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Old 07-06-2008, 12:13
mush22
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HI
I just wanted to move my modem up stairs to a bedroom which will be my study, Virgin quoted 75.00, Job looks easy I wondered if anyone new what impedance coaxial cable I need?
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Old 07-06-2008, 12:17
Ignitionnet
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75 ohm.
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Old 08-06-2008, 16:59
TheBigM
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The OP probably needs to know the resistivity rather than the total resistance of the cable given the total impedance will depend on the length of the cable used.
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Old 08-06-2008, 17:26
Ignitionnet
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Coaxial cable is sold on characteristic impedance. 75 ohm is the correct cable to use with Virgin Media, as that is the impedance of the coax coming into the home. Length of cable run and the attenuation that introduces is irrelevant to impedance, being as it is a function of capacitance and inductance on a per unit basis, this says that better:

How does coaxial cable chacteristics define the impedance ?

The length has nothing to do with a coaxial cable impedance. Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and outer conductors, and on the characteristics of the dielectric material between the inner and outer conductors.

The following formula can be used for calculating the characteristic impedance of the coaxial cable: (formula taken from Reference Data for Radio Engineers book published by Howard W. Sams & Co. 1975, page 24-21)

impedance = (138 / e^(1/2)) * log (D/d)

Where:

* log = logarithm of 10
* d = diameter of center conductor
* D = inner diameter of cable shield
* e = dielectric constant (= 1 for air)

In a nut shell the characteristic impedance of a coax cable is the square root of (the per unit length inductance divide by the per unit length capacitance). For coaxial cables the characteristic impedance will be typically between 20 and 150 ohms. The length of the cable makes no difference whatsoever in regard to the characteristic impedance.

If the frequency is much too high for the coaxial cable, then the wave can propagate in undesired modes (i.e., have undesired patterns of electric and magnetic fields), and then the cable does not function properly for various reasons.
OP really does need to know impedance, get that wrong and you introduce microreflections on to the cable network as well as degrading your own signal. The attenuation of the cable is probably irrelevant over the relatively short distance that it will traverse in home unless the signal is already marginal.

If OP just gets himself some RG59 he'll be fine, his cable drop is probably RG59 also.
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Old 09-06-2008, 10:50
spanglysteve
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or get a wiireless router. This is what we did and much much simpler!
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Old 13-06-2008, 00:06
petey286
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HI
I just wanted to move my modem up stairs to a bedroom which will be my study, Virgin quoted 75.00, Job looks easy I wondered if anyone new what impedance coaxial cable I need?
Get this router for 14.97 job done.

http://www.pcworld.co.uk/martprd/sto...&category_oid=
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