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.

 

[Tony Stewart]

The D & d electrode dimensions are the inner conductor surfaces across the dielectric. The total D+d controls the upper frequency breakpoint, while the ratio controls impedance.

Low-loss coax used air-core with a helicoil plastic spacer in 0.5" solid-Cu was used for CATV distribution amplifiers in the late 70's~80's for 300 MHz in Canada.

 

[Electroenthusiast]

If it does act like a capacitor, does it block direct current?
What is the point that a coax should be used, and why not a normal cable with two wires?

I'm just going little deep before i can dive deep into the topic.

 

[Nigel Goodwin]

If it does act like a capacitor, does it block direct current?

Obviously, you don't want current to flow between the centre conductor and screen.

What is the point that a coax should be used, and why not a normal cable with two wires?

I'm just going little deep before i can dive deep into the topic.

It provides screening, far superior to two unscreened conductors.

 

[rjenkinsgb]

If it does act like a capacitor, does it block direct current?
What is the point that a coax should be used, and why not a normal cable with two wires?

The capacitance is between the screen and inner; it's still a two conductor cable, in a sense. Any normal two conductor (or simple multicore) cable has capacitance between the conductors.

Coax, in the generic audio screened cable sense, for such as audio interconnects, guitar cables, unbalanced mic cables etc.

Coax in the RF, impedance rated types - for high frequency connections and efficient signal or power transfer, like antennas, transmitters, or interconnects in high frequency equipment.


Note that twisted pair and parallel conductor cables have some similar properties and uses:
General twisted pair cables are used for such as professional audio interconnects, phone lines and many other things. Twisted pair inherently cancels a lot of external pickup or radiation.

High frequency & impedance rated grades are also transmission lines, but balanced rather than unbalanced.

Such as 300 Ohm ribbon used for some band 2 FM antennas, also data cables (typically somewhere around 100 - 150 Ohm impedance) for RS422 / RS485 / CANbus / Profibus, Ethernet (four twisted pairs in common ethernet cable) etc.

 

[Electroenthusiast]

Nigel Goodwin What kind of screening are you talking about? Any link which explains it in much depth?

rjenkinsgb How are twisted pair have capacitance in between?

Why does impedance matching play an important role in coax cables? What happens if i neglect the fact? Didn't know that even telephone wires resist/screen external signals.

I'm not able to find the right book or tutorial where i can learn these concepts properly. Think me that i am a novice, and post a link/book name, which would help me in understanding antennas and coax in a better way.

TIA.

 

[Nigel Goodwin]

Nigel Goodwin What kind of screening are you talking about? Any link which explains it in much depth?

The outer conductor in the screening, protecting th4 inner core which carries the signal

rjenkinsgb How are twisted pair have capacitance in between?

Why does impedance matching play an important role in coax cables? What happens if i neglect the fact? Didn't know that even telephone wires resist/screen external signals.

Impedance matching is crucial, for RF to work, otherwise you get reflections - which are REALLY, REALLY bad - in an RF transmitter incorrect impedance matching can blow the transmitter.

Impedance matching is also a bad thing for lower frequencies (like audio), as while it provides maximum power transfer, that maximum is only 50% - much greater efficiency is given by a low impedance output feeding a high impedance input.

 

[rjenkinsgb]

Why does impedance matching play an important role in coax cables? What happens if i neglect the fact? Didn't know that even telephone wires resist/screen external signals.

The best parallel I know is if you imaging a wave travelling along a canal with parallel walls.
It will loose energy over time, but as long as the walls are parallel, it just travels smoothly.

If there is a sudden change in the canal width, some of the wave energy gets reflected back - that's an impedance mismatch.

Electrically, if you have a fixed source resistance or impedance and a load resistance or impedance, the lost energy is transferred to the load when the load is the same value (resistance or impedance) as the source.

Impedance matching gives optimum energy transfer.

Also, mismatches reflect energy and can cause standing waves or make the cable itself appear a different impedance, if the transmission line is more than a small fraction of a wavelength long.


A twisted pair in itself does not provide "screening" - but as both cores or wires are, on average, the same distance from any interference source, it adds equally to both wires!

It's "Common mode" signal vs "differential" signal.

So, the load at the end, only connected between the two wires, does not "see" the signal added to both wires; it does not cause any difference between the two.

(A analog normal telephone has no direct ground, so only sees the voltage across the line pair.)

That's why eg. professional audio gear can be spread out around a building or auditorium, powered from different sources, but not get ground hum or earth loops - all the connections are balanced line, screened twisted pair The only signal input each "sees" is the difference between the two wires, not any differences in voltage to ground, or noise pickup on the cables.

(Think of holding a spirit level across both hands; side to side tilt is the differential, wanted signal; The noise pickup adds the same up and down movement to both hands, without affecting the tilt).

 

[Electroenthusiast]

In that sense, i understand that the dimensions of the coax should be related to the wavelength of the signal. How can i know more about this? Because the calculator link doesn't give a practical design method for this. It appears from it that the diameters of the coax can be anything.

How does the outer sheath of the coaxial cable be grounded so that it gives screening effect from the some other RF signal that originates from a different source? (Both should be same ground).

 

[rjenkinsgb]

In that sense, i understand that the dimensions of the coax should be related to the wavelength of the signal. How can i know more about this?

If everything is properly matched, the cable length does not matter, other than longer cables mean more loss, depending on the cable type.

You can use an SWR (standing wave ratio) meter to check matching in a transmitter antenna, or an impedance (or antenna) analyser to check any antenna etc.

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With mismatches, the effect is a changing impedance that increases and decreases (or vice versa) over multiples of the signal wavelength in that particular cable (actual wavelength * velocity factor).

eg. A quarter wave stub with a shorted end has (theoretically) infinite impedance at the other end.

That's a bit like holding one end of a steel ruler solidly to a desk and "twanging" the other end; it's a resonator.

A quarter wavelength rod connected to the cable inner, with the screen to a ground plane, is one of the basic types of antenna.

Quarter wave transmission lines can be used as resonators for filtering, or "metal insulators", either literally or with capacitive ground for decoupled DC power to an amplifier stage.

A half wave with a centre ground (or no ground) acts as a resonator; it has low impedance near the mid point; a classic "half wave dipole".