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How to Check the Resistance in LMR 400 Cables

Cell phones work at microwave frequencies and typically have a wavelength of tens of centimeters. Although microwaves propagate through free space allowing wireless communication, sometimes it can be more advantageous to guide the waves down cables to reduce signal loss. Special cables are used to guide microwave signals and one such cable is the LMR 400 -- a 50 Ohm coaxial cable which is also capable of supporting radio frequency electromagnetic waves. The resistance of LMR 400 cables can be measured using a digital multimeter.

Things You'll Need

Digital multimeter

Instructions

- 1
  Plug the measurement probes into the digital multimeter. Two measurement probes should be supplied with the multimeter. Plug the red probe into the positive terminal and black probe into the negative terminal.
- 2
  Switch on the digital multimeter by rotating the dial on the front. Select the resistance function by pointing the arrow on the dial in the appropriate direction. The display should read zero. To test the multimeter is working correctly, bring the two measurement probes into contact. The display should change to a very low resistance value of approximately 0.1 Ohms.
- 3
  Connect the positive measurement probe of the multimeter to one side of the LMR 400 cable, and the negative measurement probe of the multimeter to the remaining side of the LMR 400 cable. The display on the multimeter should change and show the measured resistance.

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