Radiating mode vs Coupled mode leaky cables

KaluzaKlein

I am trying to understand how leaky cables work, and especially those that permit higher frequencies (I think about 2.4GHz), but I am quite new to electromagnetics in general. For clarification, leaky cables are usually coaxial cables that have apertures in their outer conductor from where an amount of radiation "leaks" out, thus they act like antennas.

I found this nice article "Leaky cables are a good thing" that mentions two kinds of those cables, radiating mode and coupled mode: In coupled mode adjacent apertures are spaced in a distance smaller than the wavelength, while in radiating mode the apertures are spaced almost a wavelength apart.

I have done some amount of reading, but still I am not sure I understand the concept of radiating vs coupled mode and how exactly does it relate to the wavelength. Is there an easy answer on this one?

KaluzaKlein

Here is the link for anyone that is interested:
Leaky cables are a good thing

The article also says that coupled mode cables resemble surface wave antennas. In these antennas the signal is traveling as opposed to the resonant antennas where it is a standing wave. When the signal finds discontinuities on the medium (apertures) it escapes on the outside.
In radiating mode the slots are spaced in such a manner to ensure "an in-phase addition".

Does this mean that both the radiating mode and coupled mode cables are surface wave antennas with the only difference being the strength of the signal being stronger in radiating mode because the "escaped" waves are in phase?

es1

The link was dead for me.

Just out of curiosity, it says they are a good thing, but for what?
It seems like they are suggesting using the cable itself as the antenna. No?

KaluzaKlein

Strange, it's not dead for me. Anyway, yes they do.

Like I said above, the cables have periodic slots from where radiation leaks out. And this is true for the whole length of the cable. The whole cable "radiates". You do have to be careful, though, not to let the full radiation leak out before reaching the end of the length.
Also, you can have control over the beamwidth by adjusting the space of neighboring slots. The whole process reminds me of endfire arrays, but still details, on how the fields are distributed elude me.

I found two papers about radiating and coupled mode cables. In the one about radiating cables it treats the cables as an "array of magnetic dipoles, oriented along the cable axis". The azimuthal component of the electric field is then computed.
For coupled mode, on the other hand, the cable is compared to a long electrical antenna and they calculate the radial component of the electric field.

Does anyone at least know what is a the radial and what is the azimuthal component of an electric field?

es1

Google produced these. Not sure if they match the context of the article though.
http://www.wordiq.com/definition/Azimuth
http://www.wordiq.com/definition/Radial

Algis

Hi Kaluza,

I am dealing with exactly the same issue of coupled vs radiating leaky coax.

What are the two papers you refer to?
Have you read the reknown Bell-Labs paper on this topic? It is very good and shows real life empirical results.
http://ieeexplore.ieee.org/stamp/sta...&userType=inst

KaluzaKlein

The papers are these two:
Radio propagation along a coupled mode leaky coaxial cable in tunnels
Radio propagation along a radiated mode leaky coaxial cable in tunnels

I have done some further reading but I'm still far from completely understanding. My rough conclusions are that it has to do with the radiation pattern and how much we are able to control it.
In an antenna array we can control it by properly arranging the antennas in space, changing their phases and input power. That is done in a radiating mode cable because those cables work on specific frequencies.
In a coupled mode cable radiation pattern and directivity all depend upon the operating frequency. Thus, there might be many fluctuations on the signal strength as you travel along the length of the cable.


If you are interested on the exact notions behind field distributions I suppose that any book that explains radiation from apertures, antenna arrays and surface wave antennas can help you. You can also take a look at this chapter from Orphanidis' book:
http://www.ece.rutgers.edu/~orfanidi/ewa/ch17.pdf
And there is a chapter on leaky cables at "Antenna Engineering handbook" (McGraw-Hill, lots of editions, Volakis will do)