stubby antenna - BMW Luxury Touring Community
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post #1 of 18 Old Jan 11th, 2019, 3:59 pm Thread Starter
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stubby antenna

Anyone have any suggestions for replacement of the stock antenna with a stubby online i have had mixed information?
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post #2 of 18 Old Jan 11th, 2019, 4:52 pm
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Re: stubby antenna

I bought a stubby from Amazon that seems to work as well as the original antenna. Search for CravenSpeed Stubby Antenna Replacement for BMW Motorcycles | 3.2 inches


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post #3 of 18 Old Jan 11th, 2019, 9:39 pm
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Re: stubby antenna

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Originally Posted by BMWK1200LT-AC7 View Post
Anyone have any suggestions for replacement of the stock antenna with a stubby online i have had mixed information?
An antenna that is shorter than the optimal half-wavelength dipole is going to suffer in reception. The shorter the antenna, the worse it gets. It is simply physics. Some antennas will add in an amplier to boost the signal. The problem is that this also boosts the noise, whereas a properly designed antenna will have a better SNR.

As long as you don’t mind losing reception, then the stubby is fine. Anyone who tells you their stubby performs as well as the stock antenna doesn’t know how to test an antenna. You really have to take both antennas away from the station until reception is lost to know how well they do. Many will test the antennas 10 miles from the station and say they both sound the same so they are performing equally well. That tells you nothing. All modern radios have AGC which will limit a too powerful signal. When you are close to a station, the AGC may be working hard to limit the strong signal. When you install the stubby antenna, the AGC will still let through the same amount of signal to control the level of output so that you will hear no difference.

The difference between the antennas will not be apparent until you get into an area of very weak signal where the AGC is no longer able to make the antennas perform equally and the stubby will fall off the cliff sooner. You can’t violate the laws of physics and antennas much smaller than the signal wavelength simply don’t capture as much power.
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post #4 of 18 Old Jan 11th, 2019, 10:33 pm
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Re: stubby antenna

Here is a fairly short and reasonably understandable video that tells you why a stubby antenna does not work well.


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post #5 of 18 Old Jan 12th, 2019, 4:15 am
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Re: stubby antenna

Quite right, there is no substitute for (resonant) length.

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post #6 of 18 Old Jan 12th, 2019, 6:28 am
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Re: stubby antenna

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Originally Posted by Scimitar View Post
Quite right, there is no substitute for (resonant) length.
If you ride in mostly urban areas or in flat land, the short antenna may still work just fine as you will have a lot of powerful stations to pick up. Here in mountainous rural PA we barely get reception with a great antenna and have no margin to lose. So, much depends on where you ride.

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post #7 of 18 Old Jan 12th, 2019, 6:56 am
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Re: stubby antenna

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If you ride in mostly urban areas or in flat land, the short antenna may still work just fine as you will have a lot of powerful stations to pick up.
Looks silly though! The full size aerial has benefits beside the best performance. To the uninitiated, it could be a comm's aerial on a covert Police 'bike for instance.

Best thing is a matching aerial on the other side to balance the look that could then be used for something useful like amateur radio. ;-)

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post #8 of 18 Old Jan 12th, 2019, 7:10 am
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Re: stubby antenna

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Originally Posted by Scimitar View Post
Looks silly though! The full size aerial has benefits beside the best performance. To the uninitiated, it could be a comm's aerial on a covert Police 'bike for instance.

Best thing is a matching aerial on the other side to balance the look that could then be used for something useful like amateur radio. ;-)
I added the matching antenna shortly after buying my LT. it is used for CB, so nothing terribly useful.

I got my technician class amateur license a few years ago, but haven’t really gotten into the game yet. I am hoping to do that this year. The CB antenna can probably be modified to work with a 2M rig, but I haven’t looking into the details yet.

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post #9 of 18 Old Jan 12th, 2019, 7:54 am
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Re: stubby antenna

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I got my technician class amateur license a few years ago, but haven’t really gotten into the game yet. I am hoping to do that this year. The CB antenna can probably be modified to work with a 2M rig, but I haven’t looking into the details yet.
I may be able to help. I have got a Diamond 770 (I think) PL259 fitting dual bander on the CB aerial mount bracket. I have successfully used a Yaesu VX-7R on my last bike. This newer bike has the same aerial and CB bracket fitted in a better way and a Yaesu FT-7900R unit fitted. Due to the lack of water resistance I have now swapped it out for a Yaesu FT-10SR. Currently fighting with ground loops and alternator noise on transmit.

