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How the coaxcable works

A

Avanza

Well Known Member
Transmitting on the COM-radio.

"Standard coaxial transmission lines. When this type of line is working without unwanted radiation, all currents are inside the line. The outside world is isolated from the inside of the cable by the skin depth of the shield. Voltage from the shield to "ground", or the environment around the line, ideally is zero."

Refering to the attachment, my understanding is:
This means that if the BNC connector (shield) at the back of the COM-radio, and the COM-antenna BNC connector (shield) has the same GND-potential, there will be no external radiation from the coaxial cable.
Think of it as a hose filled with water. If there is no flow of water, the water levels at the ends of the hose are equal. Lower one end of the hose and water will start to flow.
If there is a GND-potential (Voltage) difference between the BNC-connector (shield) at the COM-radio and the BNC (shield) at the antenna, a Current will
flow in the OUTSIDE of the coaxial shield.
The coaxial cable will now radiate a magnetic field and act as an antenna.
In modern aircraft there are lots of wires that are carrying 0,5-4,5 V.
If those sensitive wires are located close to a Coax cable that are carrying
a current on the outside of the shield, there will be erronous readings
on the affected instruments.
One would think that there would be no Voltage differences in a metal aircraft. There are good reasons to have only one GND-bus and have it connected to the aircraft skin.
Ideally all GND-wires use this GND-bus. (not intercom or headset GND).
If for some reason it's disired to have more than one GND-bus, it's important
to have a well sized GND-wire between the GND-buses.
Do I have to rebuild all my wiring ? No, just be aware that there may be a radiating field of some proportion arround a COM Coax cable.
Then this can be corrected or canceled with a metal braid that is grounded.
https://www.w8ji.com/coaxial_line_and_shielded_wires.htm

Good luck
 

Attachments

  • Coax.theory.pdf
    467.2 KB · Views: 192
Generally the rule is:

Noise sources: Ground both ends.

Noise Victims: Only ground one end.

In this case it is more a question of relative noise production and vulnerability to noise.
 
MechaSteve said:
Generally the rule is:

Noise sources: Ground both ends.

Noise Victims: Only ground one end.

In this case it is more a question of relative noise production and vulnerability to noise.
Click to expand...

This general rule does NOT apply to coaxial cables carrying RF power. BOTH sides must have a good ground (Radio & Antenna) or you will have a high Voltage Standing Wave Ratio (VSWR) which reduces your radiated power and can cause damage to some transmitter power amplifiers.

A well built coaxial cable will have little to no RF leakage. That is why if you don't have the proper equipment or experience making coaxial cable connections, I recommend you buy already assembled coaxial cable from a professional source. I have over 40yrs of RF broadcast experience and still prefer buying already assembled coax than making it myself. But even with professionally assembled coaxial cables, you still don't want to run it near low voltage signal wires.

:cool:
 
Last edited:
Understanding RF and transmission lines

Avanza said:
Transmitting on the COM-radio.

"Standard coaxial transmission lines. When this type of line is working without unwanted radiation, all currents are inside the line. The outside world is isolated from the inside of the cable by the skin depth of the shield. Voltage from the shield to "ground", or the environment around the line, ideally is zero."

Refering to the attachment, my understanding is:
This means that if the BNC connector (shield) at the back of the COM-radio, and the COM-antenna BNC connector (shield) has the same GND-potential, there will be no external radiation from the coaxial cable.
Think of it as a hose filled with water. If there is no flow of water, the water levels at the ends of the hose are equal. Lower one end of the hose and water will start to flow.
If there is a GND-potential (Voltage) difference between the BNC-connector (shield) at the COM-radio and the BNC (shield) at the antenna, a Current will
flow in the OUTSIDE of the coaxial shield.
The coaxial cable will now radiate a magnetic field and act as an antenna.
In modern aircraft there are lots of wires that are carrying 0,5-4,5 V.
If those sensitive wires are located close to a Coax cable that are carrying
a current on the outside of the shield, there will be erronous readings
on the affected instruments.
One would think that there would be no Voltage differences in a metal aircraft. There are good reasons to have only one GND-bus and have it connected to the aircraft skin.
Ideally all GND-wires use this GND-bus. (not intercom or headset GND).
If for some reason it's disired to have more than one GND-bus, it's important
to have a well sized GND-wire between the GND-buses.
Do I have to rebuild all my wiring ? No, just be aware that there may be a radiating field of some proportion arround a COM Coax cable.
Then this can be corrected or canceled with a metal braid that is grounded.
https://www.w8ji.com/coaxial_line_and_shielded_wires.htm

