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Old 18-11-2013, 09:58   #1
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Purpose of coax shield (braid)

A friend of mine is having some VHF radio trouble. When listening at the dock, he said he couldn't hear any of the normal chatter on 16 and wasn't sure his radio was working. He can hear the WX broadcasts but nothing else.

To test this, we tried to establish radio contact while he was sailing last weekend. I could hear him loud and clear but he could not hear me at all.

When he got back to the dock, we looked at the coax connection to the radio. The radio installation was by the builder (it is a new Bavaria), and he had gone back in and soldered the PL-259 center pin to make sure it was making good contact. We really couldn't tell whether the braid was soldered or not. Because of the type of connector the Germans use, the two little holes on the side of the connector are covered and inaccessible.

Now, I told you all of that to ask you this - While looking at his radio, I selected WX1 to get a weather broadcast. When I touched only the center pin of the coax cable to the female connector of the radio, I got reception. The shield was not required to be connected in order to receive WX1. Why is this? Is the braid on coax ONLY to shield noise? Is it not required to be connected to the radio in order to receive broadcasts?
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Old 18-11-2013, 10:09   #2
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I don't want to sound insulting, but have y'all checked the squelch?
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Old 18-11-2013, 10:47   #3
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Re: Purpose of coax shield (braid)

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Originally Posted by steve77 View Post
Now, I told you all of that to ask you this - While looking at his radio, I selected WX1 to get a weather broadcast. When I touched only the center pin of the coax cable to the female connector of the radio, I got reception. The shield was not required to be connected in order to receive WX1. Why is this? Is the braid on coax ONLY to shield noise? Is it not required to be connected to the radio in order to receive broadcasts?
At the kind of frequencies we are talking about, the antenna cable is working like a waveguide - analagous to a pipe. For it to direct the power to the antenna at the top of the mast (in TX mode), and to guide the signal from the antenna back to the radio (in RX mode), it needs to not have any breaks in it. That means a continuous shield, and centre. Any breaks in the shield mean lost signal. Even any changes in impedance mean reflections and lost signal.

I think you could get away with not connecting the shield in RX mode, as you then start using the entire cable for recieve. But in TX mode it will work very badly as the transmission will not be directed to the antenna at the top of the mast. Don't forget that the sailboat antenna is optimised in terms of directionality (transmits in a horizontal ring) to make best use of the transmitted power.

Oh, and yes, the braid does shield noise too.
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Old 18-11-2013, 10:55   #4
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The shield was not required to be connected in order to receive WX1. Why is this? Is the braid on coax ONLY to shield noise? Is it not required to be connected to the radio in order to receive broadcasts?
The radio is going to receive something with just a wire. The NOAA Wx transmitters are either 300W or 1000W and the antennas have gain. So they are transmitting at about 100 times more equivalent power than a marine VHF.

The coax shield will have more effect on transmission than reception. The radio will receive without the shield but it will not transmit well.

If your friend was heard but could not hear there are a few things to check. Squelch may be too high. Turn it all the way down and then listen. The receiver may be defective. Unlikely if Wx channels work but still possible. Another possibility is there is something onboard that is causing interference. Turn off every other device onboard and try again. Be thorough about turning off every other thing (fridge, GPS, etc.). Be especially suspicious of LED lights.
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Old 18-11-2013, 11:23   #5
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Re: Purpose of coax shield (braid)

Here's an idea - buy one of these emergency antennas. I bought mine for diagnostic purposes, when I was having receive trouble, but they are handy to have anyway :

Shakespeare 5911 10" Marine VHF Antenna The Stowaway Emergency w Unity Gain | eBay

In my case the receive issue was a bad connection at the antenna termination (mast bottom). It might be worth re-terminating the cable ends. I realise this is easier at the bottom, than at the top, of the mast.

I much prefer solderable connectors.

A favourite thing that can go wrong is to not cut the relative lengths of inner core and sheath correctly - so either the pin is too far back in the connector, or the sheath is not crimped. Assembly is most definitely error-prone, if you don't know what you're doing. Having seen plenty of badly-assembled BNC connectors on new cables, it wouldn't surprise me on your boat.
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Old 18-11-2013, 11:38   #6
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Re: Purpose of coax shield (braid)

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I don't want to sound insulting, but have y'all checked the squelch?
Yes, I had the same thought. I didn't want to ask my buddy if that could be the problem for the same reason. Verified squelch wasn't the problem when we looked at it later that day.

