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Author Topic: Receive disappears when connecting shell of coax connector  (Read 921 times)

KL7AM

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Receive disappears when connecting shell of coax connector
« on: October 09, 2022, 04:42:46 PM »

Receiver goes deaf when connecting outer shell of coax connector. It is not the coax. If the center conductor only is connected it works fine.

What do you think?
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K6SDW

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Re: Receive disappears when connecting shell of coax connector
« Reply #1 on: October 09, 2022, 05:23:02 PM »

A short or really high SWR or wrong antenna......GL
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KL7AM

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Re: Receive disappears when connecting shell of coax connector
« Reply #2 on: October 09, 2022, 06:20:23 PM »

It is not antenna or swr or the like as other radios work fine with the same connection. It is in the radio and of course it is a "short" of some sort
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WY4J

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Re: Receive disappears when connecting shell of coax connector
« Reply #3 on: October 09, 2022, 06:21:57 PM »

Your coax is shorted. Check the connector.
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AC2EU

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Re: Receive disappears when connecting shell of coax connector
« Reply #4 on: October 09, 2022, 06:54:12 PM »

 other radios working on the antenna system does not mean it's the radio. After all, you say it works fine until you connect the shell...

"other radios" seems to imply that you have a antenna switch of some type?
The problem could be there, or the short coax to the new radio.

Maybe try the the cable that the "good" radios work on.

One thing is for sure , the non working radio "sees" something different than the others!

WB6BYU

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Re: Receive disappears when connecting shell of coax connector
« Reply #5 on: October 09, 2022, 08:37:51 PM »

That's actually not uncommon, especially when the antenna is small,
or not properly tuned.

When the coax shell is not connected to the radio, the whole outside
of the coax often acts as the antenna.  It's like sticking one end of a
random wire into the antenna jack - it can pick up lots of signals.

When you connect the coax shell, then that mode should go away,
and you have only the signal from the antenna itself.  In some cases,
the antenna may pick up much less RF than the "long wire" using
the coax.  (And, of course, some antennas radiate more RF from
the coax than from the "antenna" itself.)  That might be because
the antenna is physically small, or not tuned to the operating
frequency.

The result can vary widely, depending on the coax, the antenna,
the use of an internal antenna tuner, how the coax is grounded,
etc.  By itself, I wouldn't consider it a symptom that the antenna
isn't working properly. 

But if you can't hear any signals on the antenna, when you should
reasonably expect to, then the combination of antenna and feedline
need to be evaluated to find the problem.  A good first step is to
measure the SWR at the end of the coax in the shack, without using
an antenna tuner, and compare that to the expected value for your
antenna.


There are possible quirks that can be confusing.  I have an old SWR
meter where the center contact on one of the connectors has gotten
bent out slightly (probably by PL-259s with too much solder on the
tips over the years).  Usually the center pin makes contact deeper
in the socket.  But some crimped plugs taper the center pin for the
crimp connection, and those gave only intermittent contact.

In that case, the cable would work if I plugged it directly into the
transmitter, but not if I plugged it through the SWR meter.  Other
meters worked fine.  So be methodical when testing your coax
cables - it really might be true that they work when plugged into
one radio but not into another!

KL7AM

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Re: Receive disappears when connecting shell of coax connector
« Reply #6 on: October 10, 2022, 09:29:07 AM »

It has nothing to do with the coax or switch. It is in the radio itself and it isn't the SO-239 on the radio either.
If it was something that easy I wouldn't be posting here.

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W1VT

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Re: Receive disappears when connecting shell of coax connector
« Reply #7 on: October 10, 2022, 09:42:43 AM »

A strong signal could be overloading or blocking the receiver.

This often happens during multi-transmitter field day operations.
The receiver will go deaf whenever a nearby transmitter overloads the receiver.
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WB6BYU

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Re: Receive disappears when connecting shell of coax connector
« Reply #8 on: October 10, 2022, 10:44:14 AM »

Is the coax shield grounded at any point?  Or is there a
common connection between the coax shield and other
equipment?

If so, in some fault cases, connecting the coax shield to the
radio could provide a DC (or AC) path that changes the
voltages inside the radio.  For example, a shorted DC
blocking capacitor could disrupt the biasing of the RF stage
or diode switching of output filters if the coax and antenna
provided a DC path from the center conductor to ground.

A quick check with a voltmeter across the coax jack (both DC
and AC) should detect any such issues.

AC2EU

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Re: Receive disappears when connecting shell of coax connector
« Reply #9 on: October 10, 2022, 11:15:57 AM »

It has nothing to do with the coax or switch. It is in the radio itself and it isn't the SO-239 on the radio either.
If it was something that easy I wouldn't be posting here.

If you are so certain that it's the radio, I guess you better send it out for service. No need to post here at all!
However, The only one who believes that the radio is defective so far is YOU.
No sense in asking for advise if you reject it off hand. Good luck with your "mystery"!

W9WQA

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Re: Receive disappears when connecting shell of coax connector
« Reply #10 on: October 10, 2022, 01:43:27 PM »

sounds like a job for an,

OHM METER
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KL7AM

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Re: Receive disappears when connecting shell of coax connector
« Reply #11 on: October 11, 2022, 09:53:02 AM »

The problem with an ohm meter is it is designed for DC measurements and not RF. If you have a tuned circuit on the input than a tuned circuit can show a DC short but not be a short at a RF frequency. The 50 ohms at the antenna port is a RF impedance and not a DC measurement.
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W9WQA

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Re: Receive disappears when connecting shell of coax connector
« Reply #12 on: October 11, 2022, 12:22:47 PM »

The problem with an ohm meter is it is designed for DC measurements and not RF. If you have a tuned circuit on the input than a tuned circuit can show a DC short but not be a short at a RF frequency. The 50 ohms at the antenna port is a RF impedance and not a DC measurement.

well, well, well...no kidding

still sounds like a job for an,

OHM METER
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AC2EU

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Re: Receive disappears when connecting shell of coax connector
« Reply #13 on: October 11, 2022, 03:55:08 PM »

This thread is so absurd, it's funny!
Some folks are just...well...stuck, and there is no "un-sticking" them.

N7EKU

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Re: Receive disappears when connecting shell of coax connector
« Reply #14 on: October 11, 2022, 04:25:31 PM »

Hi,

What does "works fine" mean?  For example, if you find a strong station (strong Morse or SSB signal or strong shortwave station like Radio Havana Cuba) and you receive it well with just the center connected, if you connect the shell then the station disappears?

Mark
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Mark -- N7EKU/VE3
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