How do you measure cable impedance?

S

Sam_G

Member
My Satellite Setup
Dreambox DM500 HD
Triax TDA88
My Location
Sweden
Hello everyone!

I've read that the impedance of a typical coaxial antenna cable should be 75 Ohms. I am having trouble with my new dish installation, and I would like to rule out the cable as the cause of the problem. I have done a continuity test on the copper core and the shield and it's OK. But how do you go about if you want to test the cable for impedance?

Thanks in advance!
 
I wouldn't bother - If continuity of inner and braid is ok, all you need do is check the insulation resistance between inner and braid then carry out a visual inspection for kinks or other obvious physical inconsistency.
 
There are other ways too, but all use lab equipment costing thousands of pounds, and what is the point anyway, the impedance is a design feature of the cable.

If you want to know if the cable has gone south measure the output of the LNB and then again the output of the LNB from the other end of the cable using an installer's meter. Check how many dB you are losing and check this against the cable spec. BTW, loss in the cable is normal. The only time you really need to take notice is if it is affecting the MER of the signal.
 
Also make sure you didn't use CATV coax, it doesn't have the frequency range needed for the higher frequency IF signal from the LNB, CATV coax will roll off sharply at around 1 GHz, coax for satellite work should be rated to 2.5 GHz (or better) to keep the signal loss down.

Look on the coax you used it should be marked.
 
And if someone wanted to try to measure the coax impedance of an unknown piece of coax then this link is the best one I've seen in a long time on how to do it the easy way and without some very expensive equipment.(it does require an oscilloscope though)

hXXp://www.irb.hr/users/stipcevi/ele/coaxweb/index.html
 
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