Good luck
Click to expand...
For the levels of RF at VHF frequencies that radiate from our aircraft radios the currents flowing in the shield of the coax are very very small. The resistance of the shield per foot is also very small so the voltage developed per foot of run is also very small - even if the VSWR is 2.5 or greater - is in the order of microvolts. Skin effect - where current is concentrated on the surface is a factor - but that concentration will be on the boundary of the conductor interface with the dielectric material between the center conductor and shield, not on the outside of the shield next to the outer plastic cover. For RG400 which is double shielded there will be no current flowing in the outer shield. If you want to visualize coupling to adjacent wires bundled with the coax, think more in terms of the capacitance coupling between the wires and the length of the coupling where the wires are adjacent to the coax. The capacitive term completely dominates over the inductive term for straight coupled wires at VHF frequencies and the capacitive coupling varies approximately as one over the square of the distance between the wire. Double the distance you get a quarter of the coupling. We could talk about the odd mode and the even mode coupling but it is all mute in our situation because the levels of coupling are generally so low as to be negligible . A more likely case is conducted RF getting onto the power and ground lines from the radio directly due to poor RF decoupling within the radio power supply and /or poor grounding or 12 volt dc routing and decoupling. These problem are really frustrating to try to solve without the right test equipment and an understanding of the likely source.
Keith Turner
 
My only observation is you should be really careful about the terminology used.
An antenna radiates electromagnetic energy in the Radio Frequency spectrum.
The way the energy travels along the coax depends on the frequency of the energy, as the frequency increases the wire comes a wave guide and the energy travels along the edges/walls.
The centre conductor must only make contact with the radiating element, the screen must make contact with the ground plane (at both ends), a regular BNC connector makes sure this happens.
Is it really necessary to introduce other ground planes into the discussion? To me that is only confusing a poorly understood subject.
I'm sure I don't have a full understanding of what happens and am happy to be educated.
 
Southern Pete said:
My only observation is you should be really careful about the terminology used.
An antenna radiates electromagnetic energy in the Radio Frequency spectrum.
The way the energy travels along the coax depends on the frequency of the energy, as the frequency increases the wire comes a wave guide and the energy travels along the edges/walls.
The centre conductor must only make contact with the radiating element, the +screen must make contact with the ground plane (at both ends), a regular BNC connector makes sure this happens.
Is it really necessary to introduce other ground planes into the discussion? To me that is only confusing a poorly understood subject.
I'm sure I don't have a full understanding of what happens and am happy to be educated.
Click to expand...

If we are only using and discussing Coax transmission lines we should be clear about the fundamental difference between coax and waveguide. Coax only supports TEM mode transmission (Transverse Electromagnetic Mode) which will transmit DC to whatever frequency you choose - there is no upper cutoff frequency. Skin effect and dielectric losses do cause the losses to increase with increasing frequency. Waveguide on the other hand can have a number of different modes depending on the frequency and dimensions of the waveguide. There is always a cutoff frequency and a maximum frequency for the particular mode - it is a whole different animal. I’ll look for and post some good references for a better understanding of coax and TEM mode transmission. Its a subject (like aerodynamics that is easier to understand from a mathematical viewpoint. I’ll see if I can find some references that keep the math to a managable level.

Keith Turner
 
Southern Pete said:
The way the energy travels along the coax depends on the frequency of the energy, as the frequency increases the wire comes a wave guide and the energy travels along the edges/walls.
The centre conductor must only make contact with the radiating element, the screen must make contact with the ground plane (at both ends), a regular BNC connector makes sure this happens.
Click to expand...

Actually, a fairly good explanation.

:cool:
 
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