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Author Topic: Does antenna's read short or open?  (Read 1217 times)
KT4DLB
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Posts: 72




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« on: March 14, 2013, 08:38:33 PM »

Just curious, I have a G5RV and a Diamond X-50 antenna. If you was to put  a ohm meter on the PL-259 od each antenna, would it read short or open? And is this a way to tell if you have a short on your cable coming from the antenna or not?
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N4CR
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« Reply #1 on: March 14, 2013, 11:13:06 PM »

It could be either, depending on the matching network and the type of antenna.

For example, a loop always reads a short between the terminals and a dipole always reads an open.

An ohmmeter is an insufficient test instrument nearly all measurement of antennas other than proving that you don't have any broken connections as you test each part of the system.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
K2DC
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« Reply #2 on: March 15, 2013, 03:12:00 AM »

If you disconnect the cable from the antenna and then read a short across the connector with an ohmmeter, you indeed have a short in the cable.

73,

Don, K2DC
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KM3F
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Posts: 497




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« Reply #3 on: March 15, 2013, 08:38:50 PM »

You can't rely on an ohm meter unless you know what you 'should' see.
That means knowing the feed method of any antenna.
For example, if the G5RV had no balun for mutiband operation, it should read an open, if it does, you read the balun winding as a short.
On a vertical, it depends on the feed method again. If it has a loading coil in the base and the feed is a shunt across some turns and ground it will read shorted.
Using a meter shows up gross problems DC wise but either the SWR meter or a 259 antenna analyzer shows the real situation at RF frequencies.
The other side of this is an Ohmmeter could not show an 'RF open' in a normally 'DC open circuit' as far as RF is concerned, so you could not tell in that case.
Good luck.
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WA5KM
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« Reply #4 on: March 16, 2013, 11:57:21 PM »

A "short" is a matter of degrees. You should measure at least some resistance. Put your ohm meter leads together. Do they read a fraction of an ohm or as some 6 ohms? Subtract from the reading. There is resistance in coax and feedlines but not much. Agrees with "what you should see"
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M6GOM
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« Reply #5 on: March 17, 2013, 05:58:35 AM »

Just curious, I have a G5RV and a Diamond X-50 antenna. If you was to put  a ohm meter on the PL-259 od each antenna, would it read short or open? And is this a way to tell if you have a short on your cable coming from the antenna or not?

If you've a balun you may see a short. If there's a matching section you may see a short. Certainly the matching section of the Diamond CP6 appears as a dead short.

What is a dead short to DC isn't always seen as a dead short to AC which RF is.

The only way to check for a short on coax using an ohm meter is with the antenna and any inline baluns/chokes disconnected.
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