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post #1 of 16 Old Jul 8th, 2009, 1:45 am Thread Starter
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BMW CB Antenna Tuning

I just tried my hand at tuning the SWR of the BMW CB antenna freshly installed on my '09 R12RT, along with the BMW version of the J&M handlebar CB/intercom. I'm looking to compare notes & results from those that have blazed the trail before me.

On the low power setting, the Radio Shack SWR meter is showing SWR numbers of ~1.5 on Ch. 1, ~ 1.9 on Ch. 19, and ~3 on channel 40 (suggesting to me that the antenna is still a bit long).

On the high power setting, the meter instantly pegs on Ch. 1 and Ch. 40.

I realize that this is a no ground plane antenna, and even after tuning, I should not expect the antenna's performance to be great. Nonetheless, I was curious to see whether others would care to share their tuning results, particularly at high power.

Being inexperienced in this (the last CB I installed was in 1975, when they only had 23 channels!), I didn't realize until it was too late that I had started tuning the antenna (i.e., cutting off short pieces of the coiled wire to shorten the antenna) while in the low power mode -- not sure whether I should have started in high power mode, and sorta wondering whether (i) I'm in danger of burning up the CB's transmitter when in high power mode; and (ii) I'll need to go buy another antenna mast to start over.

Thanks for any thoughts!

Mark Neblett
Fairfax, VA
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post #2 of 16 Old Jul 8th, 2009, 6:56 am
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Re: BMW CB Antenna Tuning

I've got a date Friday with the CB Shop to check my CB out. Doesn't squeal when in LP but does in HP and, I believe, the ant. was tuned in LP. 18' of coax is laid out under the rear seat; there's gotta be a better way.

I'll have to strip her down for the tech to get at the xmit end of the cable; I'll post the findings here.

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post #3 of 16 Old Jul 8th, 2009, 8:39 am
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Re: BMW CB Antenna Tuning

I have a J&M CB...the JMCB-2003-SL (solo unit) and have tuned my antenna. I started with the the stock BMW CB antenna for the K1200 LT. My initial SWR numbers were higher on the lower channels(1), versus the high channels (40), which indicated that I needed a longer mast. I was also getting some major squeel during transmit. At the advice of a board member here, I got a longer stainless steel mast (made by Wilson, and I think it was about 42" or so) for $12. Initial SWR readings with the longer mast confirmed that it was indeed long enough (as evidenced by higher readings on channel 40, versus channel 1).

For what it's worth...a perfect SWR reading would 1:1...meaning a reading of 1.0 on channel 1 and channel 40. While you can get close, it's almost impossible to achieve perfection (1:1) on our motorcycles without using a very long (about 8') mast. A reading of 1.0 means that 100% of your signal is being transmitted, while a reading of 3.0 means that 1/3rd of your signal is being tranmitted (with the other 2/3rds being fed back into the radio). Any readings below 2 are pretty good....while readings above 3 are NOT good, and in fact, can be damaging to your radio.

Things to note before you start tuning;
1) Make sure that the antenna base is sufficiently grounded. Just take a resistance reading between the antenna base, and your bike's negative batt terminal. Should be no more that about 3 or 4 ohms. If needed, either install additional ground wires (frame/neg terminal to the antenna base(bolts), or improve the ground connections you've got by cleaning, scraping off paint where the base attaches, etc. No such thing as too good of a ground connection.
2) At the end of your antenna cable, take a resistance reading between the center lead, and the outer lead at the coax connector (aka PL-259 connector). There should be no conductivity whatsoever (or infinite resistance...no short). If there is conductivity....you'll need to find out why. Possibly a bad/kinked/cut antenna cable, or possibly a poor/sloppy connection at the connector or antenna base (like maybe a couple of strands of wire from the outer layer of the anteanna cable, is contacting the inner lead.
3) SWR readings and antenna tuning should always be done with the CB set to high power.
4) Make absolutely sure that you have the SWR meter hooked up properly...both the the CB itself, and the antenna. I made a mistake once and hooked up my SWR meter to the CB only (and forgot the antenna side). The result was a full-scale (pegged) SWR meter every time I keyed up the mic. I soon realized this...and hooked it up properly.
5) Do not perform antenna tuning inside of a garage or near a building. You need to be out in the open...and no closer than about 20' to any buildings.
6) Ensure that your SWR meter is calibrated properly before recording any readings.
7) Determine your initial/starting point by checking the readings on channel 1 versus channel 40. If readigns are higher on channel 1, your mast is too short. If readings are higher on channel 40, your mast is too long.
8) I used bolt cutters to trim my stainless steel mast....a very quick and easy way to cut off small lengths.
9) When trimming....only trim a little bit at a time. You'll be surprised at the amount of SWR change with removal of small (1/4" lengths). At first...it'll seem like no change....but then when you get close the proper length (1:1 SWR)....small amountswill make big step changes. In short...it's best to take your time here. You can always cut it shorter...but once cut....there's no going back!!
10) Be sure to re-calibrated the SWR meter each time you trim the anteanna.

