# Why is Electrical length greater than physical length?

Discussion in 'General Electronics Chat' started by shopitham, Feb 14, 2011.

1. ### shopitham

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```Hai
1)What does a electrical length in transmission line means?
2)How does it differ from the physical length?
3)Why is electrical greater than that of the physical length?
some pls help me find the ans .

Thanks
Shofiya
```

shopitham, Feb 14, 2011

2. ### rob_croxfordVIP Member

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The Physical length of a coaxial cable (such as cable used in transmission lines) is the length from one end of the connector to the other.

The ELECTRICAL length of such a cable depends on the size and type of the dialectric material used within it (if im not mistaken). It is measured at the mating plane of the outer conductor and is the length of the adapter as it appears to the signal traveling through it.

Rob

rob_croxford, Feb 14, 2011

3. ### davennModerator

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The electrical length of a transmission line is dependant on 2 things...

1 ... the velocity factor of the line and
2 ... the frequency
an example ... to find the electrical length of a transmission line or dipole antenna
say for 400MHz

(300 million metres / sec (300,000 km / sec))
300 / 400MHz = 0.75m or 75cm that is the wavelength
now if we want an electrical wavelength in a transmission line that has a VF (velocity Factor) of 0.85 then....

75 x 0.85 = 63.75 cm so you can see that the electrical wavelength is shorter
than the free space wavelength

Dave

Dave

davenn, Feb 14, 2011

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