characteristic impedance of a line is square root of L/C. It does not

depend on the length. But the losses occuring in resistance, no matter

how small, is dependent on the length. Any literature to read on how

the length affects the charactristic impedance.

When doing circuit analysis, will this charactreritic impedance be

added to the series connecting element?

I have a device that says input impedance 50ohm, i am not using it.

But to use other part of the device i have terminate it with 50ohm.

How can i do it? can i just connect a 50ohm characterisitc impedance

cable and leave it?

thanks in advance

In real coaxial cable, the effect of resistance can be specified as a

certain amount of dB power-loss per foot of cable. For example, if the

cable has 10 dB of loss per foot, then 5 feet of cable would

absorb 50 dB of the total power.

Another effect to keep in mind is that as the frequency goes up, the dB

losses per foot usually go up. So if a particular cable has 10 dB of

loss per foot at 500 MHz, it may have 50 dB of loss per foot at 1.5

GHz. Note that I'm just making these numbers up.

So, to answer your question, this power loss doesn't really effect the

characteristic impedance of the cable. To a first approximation, the loss

can be modeled separately from the characteristic impedance.

Note that the above discussion assumes you have a resistive load at the

end of the transmission line whose impedance is the same as the

transmission line's impedance. For example, if it is a 50 Ohm transmission

line, you should have a 50 Ohm resistor (or 49.9) at the end as a load.

And to answer your other question, the best way to terminate a device at

frequencies up to UHF is to use a small surface-mount resistor (49.9 Ohms

in this case) as close as possible to the device. If you can't get close

to the device, for some reason,

use a 50 Ohm cable with a 49.9 Ohm

resistor at the end of it. You can also buy 50 Ohm terminators for cables

and connectors. If the device has some kind of an RF connector on it, it

may be easier to use a terminator for it.

If the frequencies are down around 100 MHz or less, then you don't really

have to be all that close to the device or do anything fancy. Just put any

type of 50 Ohm resistor across the terminals somehow.

--Mac