Twisted Pair Cable and Coaxial Cable

Well, co-ax is used in television antenna downleads running at MHz, and twisted pair in DC analog current loop control circuits, for starters.

Coaxial cables are used primarily, in RF circuits of TX & RX. Most of these have 50 or 75 ohm impedance. Twisted pairs are used mainly, in carrying low level AC or DC power.

Coaxial is used for higher frequencies, as pointed out.

Twisted pairs or twisted shielded pairs can go from DC to medium switching rates with good to excellent noise cancellation and exclusion.

Wiki

When sending video along coaxial cable, the interference is 'collected' on the shield, so the end signal reduces in quality. this is especially noticeable over long distances and where there are interference-causing machinery/cables nearby.

Use of twisted pair allows both wires to pick up equal(ish) interference and as the end signal is received by a comparator-equivalent circuit, the signal can be sent over longer distances and/or with interference-causing machinery/cables nearby without so much loss of quality.

Video for CCTV in buildings can easily be sent over CAT5 cable.

Another way of thinking about this is that shielding reduces RFI (Radio Frequency Interference and twisting reduces EMI (Electro Magnetic Interference).

The reason the latter is true is that the twisting acts somewhat like a transformer and the interference is coupled in both leads equally, but opposite in phase.

Greetings.

The use of Balum Coils on both ends of twisted pairs has been used for years since the mid 1980s in large campus environments to run high speed data to terminals, printers, etc.

A Balum coil is:

Balanced toward the high speed side and unbalanced toward the twisted pair side.

The device end this is reversed to attach to the device.

A large campus environment was totally implemented this way by AT&T in 1984.

Hi there is quite a difference between the two and it relates to the way the EM fields created by current flow, interact.

as you would expect twisted pairs is two wires twisted together and normally used to connect to a balance feeder system. the applied signals are feed 180 degrees out of phase. the fields appose each other so tend to cancel. this formation reduces signal loss and susceptibility to interference.

Coax is cable were the the fields from both conductors (inner core and shield) centre around the same point.

Both can be used at any frequency but coax is mostly used at high frequencies. Think Radio and TV transmission for coax (lowest lost). I conducted tests some years ago to find the best cable for speakers and found that Coax delivered the best characteristics for HI FI. But were interference and low noise is an issue twisted pair is often used. Think Ethernet at 1Gb/s and how ADSL services use phone wires as transport.

Test

Coax is used for broadband high frequency low current signals.

Twisted Pair for lower frequency narrow band signals. Higher frequencies are attained by combining several pairs twisted together, with each pair twisted to a different number of twists/ft. Each pair then couples differently with external signals and acts as a shield for the other pairs. That is how CAT 5 is wound. They call it self shielding twisted pair.

Coax has only one conductor and shield ground but is also consistent from end to end in construction. With the shield on the outside and the conductor centered on the inside, it behaves like a wave guide. The impedance is constant all along it's length, and it can carry many signals at the same time so long as they are different enough frequencies and each frequency is a distinct band. It is then multi-mode and behaves like many wires.

In addition to the other answers, they both are transmission lines. If used for long distance signal transmission, they need to be terminated in their natural impedance to maintain signal integrity. Coax are typically 50 or 75 ohms. Telephone companies use 600 ohm non-coax lines. The impedance is based on the diameter and separation of the conductors and the material of the insulation between the conductors. Termination is also necessary for accurate measurements of AC signals. Bad termination will result in standing waves.