CB Antenna observation... - BMW Luxury Touring Community
 
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post #1 of 9 Old Jan 13th, 2013, 7:18 am Thread Starter
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CB Antenna observation...

I bought a used OEM CB mount from a member here (thanks angelo!). I ordered the OEM coax. Yesterday I assembled everything, and to make a long story a little shorter...it didn't work well. I went to trouble shooting and discovered that the coil at the base of the antenna that threads into the mount, is not isolated electronically from the mount. A continuity test of the coil proved that there is no isolation from antenna orifice to the mounting threads.

It is my experience that the coax core, that feeds the antenna, should be insulated and separate from the braided outer portion of the coax. With "this" setup, the signal from the core wire is fed right back to ground through the base load coil. Not good for the radio. Either the coil is bad (shouldn't conduct back to ground) or there is a non conducting piece missing between the coil and the mount. I did notice that the OEM coax core that feeds up through the antenna mount had extra length that would accommodate an extra piece. I looked on the parts fiche and didn't see anything in addition to what I have, so......?
Am I missing something?

FS

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post #2 of 9 Old Jan 13th, 2013, 9:52 am
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Re: CB Antenna observation...

Using a multimeter that reads a DC short on an antenna is not conclusive there is a problem. You need to use an SWR meter to determine the antenna is faulty. Many have a loading coil that is grounded on one end and connected to the radiating element on the other. This "coil" looks like a dead short to DC (ohm meter) but is higher impeadance to an RF signal.

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post #3 of 9 Old Jan 13th, 2013, 10:29 am
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Re: CB Antenna observation...

I see that you order the cable. Did the mount you got have the rubber grommet? It shows as "cable grommet[" on the parts fische. This piece has two fingers that keeps the ground part of the cable from touching the mount when you push the cable through. I found that it is better to put the grommet in and then push the cable thru.

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post #4 of 9 Old Jan 13th, 2013, 12:18 pm
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Re: CB Antenna observation...

There is a non-conductive piece between the CB antenna and the bracket. It prevents the antenna from being grounded. I replaced my BMW antennas with Honda antennas that fold down and used this piece to mount the Honda antennas. The BMW antenna screws into this piece and this piece then screws into the bracket. Since it is not shown with the bracket, it should be part of the antenna. I cannot find this piece in any BMW fiche. Anybody find it?
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post #5 of 9 Old Jan 13th, 2013, 1:33 pm Thread Starter
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Re: CB Antenna observation...

Well...due to the available time that I had to work on this problem I decided to engineer what I thought to be the way that this setup "should be" wired. Went to the hardware store and bought a pipe fitting (male on one end, female on the other) that was approximately sized to the threaded hole in the mount, and the female end sized as close as possible to a conventional CB insulated lug. I tapped the mount to the same size, and thread count as the pipe fitting. I then cut the collar nut off of a conventional CB coax cable fitting. I ground the remaining center post (that pushes into the base of the antenna mount) so that it would fit down inside of the pipe fitting. I then took a conventional antenna fitting that accommodates the male portion of the coax and threaded it down into the pipe fitting and onto the male post. Works perfect! Had the wife go down the street with the hand held...loud and clear. Unsquelched..I can hear skip from far away. Problem solved.

Thanks for the replys and suggestions,

FS

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post #6 of 9 Old Jan 13th, 2013, 2:36 pm
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Re: CB Antenna observation...

sounds like u did a great job. u didn't mention check ring the swr tho. my experience has been that mobile cab antennas typically have the inductor, coil, at the base or mid point of the antenna for 1/4 wave antennas without a tap on the. inductor to ground. not sure about 5/8 wavelength antennas
bob

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post #7 of 9 Old Jan 13th, 2013, 6:25 pm
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Re: CB Antenna observation...


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post #8 of 9 Old Jan 13th, 2013, 11:20 pm Thread Starter
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Re: CB Antenna observation...

Quote:
Originally Posted by BecketMa
sounds like u did a great job. u didn't mention check ring the swr tho. my experience has been that mobile cab antennas typically have the inductor, coil, at the base or mid point of the antenna for 1/4 wave antennas without a tap on the. inductor to ground. not sure about 5/8 wavelength antennas
bob
I stole the fiber glass J&M antenna off of the wife's bike. Matches the am/fm antenna on the other side. Even was able to use the cone shaped cover at the antenna base. Looks like it could have been OEM.

Gotta get the wife a replacement today at the truck stop.

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post #9 of 9 Old Jan 13th, 2013, 11:23 pm Thread Starter
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Re: CB Antenna observation...

Quote:
Originally Posted by dfinazzo
I should have taken pics as I went along. I was winging it and wasn't sure how it would turn out. However...I'll post some pics of the finished product and explain what you're seeing. Might be a couple of days.

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