Tuning CB Antenna (need longer antenna?) - BMW Luxury Touring Community
 
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post #1 of 14 Old Apr 18th, 2009, 8:22 am Thread Starter
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Question Tuning CB Antenna (need longer antenna?)

OK, this is probably a really dumb question, but here goes.

Installed my stock BMW antenna and have a handlebar mount J&M CB (JMCB-2003-SL) Grounded everything really well (probably too good, if there is a such thing).
Took my SWR readings last night in an attempt to get it all tuned. My readings were as follows;

Channel 1 = 1.8
Channel 40 = 1.3

Because the reading on channel 40 is lower than channel 1, this would suggest that I need to lengthen the antenna. I've pulled it out (of the base) as far it can go, and still allow the set screws to bite. Given this, I'm assuming that in order to get a better SWR reading, I'm just going to need a new (longer) whip. Can anybody confirm?

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post #2 of 14 Old Apr 18th, 2009, 9:18 am
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Re: Tuning CB Antenna (need longer antenna?)

I'm not a CB expert, but I do know a fair amount about antennas. First of all, what is the SWR readings at 10, 20 and 30? With those, you can plot a rough curve. You can move the low point of that curve by altering the length of the antenna. With only two numbers at the band edges, you don't really have enough data, but its likely that you are in fact, a bit short.

Now let me ask you this... do you really care about a 1.8:1 SWR? Yes, keeping it below 1.5 is better, but that's not a horrible thing, especially when you're talking about an AM transceiver with 4 watts. I don't think the people you're talking to can hear the difference and I'm sure you can't hear it.
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post #3 of 14 Old Apr 18th, 2009, 9:55 am
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Re: Tuning CB Antenna (need longer antenna?)

There is some interesting reading here:
http://www.firestik.com/Tech_Docs.htm

Mike Trevelino
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post #4 of 14 Old Apr 18th, 2009, 11:58 am
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Re: Tuning CB Antenna (need longer antenna?)

Another comment about this is about what you're trying to accomplish. Are you looking for a lower SWR to reduce losses or minimize reflected power back into the radio? The losses from a 1.8 SWR are nothing compared to what losses you're getting from the antenna. Unless you're antenna is 8.6 feet long with a good ground plane, you're effective radiated power (ERP) is already in the microwatt range anyway. There is no escaping that matter of physics and lowering from 1.8:1 down to 1:1 isn't going to change that in any dramatic manner.
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post #5 of 14 Old Apr 19th, 2009, 1:22 am
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Re: Tuning CB Antenna (need longer antenna?)

234 divided my the frequency in mHz give you the length for a quarter wave antenna.

(there are some other things going on, like the K factor..but this will get you close.)

A quarter wave antenna needs a matching length ground radial to work.

Shorter antennas that show low SWR readings are just coils of wire that act like resonant antennas when the are really just coils of wire with a CAP to ground to make them appear to "tune."

I know, there are zillions of CB antenna sold that are advertised as 1/4, 1/2, 5/8 and so on that are shorter than the 8.61 foot antenna that a CB radio needs to work. These antennas are at best wishful thinking, and just plain dishonest hoaxes at worst. They are nothing but short dummy loads in most cases.

(Most dummy loads show really good SWR readings.)

It is really hard to get a good RF ground with a CB antenna on a car and I can't see how it would ever work with good efficiency on a bike.

But if you just intend to communicate a half mile or so, with an AM signal, I suppose that radiated 500 milliwatts might work.
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post #6 of 14 Old Apr 19th, 2009, 11:19 am Thread Starter
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Re: Tuning CB Antenna (need longer antenna?)

Thanks for the responses fellas. I'm no expert (obviously) and was just trying to tune the antenna for the best SWR as advised by many sites on the internet outlining "how to tune your cb antenna". I've tested the radio, and it seems to be working fine for what I need.

However, I have a squeal when I transmit, and only when the bike is running. Funny thing about this squeal is, if I turn my head (helmet) sideways (either way) whilst transmitting, the squeal disappears. Likewise, if I just lean way back in my seat whilst transmitting it also goes away...as it does when I just put my arm or hand near the antenna. Ironically, the squeal is there when I'm seated in the normal driving position, and it's worse on channel 1 then it is on channel 40. The radio works, but this is a bit frustrating. Any ideas?

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post #7 of 14 Old Apr 19th, 2009, 11:38 am
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Thumbs up Re: Tuning CB Antenna (need longer antenna?)

buy the 49 " wilson whip from clays radio shop and just put it in the stock bmw anteana holder and the swr will drop ti 1.1 and the squeal which is RFI will go away. You can listen to all the theroy stuff all day long, but the proof is in the pudding, The squeal is very common. Don't make it harder than it is. Be carefull trying to make the existing whip longer by moving it farthur out in the holder, In my attempts to do that, you can lose the actual connection between the whip and the cable intermmitently and hurt the transmitter.
I have fixed this problem many times at the dealership. It will cost you 12.95. for the whip
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post #8 of 14 Old Apr 19th, 2009, 12:44 pm
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Re: Tuning CB Antenna (need longer antenna?)

Talking "theory" may not cure your problems, but attaching a dummy load will not do it either. That "theory" happens to be simple physics and it seems to hold up pretty well.