Have fun,
Toolman

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Last edited by Toolman; Jul 8th, 2009 at 8:51 am.
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post #4 of 16 Old Jul 8th, 2009, 9:54 am Thread Starter
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Re: BMW CB Antenna Tuning

Wow! Great post -- many thanks! Let's see how many rules I violated :

Quote:
Originally Posted by Toolman
Things to note before you start tuning;
1) Make sure that the antenna base is sufficiently grounded. Just take a resistance reading between the antenna base, and your bike's negative batt terminal. Should be no more that about 3 or 4 ohms. If needed, either install additional ground wires (frame/neg terminal to the antenna base(bolts), or improve the ground connections you've got by cleaning, scraping off paint where the base attaches, etc. No such thing as too good of a ground connection.
This one is ok -- checked with ohmmeter.

Quote:
Originally Posted by Toolman
2) At the end of your antenna cable, take a resistance reading between the center lead, and the outer lead at the coax connector (aka PL-259 connector). There should be no conductivity whatsoever (or infinite resistance...no short). If there is conductivity....you'll need to find out why. Possibly a bad/kinked/cut antenna cable, or possibly a poor/sloppy connection at the connector or antenna base (like maybe a couple of strands of wire from the outer layer of the anteanna cable, is contacting the inner lead.
Ditto.

Quote:
Originally Posted by Toolman
3) SWR readings and antenna tuning should always be done with the CB set to high power.
First mistake! While I have ~1.5 SWR on Ch. 1, that's at low power -- I suspect that I have now cut the antenna too short, as SWR on high power is quite high. If that is true, the question is, do I replace the mast with another BMW mast, or go to something like a Firestik?

Quote:
Originally Posted by Toolman
4) Make absolutely sure that you have the SWR meter hooked up properly...both the the CB itself, and the antenna. I made a mistake once and hooked up my SWR meter to the CB only (and forgot the antenna side). The result was a full-scale (pegged) SWR meter every time I keyed up the mic. I soon realized this...and hooked it up properly.
No problem here.

Quote:
Originally Posted by Toolman
5) Do not perform antenna tuning inside of a garage or near a building. You need to be out in the open...and no closer than about 20' to any buildings.
Another "oops" -- done in my garage

Quote:
Originally Posted by Toolman
6) Ensure that your SWR meter is calibrated properly before recording any readings.
Calibrated at the start, but not later -- more below.

Quote:
Originally Posted by Toolman
7) Determine your initial/starting point by checking the readings on channel 1 versus channel 40. If readigns are higher on channel 1, your mast is too short. If readings are higher on channel 40, your mast is too long.
Still lower on 1 than 40 -- on *low* power. Come to think of it, I didn't look to see the relationship between Ch. 1/40 on high power -- I'll do that tonight to confirm whether I've cut too much off the BMW antenna.

Quote:
Originally Posted by Toolman
8) I used bolt cutters to trim my stainless steel mast....a very quick and easy way to cut off small lengths.
The BMW antenna requires you to pull out the fine wire from the mast tip (i.e., uncoil from the top ~2"of wire coil next to the mast tip) and snip -- very easy to control, IF you know the point in the next rule!

Quote:
Originally Posted by Toolman
9) When trimming....only trim a little bit at a time. You'll be surprised at the amount of SWR change with removal of small (1/4" lengths). At first...it'll seem like no change....but then when you get close the proper length (1:1 SWR)....small amountswill make big step changes. In short...it's best to take your time here. You can always cut it shorter...but once cut....there's no going back!!
You're not kidding! The BMW instructions say go with 2" cuts -- and never caution to go smaller as SWR gets closer. I was cutting ~1-1.5" at a time, and stopped when SWR slightly rose with the last cut (again, on low power). Thanks to your advice, I now know to go much slower as I near optimum.

