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I am using a bit of coax cable as an antenna (with the shielding off at the last 17.3 cm). However it is cheap Chinese cables and the shielding does not contain that many copper wires (the "web" isn't very thick, you can see through it).

How much would this matter? Is that sort of cable completely useless and unshielded or does it basically not matter how much wire you've got around the center, as long as its grounded and goes all the way round?

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The "density" of the shield is called shield coverage. Copper is expensive, and in a coaxial transmission line, the shield is most of the copper. So if you want to reduce cost, reducing shield coverage is a good way to do it.

The most significant impacts may be

The degree of the impact will vary significantly by frequency. As frequency increases and wavelength decreases, the "holes" in the shield appear increasingly large. This is why high frequency cables often include foil shields, or even better, solid copper.

The total loss should be specified in the datasheet, usually in decibels per unit length, at several frequencies. Though for cheap Chineese cable there may be no datasheet, and even if there is one it may not be trustworthy.

If this coax is bundled near other cables or sources of noise, more of that outside noise will couple into the signal. More of the signal within the coax will get out, which you may regard as loss; or from the perspective of other signals, noise.

Whether this matters to you or not depends on your application.

In a receive application, loss is not much of a concern since both signal and noise received by the antenna are attenuated equally, and thus the signal to noise ratio remains unchanged. Unless of course the loss is so much that the signal is attenuated below the receiver's sensitivity.

In a transmit application loss can matter quite a lot. It is in many circumstances more economical to install a higher quality feedline with less loss than it is to install a higher power transmitter to overcome feedline loss.

The significance of common-mode ingress depends very much on your situation. The received signal from the antenna will already contain noise. Are there sources of common-mode noise that are significant in comparison? Is minimizing radiation from the cable a requirement? There is no general answer to these questions.

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There is much more to the "quality" of coax cable than just the shielding. Some examples:

  • The center conductor contributes the greatest losses at HF frequencies. The gauge, the quality of the copper, and the thickness of the copper if plated all contribute.
  • The dielectric material contributes directly to losses at VHF and higher frequencies and indirectly at lower frequencies since its characteristics, along with the gauge of the center conductor, determines the inner diameter of the shield.
  • The dielectric material plays a role in the voltage and power rating of the cable.
  • Soft dielectric material can cause the center conductor to migrate from its central position which alters the impedance of the coax and reduces the power rating.
  • The RF resistance of the shield is the second highest contributor to RF losses on HF. The smaller the inner diameter, the higher the losses. The less copper in the jacket, the higher the losses. Aluminum and tin plated shields have higher losses than pure copper shields.
  • The quality of the outer insulating jacket partly determines the pliability of the cable and plays a direct role on the usable lifetime of the cable due to UV aging and moisture ingress.
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