Measurement of Resistance of an Antenna

                                                                           

                                                                                                                                                                                                                        

 Have you ever tried to measure the resistance of an antenna with an ohmmeter? As we all know that the antenna presents a load of nearby 50 ohm when we are transmitting the signal. So you will expect that ohmmeter will show 50 ohm but it won’t.  Reason????????

VHF antenna has a 50 ohm resistance only at 118MHZ to 136 MHZ, means it is AC resistance. As we know ohmmeter measure only DC resistance, so it will tell nothing about the resistance at 120 MHZ .                   You can measure either an open circuit (infinite resistance)or a short circuit(zero resistance)when checking an antenna with an ohmmeter.                                                                                                                                   The whip antenna which has just the wire connected to the antenna rod ,will of course measure as an open circuit with an ohmmeter  ,but some wide band antenna will measure as a short circuit.                        Don’t let this short circuit scare you, as it measure at DC, and at 120 MHZ the signal will see the proper load.
 If you measure a short circuit on something like whip antenna, it is probably a short in the cable somewhere, or in the connector, which is the most common problem.                                                                              The SWR measurement is the only way to check the antenna & the antenna cable thoroughly.