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Old 15-Oct-2011, 7:11 AM   #12
GroundUrMast's Avatar
Join Date: Oct 2010
Location: Greater Seattle Area
Posts: 4,773
Originally Posted by be236 View Post
Another basic question.

If I measure across the terminals of an antenna (Yagi or LPDA, etc)... using DVM, should I get about 0 ohms (closed loop) or infinite ohms (open loop)?
It depends...

The Antennacraft Y5-7-13 Yagi design will measure as a short circuit (close to 0.0 ohms) at the terminals. It uses a folded dipole. Other Yagi designs could be 'open' at DC (0 Hz).

The Channel Master CM3016 elements are open ended so you should expect near infinite DC resistance at the terminals.

The only antenna that would measure a moderate amount of resistance at the terminals (that I can think of at the moment) would be a terminated Rhombic which would be a rare custom build in the OTA TV enthusiast world.

Most 300/75 ohm matching transformers will present a DC short... but not all. So if the matching transformer is connected, the DC resistance could be near zero, then measure 'open' with the transformer disconnected.

You can usually look at an antenna and see the DC paths... The interactions between elements at radio frequencies are not as easy to observe. A typical DVM is not going to measure the RF characteristics of any of your antenna system components. The DVM is going to be limited to tasks like proving DC continuity of coax and preamp power supply voltage.
If the well is dry and you don't see rain on the horizon, you'll need to dig the hole deeper. (If the antenna can't get the job done, an amp won't fix it.)

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Last edited by GroundUrMast; 15-Oct-2011 at 7:27 AM.
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