Co-axial Cables

[Electroenthusiast]

I just ordered (a few days ago), some coaxial cables to connect to my wifi antennas. And, it works perfectly fine for my purpose. But, what i didn't understand was that being a qualified professional myself, i couldn't think of doing anything of such myself. I could had bought the spares and fixed a coax with the connectors to build a cable for myself.

I sometimes cry for this because a prof who taught me working of antennas and microwave, who is actually a doctorate from an university at a Midwestern state in US didn't explain me all of these well. And, i to this day don't understand many things on how these antennae work. be it microstrip, traditional ones, or any other.

So, here's what i wanted to ask. Why are Co-axial cables of different dimensions? How do i know which one should i use to make things work?
How do i know which coax i need to use for what purpose. Basically, i didn't understand the working principle of Co-axial cables on a google search.

 

[aliarifat794]

It's important to know the frequency range of your transmitted signal. Higher frequencies typically require cables with lower loss and better shielding.

 

[rjenkinsgb]

Basically, the ratio of size of the inside of the outer conductor to the diameter of the inner conductor defines the impedance. As long as that ratio is the same, the cable size can be anything and still have that characteristic impedance.

The type of dielectric in the gap, and the resistance of the conductor surfaces, control the cable loss at any particular frequency - there are tables for most cable types, that give losses per unit length at different frequencies.

 

[rjenkinsgb]

Impedance calculator; note the units of measurement do not matter (as long as the same unit is used) as only the ratio is relevant:

Coaxial Cable Impedance Calculator - everything RF

www.everythingrf.com

 

[Tony Stewart]

Semi-rigid copper coax for SMA's is better than some flex cables because of skin effects on the braid weave patterns.
Sat antenna use larger diameter coax for lower loss at higher f with foil wrapped braid for long feeds.

 

[Beau Schwabe]

There are parasitic capacitor effects between the center pin and the shield in combination with the inductance per length of cable that create a transmission line that work in your favor to allow transmission of signals over a great distance. Depending on the target frequency, the distance from center to shield is different based on the frequency the cable is designed for. The material used as a dielectric may also be different to create a different/desired capacitor.