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post #10 of 18 Old Jan 12th, 2019, 8:26 am
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Re: stubby antenna

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Originally Posted by Scimitar View Post
I may be able to help. I have got a Diamond 770 (I think) PL259 fitting dual bander on the CB aerial mount bracket. I have successfully used a Yaesu VX-7R on my last bike. This newer bike has the same aerial and CB bracket fitted in a better way and a Yaesu FT-7900R unit fitted. Due to the lack of water resistance I have now swapped it out for a Yaesu FT-10SR. Currently fighting with ground loops and alternator noise on transmit.
I hate to hijack this thread. Maybe we should start one for ham on an LT. Or search to see if there are any old ones that might need some fresh action.

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post #11 of 18 Old Jan 12th, 2019, 11:18 am
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Re: stubby antenna

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I hate to hijack this thread. Maybe we should start one for ham on an LT. Or search to see if there are any old ones that might need some fresh action.
There are, which is how I got the nicer aerial mounting ideas.

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post #12 of 18 Old Jan 15th, 2019, 2:38 pm Thread Starter
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Re: stubby antenna

Thanks all for the feedback, i will have to stick it out with the current antenna
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post #13 of 18 Old Jan 15th, 2019, 3:43 pm
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Re: stubby antenna

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Originally Posted by BMWK1200LT-AC7 View Post
Thanks all for the feedback, i will have to stick it out with the current antenna
Or stick out the current one.

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post #14 of 18 Old Jan 16th, 2019, 6:16 am
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Re: stubby antenna

Hi All,
right now in the middle of checking as much as possible whilst the bike is in bits and have also been looking at the antenna part, being mentioned here and there by owners. Some attention to this as I've seen a lot of comments on poor reception for the radio part (besides a few comments on the stereo unit itself... :-) ) and tried to understand more about this. Is there any special reason that the original antenna set up is less good when it comes to signal reception quality?
Not having had any real "user time" to have an opinion yet due to too few miles on the bike as it's new to me but just as a precaution I checked up all connections from the stereo unit all the way back to the "screw-on" antenna itself and after cleaning/clearing contacts and measuring again (did a "before"-test to) for resistance it all seemed at least better than before and seemed OK, Ohm-wise, comparable to a normal car/bike antenna... Not overdoing it but a good start must be to ensure good contacts, especially in the bottom contact where the antenna "rod" is attached and here was also where I found some "stuff" that was cleaned out.

Any ideas around this? Will try to remember to revisit this one once I get some miles on the bike and have some more to refer to...

Only a few months until next riding season...!!! :-)

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post #15 of 18 Old Jan 16th, 2019, 7:54 am
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Re: stubby antenna

Great idea. Every time I’ve walked past my K1200LT parked in the garage since 2003, I bump the original mast. I’ve ordered one from Amazon...Can’t wait to try it while waiting for the snow to go away. Thanks for the tip.
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post #16 of 18 Old Jan 16th, 2019, 8:50 am
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Re: stubby antenna

Quote:
Originally Posted by Bigswed1962 View Post
Hi All,
right now in the middle of checking as much as possible whilst the bike is in bits and have also been looking at the antenna part, being mentioned here and there by owners. Some attention to this as I've seen a lot of comments on poor reception for the radio part (besides a few comments on the stereo unit itself... :-) ) and tried to understand more about this. Is there any special reason that the original antenna set up is less good when it comes to signal reception quality?
Not having had any real "user time" to have an opinion yet due to too few miles on the bike as it's new to me but just as a precaution I checked up all connections from the stereo unit all the way back to the "screw-on" antenna itself and after cleaning/clearing contacts and measuring again (did a "before"-test to) for resistance it all seemed at least better than before and seemed OK, Ohm-wise, comparable to a normal car/bike antenna... Not overdoing it but a good start must be to ensure good contacts, especially in the bottom contact where the antenna "rod" is attached and here was also where I found some "stuff" that was cleaned out.

