How does an RF cable work?

Answer 1

answ2. An RF transmitter generates radio frequency waves in its circuits, and to this 'carrier signal', it adds the information part by modulating the carrier signal. This composite signal (carrier plus information) is then fed to an antenna (aerial).

The aerial induces a corresponding signal into the atmosphere, by altering the Electric and Magnetic fields at (obviously) the same frequency.

The impedance of 'free space' is few tens of Ohms to a few hundreds of Ohms.

[Impedance may be considered analogous to resistance, but with reactive properties as well.]

The power emitted by the transmitter can vary from a megawatt or so (for VLF signals) to a few watts for handheld devices.

An Rf receiver receives the signal from the atmosphere, from its own aerial.

The receiver aerial is often quite simple, and the signal level is typically of a few microvolts.

This it tunes in (gets rid of unwanted signals and amplifies only the wanted ones).

The receiver circuits then strip the information part of the signal from the carrier part, and amplify this to a useful level for audio or video.

The actual signal into the loudspeaker will be a few tens of volts. In spite of the inefficiency of loudspeakers, (often only a few %) the signal eventually appears at a level that may be heard. A background radio will be a few milliwatts of power.

Even a very loud sound is only a few watts of radiated (sound) energy!!

Answer 2

This question is too general, but here are some points that relate to the context described by the questioner on the discussion page. An RF cable specifically used to connect a cable TV provider with the Set Top Box of a subscriber consists of a single central conductor surrounded by an insulator and a cylindrical shield which is typically grounded. These conductors are normally made of copper, sometimes with silver plating. The exact dimensions of the cable determine such properties as "impedance" and "attenuation", which are important to the cable's ability to pass signals over long distances, however details of this are beyond the scope of this discussion. The single central conductor is capable of passing radio frequency alternating current signals in both directions simultaneously. In order to prevent interference between the signals as they are carried by the cable either of two primary methods may be used: FDM or TDM. FDM, or "Frequency Domain Multiplexing", sends signals across the cable on different frequencies, and filters are used at each end to split the signals out. This is the method used by Cable TV distributors. TDM, or "Time Domain Multiplexing", shares the cable by sending signals in opposing directions at different times similar to the telephone method known as "half duplex". Various methods are used to insure that signals do not overlap or "collide", causing interference and loss of signal. Ethernet uses this method. Except for various Amplifiers, Splitters, Filters, and similar electrical devices, the electrical connection across the cable is continuous between the Cable TV provider and the Set Top Box. Ordinarily, DC current will not pass across the cable because of these circuits (and could damage the system if it were attempted). It is also possible that a wireless link could be part of the circuit, for example in some rural areas. In this case "Modulators" and "Demodulators" (radio transmitters and receivers) bridge the connection and there is no continuous electrical path at all. Although I have described this system in the context of Cable TV, all or most of the comments will apply to all or most uses of RF cables.

Answer 3

An FM receiver is similar to an AM receiver as far as the RF and IF stages go. The difference here is that the frequencies are often higher. Once you get to the demodulator, however, there is a significant difference because the FM receiver must detect frequency shift, while an AM receiver must detect amplitude shift. Often, in modern receivers, there is a phase-locked loop that follows the IF output frequency of 10.7 MHz, and the control signal of the phase-locked loop is essentially the original signal that is being demodulated. Contrast this with the AM receiver, which has a peak follower that follows the envelope of the 455 KHz IF frequency.