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  #1 (permalink)  
Old 03-06-2017, 4:18 PM
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Default Ground plane antenna on RV

I recently bought a used 15 year old motorhome that was equipped from the factory with a CB radio. Upon inspection of the roof I discovered that the factory installed antenna is a common ground plane whip, which is installed on the fiberglass end cap above the drivers seat, just a couple of inches forward of the flat aluminum roof which extends back to the rear fiberglass end cap.

I am no antenna expert, but it seems like this would greatly limit the ability of the antenna to transmit in the direction of travel. Am I correct in this assumption?

thanks
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Old 03-06-2017, 7:26 PM
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Yes, having it offset that far over the ground plane will impact coverage.

Also, needing a ground plane or not depends on the exact type of antenna. Half wave designs do not require a ground plane under them to tune up properly, but they do work better with one.

Usually these CB antennas are installed by an end user who doesn't know anything about antennas, or at the factory where they really don't care.

As you are probably guessing, ideal location is going to be right in the middle of the roof giving it the most ground plane in all directions.
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Old 03-07-2017, 3:13 PM
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Quote:
Originally Posted by mmckenna View Post
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As you are probably guessing, ideal location is going to be right in the middle of the roof giving it the most ground plane in all directions.
What groundplane, those land yachts are fiberglass.

I see that firestick makes halfwave or ground independent CB antennas.
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Old 03-07-2017, 5:10 PM
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As mentioned in my initial message mine has a flat aluminum roof and aluminum sidewalls, but fiberglass end caps
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Old 03-07-2017, 5:12 PM
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Quote:
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What groundplane, those land yachts are fiberglass.

I see that firestick makes halfwave or ground independent CB antennas.
Depends...
Some have aluminum skin. Some have aluminum frames.
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Old 03-07-2017, 6:58 PM
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"Older motorhome" I missed that. OK but the aluminum skin is not always one piece.

Is the old mount threaded, probably not NMO ?
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Old 03-07-2017, 7:13 PM
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Here is a photo of the existing mount, sorry about the low quality it is cropped from a whole roof photo. The swivel mount itself is also covered with roof sealant.
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Old 03-08-2017, 7:48 AM
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Hello Isaac-1: I take it this photo is looking to the rear.

Yes there will a slight reduction of transmitted signal going forward, but a insignificant amount of signal is limited. I would insure that the antenna swivel mount and the antenna have a good connection when in the up position.

I have rented Motor Homes and temporarily mounted the same antenna as yours here ( looks like a M125 antenna or equivalent antenna ) on the Aluminum Trim or molding on the drivers side, it worked ok allowing me to talk with all the others a few miles at times.

Mounting the antenna in the center of the roof is the way to go for the best performance but not always particle.

Jay in the Great Mojave Desert
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Old 03-09-2017, 4:36 PM
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Yup, looks like the old Antenna Specialists 27 meg base loaded 49" whip antenna. And they defiantly need lots of ground plane. Not sure of the electrical contact of the aluminum sheets they may need to be bonded to get good RF ground plane..
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Old 03-09-2017, 8:11 PM
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Maybe. Pace, Midland and many other CB mfrs made very similar looking antennas and you can usually tell by the quality of the parts and whip taper. The A/S whip has a nice taper that is fairly thin at the top and many of the imports are just straight whips.
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Quote:
Originally Posted by cmdrwill View Post
Yup, looks like the old Antenna Specialists 27 meg base loaded 49" whip antenna. And they defiantly need lots of ground plane. Not sure of the electrical contact of the aluminum sheets they may need to be bonded to get good RF ground plane..
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Old 06-18-2017, 12:25 AM
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Well summertime is here, and I am getting back to this project. I am not getting acceptable SWR with the factory antenna, so I am now faced with replacing it with a No ground plane model.

Which leads to my question, I can't reach the inside mounting point of the existing antenna good enough to replace it without removing extensive amounts of overhead cabinets. I can however just barely reach the antenna lead well enough to unscrew it.

Would it be possible to replace the exterior part of the antenna, add a NGP interior cable and keep the existing swivel mount and spring?

If so can someone point me to a compatible antenna and NGP cable with a PL-259 connector on the antenna end

If not I suspect the next best thing to so will be to abandon the old antenna in place and install a side mount NGP kit.

thanks
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Old 06-18-2017, 1:16 AM
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Since you don't know for sure exactly where the issue is with the system, I'd recommend replacing the complete antenna and coax cable and not reuse any part of it.

NGP antennas don't need a special cable, although I have seen some with it. If you went with one of the half wave CB whips mentioned above, you could install a mount and new coax and be done.
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Old 06-18-2017, 12:34 PM
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Friend mounted an A-99 on top if his mobile home with no issues!
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Old 06-18-2017, 6:51 PM
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Truckers with fiberglass wind directors use dual whips to make it effectively a vertical dipole. That's probably your best bet with 11m. I have a 144/440 antenna mounted to my fiberglass top RV by gluing a 3x3 foot sheet of galvanized steel and using a magmount.
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