Quote:
Originally Posted by Toolman
10) Be sure to re-calibrated the SWR meter each time you trim the anteanna.
Didn't know this! That explains why about 1/2 way through the process I found the calibration to be off, but didn't think I'd ever bumped the cal. dial -- apparently, I didn't. This will now be part of every check step when I go back in.


So here's the big question: Anyone care to take a guess as to whether I've already trimmed too much off ?? I ended up removing ~12" of wire from the mast tip before I stopped tuning on the low power setting. Can someone give me an idea how much is "typically" removed from the BMW mast wire when the antenna is properly tuned at the highpower setting? 3"? 12"? 24"? Just trying to see whether I have a snowball's chance of salvaging the original antenna mast, or should be calling a replacement now!

Again, Toolman, a BIG thanks for the detailed response!

Mark Neblett
Fairfax, VA
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post #5 of 16 Old Jul 8th, 2009, 10:42 am
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Re: BMW CB Antenna Tuning

Quote:
Originally Posted by mneblett
Let's see how many rules I violated :
Just think about your vocation Mark, then you'll know how many you violated.

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post #6 of 16 Old Jul 8th, 2009, 11:40 am Thread Starter
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Re: BMW CB Antenna Tuning

Quote:
Originally Posted by Steve_R
Just think about your vocation Mark, then you'll know how many you violated.
If we're in that sphere, the first thing I'd have to do is question what the definition of a "violation" is. Then I'll start taking depositions ...

Which is *exactly* why I'm staying away from the 2nd career, and trying to stay in enginerd mode!. If this were a mechanical issue, no problem, but RF transmission and antenna design are weak areas for me. Hence the plea to the collective to get edumacated on the subject.

All help/reports of experience with tuning the BMW antenna still welcome!

Mark Neblett
Fairfax, VA
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post #7 of 16 Old Jul 8th, 2009, 1:11 pm
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Re: BMW CB Antenna Tuning

Mark,
I installed the Firestik antenna on my LT. I used the FS-LG3-B antenna, with the FS-K-8NGP cable and FS-K-4 mount (coupler). This is a no ground plane antenna and cable and is mounted directly to the BMW CB antenna mount. I just had to get a longer screw from Home Depot due to the thick base of the antenna mount. I initially tried to tune the antenna in the garage and moved it outside in the driveway. My readings were:

With antenna adjuster screw all the way out (up)
Channel 1 = 2
Channel 19 = 3+
Channel 40 = 3+

With antenna adjuster screw all the way in (down)
Channel 1 = 2
Channel 19 = 1.4
Channel 40 = 2.5

I want to recheck it with my bike out in the street. I think that the channel 19 reading should be between channels 1 and 40. I have the squelch turned up some and am not getting much jabber from the truckers around the 395/495 junction, so I do not know what that problem is.

Mike Trevelino
Williamsburg, VA
2008 RT
2000 LT - Totaled at 99,960 miles


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post #8 of 16 Old Jul 8th, 2009, 1:27 pm Thread Starter
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Re: BMW CB Antenna Tuning

Thanks, Mike -- I may knock on your door sometime to check out the Firestik.

As for the SWR, my understanding is that if the 1 and 40 SWR readings are relatively close, the channels in between should be somewhat lower, so the drop at 19 in your case may not be unusual -- but take that with a grain or two of salt, seeing as I have already demonstrated that I do not know what I'm doing here!

Mark Neblett
Fairfax, VA
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post #9 of 16 Old Jul 8th, 2009, 10:25 pm Thread Starter
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UPDATE -- Re: BMW CB Antenna Tuning

This evening I had a few minutes to hook up the SWR meter again. After calibrating on high power, I was pleasantly surprised to find the SWR on channel 1 was ~1.5, and ~3.2 on ch. 40.

Clipped another 1/2" of wire, recalibrated the meter, and rechecked -- ch. 1 SWR ~ 1.3, ch. 40 SWR ~2.5, ch. 19 ~ 1.5.

One last short snip after another calibration: ch. 40 SWR ~2.0, Ch. 19 SWR ~1.3, Ch. 1 SWR ~1 (when the PTT is pushed, the SWR meter needle basically pops up for a split second to 1.05 and returns to ~1).

