SWR and ground issues

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Boll Weevil
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SWR and ground issues

Post by Boll Weevil » Saturday, 02 November 2013, 17:49 PM

One of the the trucks where I work is having issues with the antenna system and every time we think we have it figured out, more questions are raised. Here's what we know so far, maybe someone can make sense of it.

1. The truck is a 2013 Mack Pinnacle. Antenna is a K40 Trucker with 18' of quality coax going into a Cobra 29 LX. Antenna, coax, and radio have been tested seperately and are in working order. The antenna is mounted on a grabhandle behind the cab, same as most trucks in the fleet. The issues we're experiencing are the same whether using the factory coax and antenna or the aftermarket.

2.SWR is good when parked and unacceptable whenever truck is moving. While the factory setup is difficult to troubleshoot, we cannot find any flaws in the K40 or coax.

3. The radio is powered using the factory cb power posts. It does not lose power when the ground wire is disconnected,but it will lose power when the antenna is disconnected from the back of the radio.

4. The cigarette lighter is not working but there are 2 other power ports that accept a cigarette light plug. Getting the power from these ports does not change the results in anyway.

Any suggestions?

Thanks.


Jim

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Re: SWR and ground issues

Post by MDYoungblood » Saturday, 02 November 2013, 21:55 PM

I am willing to say the problem is with the antenna, switch it out with another one and see what happens. It is possible that there is a cold solder connection in the load and breaks it's contact while moving. Take a couple of pics and post them so we can take a look and see if you are missing anything.

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Re: SWR and ground issues

Post by str8stroke » Sunday, 03 November 2013, 3:44 AM

I am guessing, cause when you say "loosing power", is it the power going off completely like the display is off, or "loosing power" like it won't key up?? So I am going way out on a guess... try swapping the mic?


Sure sounds strange, If the mic change doesn't help, take apart the radio and tap the board with a pencil eraser. Could be a simple loose solder joint on the board. Very common on Cobras. Especially those that bounce around in a big rig.

Good luck.


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Re: SWR and ground issues

Post by 'Doc » Sunday, 03 November 2013, 3:59 AM

From reading #3 it sounds like the radio is being grounded through the shield of the feed line and the mount of the antenna, that's for 12vdc ground. With that 12vdc ground cable connected and the antenna disconnected, does the radio still power on?
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Re: SWR and ground issues

Post by Slyguy » Sunday, 03 November 2013, 6:12 AM

I'm with doc. Check continuity from the negative post of the "cb power posts" and truck chassis. Bet it's opened.


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Re: SWR and ground issues

Post by MDYoungblood » Sunday, 03 November 2013, 7:28 AM

I took it as in #4 using the power port sockets the radio had no issues, SWR is the problem.

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Re: SWR and ground issues

Post by Circuit Breaker » Sunday, 03 November 2013, 19:31 PM

Boll Weevil wrote:3. The radio is powered using the factory cb power posts. It does not lose power when the ground wire is disconnected,but it will lose power when the antenna is disconnected from the back of the radio.
This tells me the radio is getting its negative/ground connection through the antenna feed line instead of through the black (negative) power lead like it should. Not really a good thing.


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Re: SWR and ground issues

Post by jessejamesdallas » Sunday, 03 November 2013, 20:03 PM

Get rid of the radio and antenna...Go back to using two tin cans and a string like in your avatar... They don't require any electrical power, so when the truck starts up, "No worries". :cheers:


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Re: SWR and ground issues

Post by Boll Weevil » Sunday, 03 November 2013, 21:23 PM

Thanks for the replies. I though we had tried unplugging the antenna but we've tried so many other things it's all a blur. We'll give that shot when we have time and see how it works with a different ground.

Str8stroke: I wasn't clear on that but the issue was that the radio should've went dark. The output appears to be fine.
jessejamesdallas wrote:Get rid of the radio and antenna...Go back to using two tin cans and a string like in your avatar... They don't require any electrical power, so when the truck starts up, "No worries". :cheers:
That's my backup unit. I did a noise cancelling mod to it by filling the cans with toilet paper.


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Re: SWR and ground issues

Post by 'Doc » Monday, 04 November 2013, 6:25 AM

Oooo... I'l bet those toilet paper filled cans could come in handy for more than one reason. Wish I'd'a thought of that.



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Re: SWR and ground issues

Post by Boll Weevil » Monday, 04 November 2013, 8:13 AM

'Doc wrote:Oooo... I'l bet those toilet paper filled cans could come in handy for more than one reason. Wish I'd'a thought of that.
I tried a few other things, but the reception was crappy.


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Re: SWR and ground issues

Post by str8stroke » Tuesday, 05 November 2013, 3:07 AM

Ur toilet paper mod gave me a idea. I think I will use some old packing foam to do the same thing. Cool idea.


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Re: SWR and ground issues

Post by Boll Weevil » Tuesday, 05 November 2013, 5:29 AM

'Doc wrote:From reading #3 it sounds like the radio is being grounded through the shield of the feed line and the mount of the antenna, that's for 12vdc ground. With that 12vdc ground cable connected and the antenna disconnected, does the radio still power on?
- 'Doc
I hope this answers your question. I just discovered this today.
The ground at the power port is good. The radio will power on with the coax AND/OR the ground lead connected. The only time it will power off is if BOTH are disconnected.

The design of the truck can be ruled out as we have 3 more of these that rolled off the assembly line within sight of one another. These issues are limited to one truck.

Next episode: try another radio. I used my radio to compare my SWR to that of the radio installed in the truck and had similar results. The power/ground issue mentioned above is a new discovery so we'll see what another radio will do under the same circumstances.

Tuesday, 05 November 2013, 7:31 AM
str8stroke wrote:Ur toilet paper mod gave me a idea. I think I will use some old packing foam to do the same thing. Cool idea.
Now if I could only get the string to quit breaking hittin' me upside the head.

Wednesday, 06 November 2013, 18:18 PM

Today we found out that if the radio is on, with power and ground being provided by the posts, there is a spark when trying to connect the antenna. We then tried a different radio and this did not happen.

Something loose, broken, or shorted inside the radio at the coax connector is my guess. Any othe ideas?

Wednesday, 06 November 2013, 18:26 PM

Today we found out that if the radio is on, with power and ground being provided by the posts, there is a spark when trying to connect the antenna. We then tried a different radio and this did not happen.

Something loose, broken, or shorted inside the radio at the coax connector is my guess. Any othe ideas?

Wednesday, 06 November 2013, 18:27 PM

Today we found out that if the radio is on, with power and ground being provided by the posts, there is a spark when trying to connect the antenna. We then tried a different radio and this did not happen.

Something loose, broken, or shorted inside the radio at the coax connector is my guess. Any othe ideas?

Wednesday, 06 November 2013, 18:29 PM

So, that's where my replies went, but I didn't hit "quote". :confused:


Jim

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