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Does a coaxial cable have any advantages in transmitting lower frequencies? If it does, what are those advantages?

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  • \$\begingroup\$ Advantages compared to what alternative? \$\endgroup\$
    – The Photon
    Apr 23, 2020 at 18:21

3 Answers 3

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Coax does two things.

  • Provides a constant impedance so the cable acts as a transmission line

  • Prevents outside EM interference from coupling into the signal line

First point is a high-frequency phenomenon. You achieve no benefit in your application

Second point you WOULD get benefit from. However, you can get the same benefit from a cheaper (and much more flexible) audio-grade shielded wire. Source a (good quality) guitar or microphone cable and you'll be in business.

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Just a few advantages as I see them: -

Externally produced magnetic fields are zero

With coaxial cables (at any frequency suitable for the type of coax), the external magnetic field produced by the inner conductor signal current is exactly cancelled by the external magnetic field due to the same signal current returning on the outer shield.

Internal Magnetic field (shield to inner)

The internal magnetic field produced by the shield return current is zero because a current tube cannot have an internal magnetic field. However, there is still an internal field that exists and that is solely due to the inner conductor current.

Shield inductance

Because the outer magnetic fields are zero and only the inner wire produces the internal magnetic field, the shield has to have zero inductance. This means it can be earthed with a fair degree of impunity without a serious degradation of the inner signal. It can even have multiple earth points providing there is no earth fault currents present.

Advantages

This also means that when using a differential amplifier to receive signals remotely transmitted, the integrity is rather good. This is all down to the shield having a very low inductance compared to the inner wire.

It is simpler than twisted pair in this respect and somewhat cheaper to achieve the same performance because to get close, twisted pair would also need an outer shield.

External surges

Because of the nature of coaxial cable, an external surge (such as an indirect lightning strike) will travel along the outer shield and induce a near identical voltage on the inner wire hence, if a balanced receiver is used, you will find it easier to manage such things as TVS diodes because they can be attached to the shield only (at both ends) without worries about upsetting signals on the inner.

So, with this type of EMI, rather than it being prevented from coupling to the inner wire, it is near perfectly coupled and hence, when using a balanced receiver, it overcomes serious signal degradation.

Other types of EMI

For other types of EMI such as capacitively coupled electric fields, the outer shield does a decent job of shunting interference to ground.

For external magnetic field interference, due to the outer shield coupling near 100% magnetically to the inner Wire, a balanced receiver does the trick of significantly reducing the impact of interference.

You do need to use a balanced receiver when using coax for best performance and minimising ground loops.

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Coaxial cable provides shielding for the signal, assuming the shield is properly grounded. Coax also provides close to the smallest loop area possible, further reducing noise pickup.

For signals in the hundreds of KHz and up, properly-specified coaxial cable provides a controlled impedance to improve signal integrity.

The venerable 3-way RCA cable for audio and video is an example: coax cables for the audio (low frequency, high impedance) and video (~5 MHz, low impedance).

On good A/V cables the video cable will be thicker than the audio because the video needs controlled 75 ohm impedance, and so needs a thicker dielectric. The audio doesn't care about impedance, but can work just fine with the video 75 ohm type. The reverse isn't true: using audio coax that isn't 75 ohm will distort video.

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