became available and was a strong contender at a time when complicated FM demod was relatively expensive. A significant advantage for FM is that your AM broadcast transmitter power / modulator tended to be larger and sweatier. There are many facets to choice of mod systems.
     
  11. Jul 7, 2016 #10

    sophiecentaur

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    Yep. That is the well known 'capture effect'. When an interfering station only a bit lower in strength than the wanted station, the FM demodulator tends to demodulate the wanted signal and the interfering carrier, which just produces a small amount of high frequency phase mod, is demodulated as mostly high frequency baseband, and, like the demodulated noise, appears largely out of the audio frequency spectrum. A very clever system for broadcast quality - but not much use for 'chat' Comms, where (as with AM citizen's band,) you actually want to know who else could be calling you., Horses for courses. It will be a long time before analogue transmissions die a death and digital takes over for everything, I think. But it will happen.
     
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