# Voltage measurement on RF cable - RF Cafe Forums

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Ted1234
 Post subject: Voltage measurement on RF cable Posted: Thu Oct 20, 2005 4:03 pm
I have a piece of RG58 going into an into an o-scope with a 100 MHz BW. The signal is a sinewave (almost) and I know the power out of my source (10 MHz) is +10 dBm. Problem is the o-scope is reading about exactly twice the voltage I expect to see (which is 2 Vpk-pk). I'm measuring 4 Vpk-pk. Any ideas what might be going on. Thanks.

--- Ted

kanling
 Post subject: Posted: Thu Oct 20, 2005 4:08 pm
 Colonel

Joined: Mon Jul 18, 2005 4:31 pm

Posts: 40

Location: Baltimore, MD

I'll bet your scope's input is set to the high impedance mode. Make sure it is set to be a 50 Ohm load. You are exactly right to be expecting about 2 Vpp.

Guest
 Post subject: Posted: Fri Oct 21, 2005 7:17 am
Agreed, your scope impedance is not 50 ohms. If you do not have an option for 50 ohms, place a 50 ohm resistor in parallel between clip leads.

Ted1234
 Post subject: Posted: Fri Oct 21, 2005 10:24 am
 kanling wrote: I'll bet your scope's input is set to the high impedance mode. Make sure it is set to be a 50 Ohm load. You are exactly right to be expecting about 2 Vpp. Thanks kanling - you are exactly right I can't believe I was dumb enough to not think of that. Ted

Posted  11/12/2012