Any ideas around this? Will try to remember to revisit this one once I get some miles on the bike and have some more to refer to...

Only a few months until next riding season...!!! :-)
What do you mean by “less good?” Less good as compared to what? An antenna on a car?

If you are comparing to a car, the answer is very easy: ground plane. Try to find the ground plane on a motorcycle.
https://www.electronics-notes.com/ar...ory-design.php

Most cars have lots of fairly flat metal around the antenna which helps a lot. Now many newer cars have antennas built into the rear window or other such hidden place, but almost all of them perform much more poorly than a conventional whip antenna.

For example, my 06 Sonata has a hidden antenna. It’s radio reception is barely half as good distance-wise as my Chevy truck which has a good old whip antenna mounted beside a big, flat steel hood. I can pick up the same station nearly twice as many miles away than in my Sonata. Given that radio waves diminish with an inverse distance squared function, that means that my truck antenna is picking up about 4 times the signal as compared to my car hidden antenna.

If you replace your stock BMW antenna with a stubby antenna, you will likely have about the same result. You will probably cut your radio reception range about in half and motorcycle antennas aren’t that good to start with. A better solution is probably to just get satellite radio where the wavelengths are so short that a short antenna is the right sized antenna.
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post #17 of 18 Old Jan 16th, 2019, 9:12 am
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Re: stubby antenna

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What do you mean by “less good?” Less good as compared to what? An antenna on a car?

If you are comparing to a car, the answer is very easy: ground plane. Try to find the ground plane on a motorcycle.
It's right where the aerial is mounted! Assuming that the factory whip is a loaded quarter wave, the "missing" counterpoise forms an image in the ground plane. I would have thought that the frame of the 'bike would allow a decent enough image to be formed at VHF? However using Medium and Long Wave bands would be a whole new kettle of worms of course but who still uses those bands for entertainment?

Quote:
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Most cars have lots of fairly flat metal around the antenna which helps a lot. Now many newer cars have antennas built into the rear window or other such hidden place, but almost all of them perform much more poorly than a conventional whip antenna.
Agreed. Inadequate ground plane which is more like screening in fact and non resonant aerial section for any broadcasting band!


Quote:
Originally Posted by Voyager View Post
If you replace your stock BMW antenna with a stubby antenna, you will likely have about the same result. You will probably cut your radio reception range about in half and motorcycle antennas aren’t that good to start with. A better solution is probably to just get satellite radio where the wavelengths are so short that a short antenna is the right sized antenna.
There is that, but the all round best solution is to service the factory aerial and learn to stop trying to stick it up your nose!

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post #18 of 18 Old Jan 16th, 2019, 12:46 pm
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Re: stubby antenna

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Originally Posted by Scimitar View Post
It's right where the aerial is mounted! Assuming that the factory whip is a loaded quarter wave, the "missing" counterpoise forms an image in the ground plane. I would have thought that the frame of the 'bike would allow a decent enough image to be formed at VHF? However using Medium and Long Wave bands would be a whole new kettle of worms of course but who still uses those bands for entertainment?



Agreed. Inadequate ground plane which is more like screening in fact and non resonant aerial section for any broadcasting band!




There is that, but the all round best solution is to service the factory aerial and learn to stop trying to stick it up your nose!
The problem is that the ground plane or counterpoise greatly affects the antenna radiation pattern. Since the LT antenna effectively has no counterpoise to the “outside” and rear of the antenna, only maybe 1/3 the antenna length of counterpoise to the “inside” of the antenna, and MAYBE a sufficiently long counterpoise to the front, although keep in mind that the metal subframe is not continuous to the front of the bike, the pattern will be very skewed towards the front of the bike. The problem here is that you now have one or two mostly water filled masses (aka rider and passenger) between the antenna and its counterpoise which doesn’t help.

So, no matter how you look at it, for wavelengths in the FM band, a motorcycle is not a good location for an antenna. Unless you mount a large circular metal sheet above your bike and put the antenna in the middle of it.

https://www.usna.edu/EE/ee434/Handou...ndamentals.pdf
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