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Fibre optic cable for antennas?

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raiya_23

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Can we use fire optic cable for connecting antennas to communication equipment? Why do we use only coaxial cable for connecting an antenna to a transceiver and fibre optic is used only in data communication?
 

davenn

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Can we use fire optic cable for connecting antennas to communication equipment? Why do we use only coaxial cable for connecting an antenna to a transceiver and fibre optic is used only in data communication?
and how do you expect the small received RF current from the antenna to get into the optical fibre ?
or the large Rf signal from a transmitter to get into the optical fibre ?

basically....
you will need a receiver at the antenna to recover the modulation and that will drive a fibre laser diode transmitter at the other end you can recover the modulation out of the fibre and use it as needed.
On the transmit side, you will need a modulator driving a laser diode to send the modulated signal up the fibre to the antenna on the mast.
At the antenna end you will suce that modulation to modulate a transmitter to feed the antenna

see how much of a major effort it would be ?
thats why we use coax or waveguide

light in a fibre ISNT an RF signal

Dave
 
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jiripolivka

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and how do you expect the small received RF current from the antenna to get into the optical fibre ?
or the large Rf signal from a transmitter to get into the optical fibre ?

basically....
you will need a receiver at the antenna to recover the modulation and that will drive a fibre laser diode transmitter at the other end you can recover the modulation out of the fibre and use it as needed.
On the transmit side, you will need a modulator driving a laser diode to send the modulated signal up the fibre to the antenna on the mast.
At the antenna end you will suce that modulation to modulate a transmitter to feed the antenna

see how much of a major effort it would be ?
thats why we use coax or waveguide

light in a fibre ISNT an RF signal

Dave
Still, with all difficulty described above, the "RF over fiber" technology is now widely used in microwave and satellite communication systems. Fiber is advantageous in that its transmission loss is much smaller than RF coaxial cable or microwave waveguide, and its capacity is much more than one can imagine. Adding more RF channels to one fiber is easy, and now all of it is cheaper than the earlier RF technology.
 

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pstuckey

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There are a number of companies marketing some type of RF over fiber solution. I believe that most (all?) of them currently have a similar limitation though, the loss in the optical modulation/demodulation is large (30-40 dB), and the corresponding noise figure is high (40+ dB). Many companies get around this by putting a large LNA in front, which results in net gain close to zero, and much lower system noise figure. However, now there is 30-40 dB of gain before the optical components, and that limits dynamic range. In some cases, such as satellite comms as pointed out above, you may know the desired signal is weak, and there are no large interfering signals, therefore the high gain LNA is not a drawback. For other situation, particularly in cluttered environments with large interfering signals, the high gain LNA is a significant problem.
 

biff44

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FO cable with RF on it can be done. but it typically is expensive, has intermodulation product problems, and could have phase noise problems for the more basic systems. A coax cable costs 20x less, so unless you have a really special need (like having to send the signal many km) you would never see it.
 

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E Kafeman

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Optical glass fiber is a waveguide, similar as for radio frequent waveguides. If it had been possible had it been an advantage to have a tuned hollow center and total reflecting walls also for optical fiber if it had been possible to control center diameter. Drilling inner diameter lambda/2 for light waves is however a a bit tricky to do, in a long fiber cable. Low loss optical glass fiber is instead done in two or several more layers of glass (single or multi mode), each layer outwards with increasing refractive index forcing light waves to travel only in center of the the fiber cable, causing as few reflections in the wall(s) as possible.
Even sound waves can be distributed low loss over great distance with a waveguide of right dimension and high refractive index in the walls. Both these parameters say that a fiber with dimensions for light waves is a poor selection, but RF waveguides can have rather low loss also for sound waves.
Sound waves in optical fiber is common but first after that the sound signal have been modulated to a optical frequency. Same rules for RF signals in optical fiber.
Laser as light source/modulator is a special case as it can travels with lower loss without cable if it is good focused and if it not is raining.
 

biff44

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Optical glass fiber is a waveguide, similar as for radio frequent waveguides. If it had been possible had it been an advantage to have a tuned hollow center and total reflecting walls also for optical fiber if it had been possible to control center diameter. Drilling inner diameter lambda/2 for light waves is however a a bit tricky to do, in a long fiber cable. Low loss optical glass fiber is instead done in two or several more layers of glass (single or multi mode), each layer outwards with increasing refractive index forcing light waves to travel only in center of the the fiber cable, causing as few reflections in the wall(s) as possible.
Even sound waves can be distributed low loss over great distance with a waveguide of right dimension and high refractive index in the walls. Both these parameters say that a fiber with dimensions for light waves is a poor selection, but RF waveguides can have rather low loss also for sound waves.
Sound waves in optical fiber is common but first after that the sound signal have been modulated to a optical frequency. Same rules for RF signals in optical fiber.
Laser as light source/modulator is a special case as it can travels with lower loss without cable if it is good focused and if it not is raining.
Not sure what you are trying to say! You, of course, would modulate a laser at one end with the RF signal, and use a photodiode at the other end to demodulate that signal
 

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E Kafeman

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Original question:
Can we use fire optic cable for connecting antennas to communication equipment?
As I understand it, is then NO electronic active circuits allowed between optical fiber and antenna, such as (de)modulators.
The optical fiber is also assumed to have a effective diameter much less then common radio wavelengths.

It is not impossible but very ineffective to use a conventional waveguide designed for optical frequencies for transferring RF waves.
Cladding in a glass fiber is normally carbon based that absorbs RF signals and fiber core is too thin for most RF wavelengths.
Another hypothetical possibility require that RF energy must be high enough to be able to charge and emit photons or other elementary particles.

There are however several more realistic solutions that not have active electronics at a receiving RF antenna end. A active electronic demodulator exist, but not at antenna location.
A few years ago was following two similar systems hot topic:
EMPiRe - Electro-Magnetic Pulse Tolerant Microwave Receiver (DARPA)
ADNERF - All Dielectric Non Electronic Radio Front End
Main advantage is that RF antenna can be placed several miles away from RF receiver and the receiver can be placed in an EMP protected environment.
System loss is around 10-20 dB, but as R&D is mostly of military interest, improvement can have been done that not is official.
ADNERF.jpg
By adding RF sensitive nano structures, in the fiber core or cladding or at fiber end can similar effect be achieved and it is ongoing research in this field as it can be an cost effective alternative for modulating/transferring commercial +100GHz wide band RF.
ef_Pockel.gif
 

davenn

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Try "www.jackdaniel.rfwise.com, a short tutorial on RF over fiber technology.
yeah but I dont think that was what the Op was asking.

Can we use fire optic cable for connecting antennas to communication equipment? Why do we use only coaxial cable for connecting an antenna to a transceiver and fibre optic is used only in data communication?
the way he asked the question was clearly inferring that the fibre got connected to the antenna as coax would and the RF was pumped directly into the fibre and that answer was as I said in my first post.... no

but I went on to tell the basics of what is done with equip at the fibre and coax nodes which is what your post and a couple of others confirmed.

having coax-fibre headend equip is pretty common these days for distributing cellular and all sorts of other RF comms deep into buildings where signals wont penetrate from outside

Dave
 
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