At this point I chickened out and put the meter away

So, what do you all think? The fact that SWR on ch. 1 is lower than ch. 40 sez (based only on what I've read) the antenna is still too long. On the other hand, I wonder whether the SWR of essentially 1 on ch. 1 is real -- i.e., a misleading false indication.

Here's where I'm completely at the end of my knowledge, and I'm looking for other folks' experience -- should I continue trimming, or just take what I have now and not go too far?

Thanks again for any help!

Mark Neblett
Fairfax, VA
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post #10 of 16 Old Jul 8th, 2009, 10:53 pm
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Re: BMW CB Antenna Tuning

You can probably go another 1/4" or so to fine tune things. One thing I forgot to mention is....if you know you'll be talking on one partiucular channel all the time, you could optionally tune the antenna to be optimized (reading of 1) on that channel. But if you're like me....you don't know what channel(s) you'll be using...so I just fine tuned it such that channel 1 and 40 were pretty close...and I managed to get 'em both down to about 1.1 each. Sounds like you've got it all figured out...and the radio should work just fine.

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post #11 of 16 Old Jul 9th, 2009, 5:19 am
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Re: BMW CB Antenna Tuning

I would leave it as is. You are down to 1.3 on channel 19. Basically you will be just listening to channel 19 for the trucker jatter. If you are talking to friends in a group, you will most likely be using a channel between 1 and 19, and you are showing even a lower swr.
On a different note, I did lower my squelch a little more and seem to be picking up more people talking. Previously, with the squelch set about to the mid point, I was still getting a lot of noise, thus having to turn it up more and hear less people further away.

Mike Trevelino
Williamsburg, VA
2008 RT
2000 LT - Totaled at 99,960 miles


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post #12 of 16 Old Sep 7th, 2009, 11:15 am
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Re: BMW CB Antenna Tuning

I am going to sound like a real newby here.

My dealer in Chicago installed the BMW system and BMW antenna on my R1200RT, but I bet he hasn't tuned the antenna because the transmit is useless.

Does anyone know where I can go to get the antenna tuned and someone to get rid of the squark on transmit?

Thanks
Kevin
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post #13 of 16 Old Sep 7th, 2009, 11:20 am Thread Starter
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Re: BMW CB Antenna Tuning

Quote:
Originally Posted by kdoddrell
I am going to sound like a real newby here.

My dealer in Chicago installed the BMW system and BMW antenna on my R1200RT, but I bet he hasn't tuned the antenna because the transmit is useless.

Does anyone know where I can go to get the antenna tuned and someone to get rid of the squark on transmit?

Thanks
Kevin
Two options come to mind: Pick up an SWR meter and tune the antenna yourself, or hit the local yellow pages for a CB dealer/installer in your area and ask whether they can tune a "no ground plane" antenna on a motorcycle (i.e., let them use their SWR meter to tune your antenna).

Mark Neblett
Fairfax, VA
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post #14 of 16 Old Sep 7th, 2009, 11:37 am
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Re: BMW CB Antenna Tuning

Mark,

I have a 12 foot obsolete antenna cable under my tank and seat. Would that interfere with the transmit, do you think?

Thanks
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post #15 of 16 Old Sep 9th, 2009, 6:47 pm
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Re: BMW CB Antenna Tuning

Well, I am going to try the easy way. The dealer says he can tune the BMW antenna, or failing that, put the firefox back on and tune that.

Thanks for the comments. I will let you know how it goes.

kevin
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post #16 of 16 Old Sep 22nd, 2009, 11:27 am
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Re: BMW CB Antenna Tuning

I'm always a bit baffled by some of the focus on SWR with CB antennas. These things only put out 4 watts of power and only as an AM mode. If you have an SWR of 2:1, you're really only putting out 2 watts. Does anybody think they could tell the difference between 2 watts and 4?

"But 4 watts is twice as powerful as 2 watts!" you may be tempted to say. Yeah, but do you know what that means on the receive end? You're down 3DB which is half an "S unit" which is almost nothing. On an AM transmission, there is no way the other guy could tell if you were at 2 or 4 watts. It would not increase your usable range either.

The only benefit to messing with the SWR is to make sure you don't have enough reflected power back into your radio to damage it. Get it below 3:1 through the whole band and you're fine.
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