High Frequency rated cables

[Braeden Hamson]

I'm probably going to explain this poorly because I really don't understand it. So I was in my school's electronics lab and I was looking at a spectrum analyzer. It was brought to my attention that the cable it uses is special because it can handle the frequency at which the instrument operates. So I'm wondering what makes those cables so special my guess is shielding. But is there more? Like resonance and echos? I'm just spitballing.

 

[(*steve*)]

Three are three ways power can be lost from coax:

1. Resistive
2. Dielectric losses
3. Radiation losses

Resistive losses are simply due to the electrical resistance of the conductors. Because of skin effects, the total mass of the conductor is less important than its surface area. The centre conductor in high power coax can be made of aluminum with a thin coating of copper.

Dielectric losses are losses in the coax's dielectric. The coax is like a long series of inuctances and capacitances. Every time the capacitance is charged and discharged there are losses. Different insulators have different losses. Larger diameter coax often uses air as the dielectric. Air is less lossy than many other dielectrics.

Lastly, imperfection in the shielding can allow the coax to act like an antenna. Better shielding (for example solid metal rather than braid) and keeping the centre conductor in the centre help to minimise this.

Other sources of loss are impedance mismatches which cause some of the signal to be reflected.

 

[Harald Kapp]

I'm wondering what makes those cables so special

It is mostly the 'matched impedance' that makes these cables 'special' (c.f. Steve's last point). At high frequencies it is important to match impedances of source (sender), transmitting medium (cable) and sink (receiver). If there is a mismatch, signals get reflected, thereby distorted up to the point where no proper transmission of information is possible. On old analog TVs this could be seen as 'ghosting' where a reflected signal led to a second image, shifted across the screen.

Typical impedances are 50 Ω and 75 Ω for coaxial cables. Also used are 150 Ω and 600 Ω.
Laboratory equipment like oscilloscopes, signal generators etc. typically operate at 50 Ω. The cables used to connect these insruments need to be matched i.e. they need to have an impedance of 50 Ω in the respective frequency range, too.

 

[Braeden Hamson]

Very informative
Thanks guys