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Is it normal for the antenna conector of the radio to have 18ohm resistence between the inner socket(signal) and the thread (ground)?

When I connect the antenna works fine if only the inner plug makes contact, if the outer metal part of the antenna touches the radio it stops receiving anything.

The radio is a Yaesu FT-747 and the antenna is a Sirtel Santiago 1200

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  • $\begingroup$ What band are you on when this happens? I'll bet it wasn't on 10 meters. (See my comment below). $\endgroup$ – Mike Waters May 21 '18 at 23:48
  • $\begingroup$ In this case I was lintening to 40m QSOs, so, not in the antenna for the band, but the signal was ok. In 10m happened the same, even if there were no comunications to listen, as soon as the coax ground touched the radio the sound became all white noise in any band. $\endgroup$ – Terrik May 22 '18 at 12:00
  • $\begingroup$ Try it on 10 meters or CB. They are mostly dead now, but you might pick up a beacon on 10 or a trucker on CB. $\endgroup$ – Mike Waters May 22 '18 at 14:01
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Very loosely speaking, you should expect the radio's antenna connector to measure 50 ohms. But that is only true if you are measuring it using an RF signal in the frequency band the radio is expecting, not using the DC output of an ohmmeter, so 18 ohms is entirely reasonable.

The fact that making the shield connection causes you to stop receiving signals is a very good indication that either the antenna is defective or that it is completely unsuitable for the band you are trying to receive.

For example, you'd make exactly the same observations if you were to partially plug in a piece of coax with no antenna on the other end. This is because, without the shield connected, the coax might as well be a plain wire, and any old wire stuck in the antenna port will make a receiving antenna of some effectiveness.

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    $\begingroup$ Looks like the OP measured the socket on the radio, not on the coax. $\endgroup$ – mike65535 May 21 '18 at 23:35
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    $\begingroup$ @mike65535 Oops! Edited. $\endgroup$ – Kevin Reid AG6YO May 22 '18 at 0:16
  • $\begingroup$ I'm afraid that, even if 18ohm is reasonable. It doesn't rule out the posibility of something bad in the radio. I'll do further testing to see if I can find the fault. Thanks $\endgroup$ – Terrik May 22 '18 at 12:04
  • $\begingroup$ Completely unsuitable for 40 meters, that's it. That is a 5/8 wave antenna for 10 meters. $\endgroup$ – Mike Waters May 22 '18 at 13:58
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I did some more testing and it turns out the fault is in the coax wire.

It's not a normal cable, it's a vertical magnetic mount and when I use this cable conecting both live and ground it kill all RF signal but it's not shorted.

Thank you for your answers.

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You may have a short in the connector on the coax such that when only the centre connector is attached the coax+antenna forms an antenna but when the shield is connected you just have a short across the antenna terminal.

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  • $\begingroup$ No. I tested, and both the antenna and the cable give open circuit between the inner and outer part of the conection in the multimemter. That was my first idea too:) Thanks anyway. $\endgroup$ – Terrik May 22 '18 at 14:23
  • $\begingroup$ Did you test with it screwed in to a socket? I’ve wired up PL-259s before and accidentally left a stray piece of copper loose which only made contact with the centre pin when it was connected in a socket. That was difficult to trace down! $\endgroup$ – MattC May 23 '18 at 19:42

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