danickstr wrote on Mon, 30 May 2005 19:32 |
to test for 75 ohm accuracy, you have to put the terminator on the end first. the terminator is what makes the cable's resistance 75 ohms.
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Well, sorry to be pedantic here. I feel like an old teacher saying it, but it can actually be very important.
As written above, it is only part of the truth. The 75 Ohm really should referr to the characteristic impedance of the cable, not the resistance. They share the same unit, Ohms, but describe different things. Impedance is an indicator of how the cable behaves with alternating current such as radio frequency signals, resistance is how things behave with direct current. Standard multimeters generally measure resistance using direct current.
Characteristic impedance of a coaxial cable in general is dependant on the size and spacings of the inner and outer conductor, and the dielectric in between. The length of the cable makes no difference. Hence it is sort of built into the cable at manufacturing time.
Gunnar.