The point that BarryG and myself were making is that a 1:1 SWR isn't the ultimate goal. Radiating a signal is. Getting a 1:1 SWR is very easy. All it requires is a simple dummy load. That will also cure your RFI problems. Of course, you can't be heard more than a few feet away, nor can you hear anything, but hey... your SWR is nice and low.

There are two problems that Toolman has described. One is not really a problem and that's a 1.8:1 SWR. The other is an RFI problem. RFI can come from a number of things, but a 1.8:1 SWR isn't one of them. It could be a ground issue. It could be wiring, bad insulation, bad connection, the antenna interacting with other nearby metal objects and a variety of other things. Replacing the antenna might help, but only if the original has defective and didn't have a good conntection at some point.

From the description, it sounds like its tied to the ignition system or alternator. Since it doesn't happen when the bike is not running, that's a pretty good clue. You could always install a dummy load and get rid of it, but I'd suggest looking to other solutions.
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post #9 of 14 Old Apr 19th, 2009, 5:43 pm Thread Starter
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Re: Tuning CB Antenna (need longer antenna?)

Again thanks. I might just get that 49" Wilson whip and try it out. I've looked on Clay's Radio Shop wesite, and yikes, the shipping costs more that the whip. Whip is $11.95, but shipping is $13.00. Might see if I can source it locally.

A couple of responses have indicated that my SWR is 1.8:1. How is this calculated when my SWR on channel 1 is 1.8 and my SWR on channel 40 is 1.3? Does the 1.8:1 simply mean that I have a reading of 1.8 on channel 1? Sorry for the dumb question....but I'm always intertested in learning more.

Bottom line is, my radio does seem to work OK. I tested it with a friend who was about 1/4 mile away. As the primary purpose of this radio is to communciate with my riding group, I think it'll be fine. I would, however, like to eliminate the squeal, or RFI.

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Last edited by Toolman; Apr 19th, 2009 at 6:08 pm.
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post #10 of 14 Old Apr 20th, 2009, 10:03 am
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Re: Tuning CB Antenna (need longer antenna?)

Quote:
Originally Posted by Toolman

A couple of responses have indicated that my SWR is 1.8:1. How is this calculated when my SWR on channel 1 is 1.8 and my SWR on channel 40 is 1.3? Does the 1.8:1 simply mean that I have a reading of 1.8 on channel 1? Sorry for the dumb question....but I'm always intertested in learning more.
SWR means "standing wave ratio" and its expressed as a ratio. You indicated that your's is 1.8 which is really 1.8 to 1, or simply 1.8:1.

Its a ratio of the impedence between the radio and the antenna. If the radio is 50 ohms and the antenna is 100 ohms, you have a 2:1 SWR.
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post #11 of 14 Old Apr 20th, 2009, 11:45 am Thread Starter
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Re: Tuning CB Antenna (need longer antenna?)

Makes perfect sense. Thanks for the ratio explanation.

For what it's worth, I managed to find a 42" Wilson whip at a local CB radio shop (in a Truck Stop) for $12, so I'll be going that route instead of getting it from Clay's Radio Shop. I'll report back when I've had a chance to test it out with the new whip.

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post #12 of 14 Old Apr 20th, 2009, 8:31 pm Thread Starter
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Re: Tuning CB Antenna (need longer antenna?)

I'm absolutely thrilled to report that it seems like the 42" whip did the trick. I haven't taken new SWR readings with it yet, but I can transmit successfully (my friend says it sounds perfect), and no more RFI noise. Can't thank you enough rocketman007.
Many thanks,
Toolman

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post #13 of 14 Old Jun 14th, 2010, 6:20 pm
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Re: Tuning CB Antenna (need longer antenna?)

Quote:
Originally Posted by Toolman
I'm absolutely thrilled to report that it seems like the 42" whip did the trick. I haven't taken new SWR readings with it yet, but I can transmit successfully (my friend says it sounds perfect), and no more RFI noise. Can't thank you enough rocketman007.
Many thanks,
Toolman
I want to confirm that you got a 42" whip? I went to the Clay's Radio site and they list 36", 49" and longer whips, but no 42.

Also, the antenna appears to be black in color with a top that looks somewhat like the stock one I bought from BMW. Can you comment as to how well the Wilson whip looks as compared to your AM/FM radio antenna, other that being longer obviously.

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post #14 of 14 Old Jun 27th, 2010, 8:24 pm
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Re: Tuning CB Antenna (need longer antenna?)

Quote:
Originally Posted by Toolman
Makes perfect sense. Thanks for the ratio explanation.

For what it's worth, I managed to find a 42" Wilson whip at a local CB radio shop (in a Truck Stop) for $12, so I'll be going that route instead of getting it from Clay's Radio Shop. I'll report back when I've had a chance to test it out with the new whip.
I'm on a short road trip to take my daughter for her college orientation day. We had supper at a truck stop restaurant so just for grins I wandered into the store attached to the restaurant. They had the 49" whips on the rack and for only $15. This is only slightly higher than Clay's price and even with tax was still much less than Clay's price plus shipping cost. So, I now have one to try out on my LT.

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