2007 BMW 328xi Power Hookup

[KD8KSN]

Well, I am not completely sold on the mag mount yet, so maybe I will look into a different antenna setup before installing a filter. Drilling a hole is not an option, but a lip mount might be. I have a feeling I might run into the same problem with one of those mounts as well.

Thanks for your help.

[K0BG]

Playing the Devil's advocate....

Your Bimmer is almost 7 model years old, and you don't want to drill a hole? Hum!

About five months ago, I drilled a hole, and mounted a Larsen NMO2/70BK in a brand-new, bright-red, BMW, M5, right in the middle of the trunk lid. Not one word was said, albeit the owner thought I would be using a clip mount. Well, they're just as bad as a mag mount, except they leave a wrinkle in the sheet metal, instead of leaving a moony in the paint.

[KD8KSN]

Still think I would take a wrinkle over a gaping hole, 7 years old or not.

[N9RAM]

Still think I would take a wrinkle over a gaping hole, 7 years old or not.

It's not a gaping hole once you fill it with an antenna. On my '07 Mazda3 I drilled the hole and used the exact Larsen mount and antenna K0BG referred to above and haven't looked back. If you're going to keep your 328 forever, what do you have to worry about? They never leak. That said, wiring to the roof in a recent-model BMW might be a pain in the neck, what with all the air bags and accessory wiring.

[K0BG]

My web site has been up since late April, 2004. In that time, I have no idea what my e-mail inbox count is. It has got to be in the mid 5 figures. There are about 20 reoccurring themes. They appear so often, I have actually thought of making up a few panned messages to save me a few hours per week. One of those reoccurring themes is the issue we're dealing with—an unbelievable aversion to drilling any sort of hole in the sheet metal of a vehicle, no matter the age! The aversion is supported by an equally verbose set of excuses.

Most assuredly, the most prevalent is the wife excuse! The first question I ask is, who paid for the vehicle? If she did (seldom the case), then you have a legitimate excuse. If you did, then you don't have a leg to stand on!

The lease issue is the second most used excuse. The fact is, leasing companies lease vehicles to all manner of commercial entities. While cellphones have replaced a lot of two-way mobile radio business, the big guys still use commercial radios for communications. If drilling a hole was a real issue, then everybody would be using cellphones. They don't!

Then there is the other side of the coin, which this post relates to—an RFI problem. The solution is simple: Drill a hole, and mount the NMO-based antenna correctly. No workarounds or patches (in-line, brute-force filters for example), but good, sound, engineering practices. But the pushback remains. While this stance rings of the Devil's advocate standpoint, it is, nonetheless, the true fix. It is not a patch or workaround!

Here's a photo of what clip mounts do. Tell me this: What is more expensive to repair? What is shown in the photo, or a hole? If you said hole, you don't know much about body work.


lipmount.JPG

[K7JEM]

One of these days, when you remove the mag mount, and see the moon that it has left, you'll get part of the picture.

I agree that a permanent NMO id the best solution, but for a very small antenna like that dual band antenna, a trunk lip mount would probably work fine. The problem with mag mounts is that they will damage the surface of the car, if left on for any length of time. A trunk lip mount can be installed so that no visible damage occurs, which seems to be the goal here.

As for the voltage drop. Most nominal 12 volt transceivers are designed to operate at 13.8 vdc. Drop that one full volt, and a 50 watt radio will drop to about 35 watts. You can get by with this on FM perhaps, but as the voltage goes down, the IMD goes up. At battery resting voltage (12.2 vdc) none of the popular mobile transceivers will me the FCC's -42dB standard.

IMD is not an issue with an FM transmitter. Most FM mobile radios will work fine down to 11v at the radio, they will suffer some power degradation at that point, however. The alternator filters can work well on radios of this power level, if normal grounding procedures do not work. I am not sure about the particular unit linked to, it may be good, or not. Hard to say. I have a friend that recently put one of the Powerwerx filters on his FT-1802 that he just couldn't get all the whine gone from. It fixed the problem instantly, but it does cost close to $30 instead of the $15.

My recommendation would be to get a trunk lip NMO mount, and an NMO short dual band antenna, like in the photo. Then add a filter, if normal grounding doesn't cure the whine. A good filter will have less than a volt of drop, and should not be an issue for the radio, with the exception that your power out might be a little less. On FM, that will never be noticed.

Joe

[K0BG]

I have an Icom IC-208. It is rated at 55/50, VHF/UHF at 13.8 vdc. It actually puts out exactly what Icom says it should according to my Bird 43. With the power supply turned down to 12.2 (the resting voltage of a LA battery), the output is just over 30/26. That's nearly a 50% drop! In any case, it probably isn't noticeable, unless you're operating fringe-area simplex.

By the way, if the input drops below 11.8 vdc, the IC-208 just shuts itself off.

[KD2AXU]

I have an Icom IC-208. It is rated at 55/50, VHF/UHF at 13.8 vdc. It actually puts out exactly what Icom says it should according to my Bird 43. With the power supply turned down to 12.2 (the resting voltage of a LA battery), the output is just over 30/26. That's nearly a 50% drop! In any case, it probably isn't noticeable, unless you're operating fringe-area simplex.

By the way, if the input drops below 11.8 vdc, the IC-208 just shuts itself off.

Decades of experience wiring BMW's here (almost exclusively). Seen just about everything under the sun. Run the power direct from the engine side power distribution bock with a fuse (Red cap with the + on it). Use as short of a ground length you can conceive from radio to "bare" metal. No splices... no pigtails. Avoid the temptation to "share" a ground point. Solder all connections. Grounding is the most important topic as with the shack in your house. Total isolation is the key.