Would have been a simple solution!
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Old 18-11-2013, 11:50   #7
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Re: Purpose of coax shield (braid)

Thanks, all!

A common thread of the responses is the shield is required for transmission. It may or may not be required for reception depending on the strength of signal.

In my buddy's case, he could transmit fine, so this implies the shield is connected properly at the antenna and the radio. Is this correct?

So, if it's not a problem with connectors, what else? We will check for electrical interference, but that seems like a long shot to me. I don't remember a lot of noise coming over the speakers, like it were masking other signals. It was pretty quiet.

The emergency antenna idea will isolate the problem between the radio and the cable/antenna. I'll check to see if he has one we can try. Like you said, it would be a handy spare to have on board.
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Old 19-11-2013, 05:03   #8
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In my buddy's case, he could transmit fine, so this implies the shield is connected properly at the antenna and the radio. Is this correct?
A check with a VSWR would tell for sure but if he can be heard several miles away then probably the shield is ok.
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So, if it's not a problem with connectors, what else? We will check for electrical interference, but that seems like a long shot to me. I don't remember a lot of noise coming over the speakers, like it were masking other signals. It was pretty quiet.
It is not easy to tell the difference between interference and normal background noise. VHF receivers should not be "quiet". If he turns the squelch all the way off the radio should be noisy. If it is quiet there is something wrong with the receiver or speaker. If there is an external speaker unplug it from the radio and see if anything changes.

If the radio has a PA function make sure that is turned off.
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Old 19-11-2013, 06:14   #9
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Re: Purpose of coax shield (braid)

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Originally Posted by steve77 View Post
So, if it's not a problem with connectors, what else? We will check for electrical interference, but that seems like a long shot to me. I don't remember a lot of noise coming over the speakers, like it were masking other signals. It was pretty quiet.
At this point, it seems that the VHF radio itself is suspect. Easiest thing might be to connect and try another radio on your friend's boat.
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Old 19-11-2013, 07:20   #10
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Re: Purpose of coax shield (braid)

We have some trouble-shooting to do. It'll be after Thanksgiving, but I'll let you know what we find.

I have another antenna, and I can take the VHF from my boat over to his, so we should be able to isolate the problem. Come to think of it, I THINK I have a handheld with a removable antenna, so maybe I can kluge together something for testing.

Thanks, everyone!
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Old 19-11-2013, 10:08   #11
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Re: Purpose of coax shield (braid)

Steve,
1) The very first thing to suspect when dealing with on-board VHF radio reception (or transmission) issues, is the coax connectors!!!
And, in your situation, this is almost certainly the case...(although it may be the connector at the masthead, and/or connector at the back of the radio, and/or any coupling connectors at the mast base...)
Quote:
Originally Posted by steve77 View Post
A friend of mine is having some VHF radio trouble. When listening at the dock, he said he couldn't hear any of the normal chatter on 16 and wasn't sure his radio was working. He can hear the WX broadcasts but nothing else.

When I touched only the center pin of the coax cable to the female connector of the radio, I got reception. The shield was not required to be connected in order to receive WX1. Why is this?
2) If I understand your description correctly, your friend's radio gets better reception with only the center pin of the coaxial connector plugged in???
If that is correct, that is a common symptom of a shorted antenna, and/or a shorted cable, and/or shorted coax connector(s).....
(most often a shorted coax connector)....

3) Your friend's reception difficulty (although, with some transmit ability) is also very indicative of a coax / connector short (or antenna short / fault)....

4) Being a "new" boat, I suspect connector shorts....even the guys that are supposed to know how to assemble / install connectors can get it wrong!!


So, my best advice is to check the connectors, and if you do not have the experience in repairing/replacing these coaxial connectors, have your friend hire a pro to do it....
And, if it is like you say, a new boat, ask the dealer / broker / factory rep to get it done, and get it done correctly by a professional, ASAP!!!
As, a VHF radio system on-board is part of your on-board safety system...


I hope this helps..

