Role of Coax Cable in bringing TV signal to our TV sets

Question

We know that we receive the programs/shows of TV Channels from the producers of programs through

1. CATV Service providers (almost obsolete, just wanted to get clarified on its working process)
2. DTH Service.

Consider that a TV Channel produces programs and wants its programs to be telecasted to a wide audience. It will request a CATV Service provider to telecast its programs. After an agreement, the producer/channel will send his programs to the service provider. The service provider will then allocate a unique number to that channel. Transmission of the programs produced by the channel will be enabled through a Coaxial Cable.

Now, a single RG-6 coax cable can carry multiple channel programs with the concept of Frequency Division Multiplexing whereby, each channel will be allocated a bandwidth of 6-7 MHz. Typically 166-200 channels will be transmitted through the same single RG-6 cable.

My queries are

1. Will the Service provider transmit an AC of radio frequency for all the channel programs into the cable, which gets converted to EM waves and reaches the 3-way splitter?

2. Is it just an assumption that the two currents in the coax conductors need to be equal and opposite in order to not radiate outward?

3. Provided that each channel can occupy only 6-7 MHz band, is it obvious that each program produced by the channel will be transmitted at frequencies within this band? And, if 0-6 MHz is allocated for channel A, 7-12 MHz will be allocated for channel B? To avoid overlapping of frequencies?

Answer 1

Will the Service provider transmit an AC of radio frequency for all the channel programs into the cable, which gets converted to EM waves and reaches the 3-way splitter?

Yes, but there's no "conversion" per se, only manipulation. Any change in voltage or current in any circuit necessarily begins an electromagnetic wave. If you trace the received signal back to the source, it's probably electromagnetic waves all the way back to a hard drive which generated waves from rapidly spinning, magnetized cobalt, and an electric power plant burning hydrocarbons to turn electromagnetic generators.

Is it just an assumption that the two currents in the coax conductors need to be equal and opposite in order to not radiate outward?

That's mostly true. The current and voltage on each of the shield and center conductor are associated with a magnetic and electric field around them. When the conductors are arranged concentrically, and the fields are equal and opposite, the fields cancel outside the cable. If there are no electromagnetic fields outside the cable, it can't be radiating.

If the fields are not equal but opposite, the fields don't cancel. A field outside the cable means it can radiate.

Note there still may be a DC bias to the cable. DC does not radiate, so it does not cause an issue.

Provided that each channel can occupy only 6-7 MHz band, is it obvious that each program produced by the channel will be transmitted at frequencies within this band? And, if 0-6 MHz is allocated for channel A, 7-12 MHz will be allocated for channel B? To avoid overlapping of frequencies?

The frequencies are allocated in non-overlapping bands, but there's a "guard band" between them. There are also backwards compatibility concerns that dictate where each channel is placed.

Answer 2

"Will the Service provider transmit an AC of radio frequency for all the channel programs into the cable"

Yes, the classic CTV distribution provides all channels simultaneously.

"which gets converted to EM waves"

No.

"and reaches the 3-way splitter?"

Which splitter?

"Is it just an assumption that the two currents in the coax conductors need to be equal and opposite in order to not radiate outward?"

Yes, that is a weird assumption. There could very well be a DC imbalance between the two currents. The point is that the outer conductor is at ground potential, and encloses the inner conductor.

"Provided that each channel can occupy only 6-7 MHz band"

which is roughly correct

"is it obvious that each program produced by the channel will be transmitted at frequencies within this band?"

yes

"And, if 0-6 MHz is allocated for channel A, 7-12 MHz will be allocated for channel B? To avoid overlapping of frequencies?"

The frequency bands will be non-overlapping, but to match the TV receiver HF part, the frequencies will be choosen to fall into the old over-the-air transmission bands. And there will be some considerable margin between the bands.

Answer 3

One way of distributing TV over Coax is DVB-C, see: en.wikipedia.org/wiki/DVB-C

This describes the signals that are used. A DVB-T signal can be distributed through a coax cable but it does not have to. In my country only the last meters to the houses are coax, optical fibres are used for long distance distribution.

1. There is no explicit "conversion to EM waves", all the bitstreams containing TV programmes are converted (modulated onto a carrier frequency) and combined into one signal which needs to be distributed. It can be directly fed into a Coax cable or modulated on a light beam and distributed using an optical fibre.

Before it reaches all the TVs the signal will be amplified and splitted many times.

2. Coax is shielded, it does not radiate. Study how transmission lines work, Coax is just a T-line in shape of a cable

3. Roughly yes but actually more complex, study DVB-C and OFDM etc.