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The relation between cable losses and cable length in coaxial cables

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alqasim

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In coaxial cable, is there any relation between cable length and losses , i mean does the reflection coefficient have an effect on the cable losses
 

flatulent

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Re: cable losses

Yes. The loss of cables is in dB/m so the length is a factor. Also the reflection loss is a function of load impedance and cable impedance.
 

alqasim

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Re: cable losses

is the loss a function of reflection coefficient
 

Azulykit

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Re: cable losses

Insertion loss has a number of components including conductor and dielectric loss as well as loss due to reflected power (described by reflection coefficient).

The literal answer is that insertion loss is not a function of reflection coefficient (because insertion loss also has additional components).
 

vfone

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Re: cable losses

Yes, there is a relation between Reflection Coefficient Γ and Loss

Reflection coefficient Γ = (VSWR – 1) / (VSWR + 1)

Missmatch Loss (dB) = – 10*LOG (1 – Γ²)
 

alqasim

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Re: cable losses

Is there any possibility that the losses will be less for a longer cable length depending on VSWR ( i am talking about the same cable type and frequancy )
 

Azulykit

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Re: cable losses

Cable insertion loss increases with length. The greater the VSWR the greater the reflection loss.

I imagine that one might contrive a situation where an improvement in VSWR for a longer cable might give the appearance of lower overall loss, but this is not the normal situation.
 

alqasim

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Re: cable losses

I want to ask a question in this way, if I am working on 200 MHz the wave length will be 1.5 m, so the half wave length will be 0.75 m.

If I have two coaxial cables 30 m & 28.15 m respectively, the cable loss for 28.15 will be higher than the cable loss for 30 m. Am I right?
 

fernetbranca

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Re: cable losses

The wavelenght inside the coax is not the same as in the free space, so it's not c/f.
Coax have a "velocity factor" (0-1) wich means that the wave velocity is less than "c", the speed of light. From what I know, wavelenght should be calculated this way.

If you have two identical coax, working with the same load connected, at the same frequency (and bandwidth), the attenuation will be higher in the longer coax (more dB/m I think)

(Please correct me if you think I'm wrong)
 

FvM

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Re: cable losses

If I have two coaxial cables 30 m & 28.15 m respectively, the cable loss for 28.15 will be higher than the cable loss for 30 m. Am I right?
I think, generally not. First of all, any effect of impedance mismatch shouldn't be designated cable loss. Cable loss is basically independant of impedance matching, a frequency dependant (approximately proportional to squareroot of frequency) and length proportional attenuation.

Vfone has presented some formulas for reflection or mismatch loss. As a problem, the formula only applies if the other cable end is matched. With two-side mismatch, you get a complicated situation, where also the cable length (as multiply of wavelength) matters. For each pair of source and load impedance, you get alternating maximum and minimum transmission factor with varying cable length.
 

alqasim

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Re: cable losses

thank you for all participations

Cable loss is basically independant of impedance matching, a frequency dependant (approximately proportional to squareroot of frequency) and length proportional attenuation.

Can you give me any equation about this ?
 

g579

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Re: cable losses

Perhaps you would examine your measuring setup. If it is not correct, the results will also be incorrect
 

FvM

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Re: cable losses

Can you give me any equation about this ?
This can be seen from cable manufacturers data, e. g. suhner.com. The squareroot relation is valid for a region where cable resistance (skin effect) losses are dominant.
 

amaral.eel

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Re: cable losses

Beyond of to be dependent of frequency and of lenght, cable losses is dependent of impedance matching between cable and connector.
 

ulasaci13

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Re: cable losses

sure it is about the length of the cable..it changes but if you want to use a standart loss value; you should take 0.22 dB / m for coaxial.
 

pankajdudulwar

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cable losses

As discussed in above post I am expressing my view if I am wrong plz correct me.The cable characteristics given by manufacturer provides the maximum frequency of operation of the cable. So the wavelength and length of the cable might have relation but generally if it specifies that the cable is working for the specified frequency than it work for small as well as large cable length only the losses will differ with the length.
 

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