John
s/v Annie Laurie




{P.S. YES.....the shield is an important part of the cable, NOT just to shield out noise....
Think of 12vdc wiring....there's a positive and a negative....are they both needed???
Or 120vac wiring, you have a hot and a neutral/ground....are they both needed???
Same for RF transmission lines....two wires are needed....
There is no "magic" as to how a coaxial transmission line works...
(And while some on-line sources may say that you can think of coax like a waveguide, in reality, at the freqs that we use, it isn't...) }
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Old 19-11-2013, 12:41   #12
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Re: Purpose of coax shield (braid)

Steve,
Just to clarify things a bit...
Quote:
Originally Posted by steve77 View Post
A common thread of the responses is the shield is required for transmission. It may or may not be required for reception depending on the strength of signal.
Proper connection of the shield of the coax (as well as the center conductor) is required for both effective transmission and reception!!
The reason you see a difference between reception and transmission is the overwhelming link-budget margin of most VHF-FM Marine transmissions, especially those from shore stations / NOAA weather stations, etc..
Like Dan wrote, you can receive most of these NOAA weather broadcasts with just a wire stuck in radio...





Quote:
Originally Posted by steve77 View Post
In my buddy's case, he could transmit fine, so this implies the shield is connected properly at the antenna and the radio. Is this correct?
Actually, no this is not the case....as we've mentioned, the symptoms you described are indicative of a connector / cable short (and/or a antenna short, but that is unlikely).







Here are some easy troubleshooting tips...
Quote:
Originally Posted by steve77 View Post
I don't remember a lot of noise coming over the speakers, like it were masking other signals. It was pretty quiet.
Tune in to one of the NOAA weather channels that you CAN hear, turn the volume up to a normal listening level, turn the squelch all the way down, and then tune to an empty/unused channel....and you should hear lots of noise, typically louder than the computerized voice of the NOAA weather broadcast you were just listening to,,,
If you do NOT hear lots of noise, the radio is suspect....
If you DO hear this noise, the radio is probably fine....

In my opinion, searching for RFI/noise sources as possible causes for this problem is probably a waste of your time...certainly a waste, until you've ascertained that the cable / connectors (and antenna) are okay (see above)...






I hope this clarifies things...

John
s/v Annie Laurie
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Old 20-11-2013, 06:34   #13
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Re: Purpose of coax shield (braid)

Thanks, John.

Some answers -

His boat is not in warranty. I say "new" but it's 4 or 5 years old. It's a lot newer than any boat I've ever owned!

I wouldn't say that we got "better" reception with only the center pin making contact. The fact we got anything at all is what surprised me.

Another thing for me to confirm is whether the radio used to work in the past and failed, or if it has always been like this. We'll check it out next week. We only had a few minutes to look at the problem last time, so this time we'll see if we can't find the problem.

By the way, really appreciate the effort you took to make the videos on the M-802. Thanks!

Steve
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Old 01-12-2013, 20:47   #14
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Re: Purpose of coax shield (braid)

I said I would let everyone know the results of our troubleshooting, so...

We removed the radio connector from the back of his radio and tried a good piece of RG-213 with good connectors and a new antenna I had not installed on my boat yet. No joy.

Pulled my buddy up the mast to inspect the connector. Checks with a VOM all looked good. Replaced the antenna temporarily with my new antenna. Still nothing for reception.

Put everything back together at the masthead, went to my boat and removed my VHF radio. Took it to his boat and plugged it in to his cable/antenna. Reception was excellent.

Conclusion: his radio is the problem, and his cable, connectors, and antenna are fine. Thanks to everyone who chimed in on this one. Although it sure looked like the connectors were suspect, the radio was the culprit.

Steve
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Old 02-12-2013, 12:22   #15
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Re: Purpose of coax shield (braid)

Steve,
Thanks for returning to let us know what your final results were!!!!



Quote:
Originally Posted by steve77 View Post
I said I would let everyone know the results of our troubleshooting, so...
Conclusion: his radio is the problem, and his cable, connectors, and antenna are fine. Thanks to everyone who chimed in on this one. Although it sure looked like the connectors were suspect, the radio was the culprit.
It's rare, but receivers do fail...

And, vhf marine radio front-ends are susceptible to damage from the EMP of close lightning strikes....so 'ya never know, perhaps your friend's boat was in the wrong spot at the wrong time, at sometime in the past that contributed to his vhf radio receive issue???

Whatever the case, it's good that he had a friend like you to help him figure it out!!



Fair winds...

John
s/v Annie Laurie
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