BMW CB Antenna Discovery -- Range-- - BMW Luxury Touring Community
 
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post #1 of 4 Old Dec 4th, 2007, 9:54 am Thread Starter
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BMW CB Antenna Discovery -- Range--

If your LT is having limited range on your cb it may be due to what I found with my BMW antenna. The positive line in the antenna coax cable was not connected to the antenna. An antenna coax has an outer protective cover under which is a mesh shield (the cables negative or ground wire) then an insulated protected center wire which is the positive line that carriers the signal from the radio to the antenna.

There are two simple ways to check this assuming there are no crimps or breaks in the antenna cable.

1. If you have an ohm meter simply uncouple the antenna cable from the CB cable (should then have an exposed male plug coming from the antenna) Check the continuity of the antenna itself to the male center of the plug. If the needle does not move then the antenna cable positive line is not making contact with the antenna.

2. The second way to determine this is take the antenna base apart. This can be done at the base with out taking the mount off the bike. Slide the plastic cone cover up the antenna exposing the base, The antenna base has four set screws (small metric allen wrench is needed) Two set screws at the top hold the actual antenna in position i.e. where you fine tune the SWR. A third one is about half way down this is the one that connects the positive wire from the coax cable to the antenna. A fourth one is near the bottom and it holds a brass fitting in place thru which the cable extends up into to mount as well as grounds the antenna cable to the bottom of the base. You are interested in set screw number three. Loosen that set screw. Then just below that set screw the mount can be taken apart unscrewing the upper part from the base. Mine was hand tight. With is off (which is a non conductive hard plastic or something that keeps the antenna cable from grounding to itself) you should see a small wire insulated in clear or white cover. This is the positive wire that carriers the radio signal. Position the top part to see is this wire is long enough to be mashed by the set screw number 3. Mine was not. If not then the signal is not getting to your antenna.

Easy fix but you have the take the rest of the antenna base off, remove the top case etc. With the mount off remove the plate (4 screws) on the bottom. Loosen set screw number 4 the bottom one, The antenna cable should slide out. You will see a brass fitting thru which the cable goes and the small positive wire sticks out the other side. Take a pair of plyers hold the fitting and with the other hand pull the cable out of the fitting. You will see how the shield i.e. silver or copper mesh wire is folded back and was between the fitting and the outer casing of the coax. This is important when reassembling the fitting.

Being careful not to break the protective cover on the center positive wire strip the cable back far enough so the exposed positiive wire can be engaged by the set screw number 3. For me this was about 3/8 of an inch. It is important to leave enough excess of the shield intact so it can be folded back as before along the out side of the cable between it and the brass fitting. Force the cable back into the brass fitting as before. Slide it up into the base and secure with set screw number 4 as before. You may want to crimp this connection tight but do not crimp hard enough to damage the small center positive wire. If you have an ohm meter double check the continuity of the postive wire as well as check to be sure the ground (shield) and the positive wire are not touching. Do this by reassembling the bottom of the base and mount it on the bike as before. If you did not sand the area under the mount where the mount ground wires go this is a good time to do it. With the ohm meter check the small positive wire sticking up thru the base to the tip of the male plug of the antenna cable that connects to the radio antenna cable. Needle should show continuity. Then check the same little wire to the brass fitting and then to the set screw number 4. There should be no continuity at either of these. If this checks out the reasemble the rest of the antenna base. The little wire should extend far enough up the base so it is engaged by set screw number 3. Tighten set screw number 3. If the outer cover of the small wire is very hard you may want to shave it thinner to insure the set screw penatrates it for the connection. Now with it all back together and antenna secure with set screws 1 and 2 if you have an ohm meter recheck the continuity of the antenna itself to the tip of the male plug of the antenna cable. Needle should show continuity. Then check antenna to set screw number 4. There should be no continuity. From set screw number 4 to the ground wires connection at base or the other ends under the seat should show strong continuity. This is why the shield is between the outer casing and the brass fitting, it completes the grounding of the antenna cable.

Reconnect the antenna cable to the cb and proceed with normal SWR tuning.

I hope this is of help to others. I determined the positive wire was not long enough to connect to the antenna by accident after I managed to crimp the antenna cable causing a break inside of it the shorted the positive wire to the shield. SWR was way high. Prior to this I had very short range but could hear fine. No wonder as instead of and long metal antenna I had a short little stubb sticking up. (Having been dealing with a short little stubb for years but that is a whole nuther story and has nothing to do with a cb antenna.....)

Gene
'06 K1200LT
IBA 31499


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Last edited by GBaker; Dec 4th, 2007 at 1:03 pm.
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post #2 of 4 Old Dec 5th, 2007, 1:14 pm Thread Starter
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Wow

With 58 views and no response I must have been the only dummy to trust BMW's premade antenna wire and not check to see if the center wire was indeed long enough.

Either that or there is a lot of people busy checking theirs.....

Gene
'06 K1200LT
IBA 31499


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post #3 of 4 Old Dec 5th, 2007, 6:30 pm
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Before you connected the center conductor of the coax to the antenna, what was your SWR reading ??? It shoulda been pretty bad (lousy transmitter), but the CB could still have been an acceptable receiver.

- Bob

Cowboy Bob Menton
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Greenville SC USA
'07 K1200GT, '83 R80S (nee '83 R80RT)
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post #4 of 4 Old Dec 6th, 2007, 8:41 am Thread Starter
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As it happened I had an old SWR meter that turned out not wrking properly so I do not know what actually it was.

Now with a new meter it is 1.6 at ch 1; 1.4 at ch 20; and 1.2 at ch 40. that is with the antenna rod out as far as I dare re so both the set screws holding it have a grip on it. I may try to find one about 3/8 to 1/2 inch longer.

Dave, were you able to use the BMW mount with the FireStick. If so which antenna did you get?

Thanks

Gene
'06 K1200LT
IBA 31499


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