Mobile Antenna and grounding plane?

[thercman]

Okay... I ordered an antenna mount to attach to my kayak rack. Apparently this is going to limit my antennas performance due to a lack of a grounding plane. WTF?!? I told the guys at the on-line shop this is my first radio and I have already had to call back twice to change my order based on what I realized I needed after the fact. So the local radio shop said I should either try to find an antenna that will work with that mount or drill a hole in the car. Two issues with that are, I have a sun roof in the car which limits placement, and the kayak rack takes up a lot of placement space. Is there a way to make the rack mount work? I do have a small NMO antenna and mounting hardware I just bought so I can drill and go with it. The bad thing is it will probably end up on the truck limiting its range.... HELP! Suggestions please...

Edit: So does an antenna such as the Comet SBB-7 require a grounding plane (such as the roof of the car) for optimal performance? If so I need to send the $35 bracket back and order the NMO version. Or is there an antenna that doesn't need to have the grounding plane for optimal performance? Hope that clarified my rant a bit....lol

 

[thercman]

Okay, this has been eating at me all day so I have been trying to research it and here is what I found on Comet's website... Boy that dude really got me wound up...

"Q. Should I drill a hole in my roof/trunk or use a lip mount?
A.In the past, most people preferred drilling a hole in the center of the roof or trunk to provide the best ground plane. This made a lot of sense when the repeater systems were poor and most antennas were single band and needed the ground plane. Current antenna design produces antennas that are ground independent. The sensitivity, coverage area and height of most modern repeater systems is outstanding. The antenna no longer needs to be in the middle of a ground plane for excellent coverage.
The benefit of drilling a hole and mounting your antenna in the middle of the roof or trunk is minimal."

 

[Mudfoot]

I'm a fan of drilling holes in the roof. I've seen a few dudes use the "rack" type antennas and they all liked them.

As long as they could hit local repeaters, they were happy.

 

[W5LZ]

I'm really surprised that 'Comet' would make such a statement, it's simply not true. The fact that repeaters are well placed or not has very little to do with where a mobile antenna is placed and how it's mounted.
If your rack is metal and if it's making electrical connection to your car's metal body, then that rack mount should work fine. If some part of the above is not true then that antenna isn't going to work correctly because it still needs a 'groundplane' or an electrical connection so that the car's body provides the 'other half' of the antenna. That is not a NGP, no-ground-plane antenna.
Something else to consider is that SBB-7 antenna is something like 5 feet long. It is not very 'limber' and you will be whacking a lot of things with it.
I've seen it mounted on one of the 'lip-mounts' and it does work even if it isn't center of the roof. I've also seen the same antenna mounted on a fender and it worked okay. An antenna doesn't have to be mounted in the center of a roof to work well.
- 'Doc

 

[thercman]

Thanks Doc...

My roof rack is a Thule. While it is clamped pretty tightly to the car it does have rubberized coating on the bars. Essentially, the bracket would be clamped to it and not have a grounding plane. I just purchased locally a Diamond NR72BNMO antenna. It is a short 13" cell phone looking antenna. It has gotten pretty good reviews. I figure that will allow me to hit the local repeaters while getting around locally but once I start camping and get away I am probably going to need something taller. I can always keep it in the trunk and put it on when I need it.... Do you have any recommendations for a larger mobile antenna? Also, you are saying that my NMO mounted antenna does not necessarily need to be mounted on a large surface area to get good results? For example right in the middle of the roof? I could put it off to the side and still get good signal?

Regards,
Greg

 

[W5LZ]

The centrer of the roof means a pretty symmetrical radiation pattern. If you move away from the center the pattern gets sort of lopsided. The likelihood of that lopsidedness being 'extreme' is very unlikely. Some sure, but nothing hugely drastic. Most of the time you'll probably never notice it.
- 'Doc

 

[thercman]

Well, I am going to cancel my order on the Comet since it is back ordered. Maybe I'll just wait on a taller antenna to see how this one fairs. I am on my way to the hardware store to pick up a 3/4" metal hole saw. I'll pull the liner back a bit, open the sun roof all the way up to see where the best place to mount it will be. I could move the kayak rack to one side which will make it easier to load, however I would rather be able to use the center front and rear latches to secure the 20 footer. Thanks again!

Greg
~ ~ ><{{{}}}">

 

[W5LZ]

One Comet antenna I've had a lot of luck with is the 'CA2x4SR' dual band antenna. It doesn't have the most gain, but it does very well for me. Your 'wants' are going to be different than mine, so you have to decide what you want.
- 'Doc

 

[thercman]

So I called to cancel the larger antenna and the guy on the phone was really helpful. As you guys have mentioned the 7 is pretty large. He suggested I use the SBB-5 which is 38" tall with a folder-over mount and use a trunk lid attachment. He highly recommended against drilling since it will ultimately leak, rust, and give me problems if I choose to trade it in. Makes sense. He also said that the mount will be grounded to the underside of the trunk lid providing the ground plane. So based on all that, I said okay. At least no pulling back the head liner and cutting holes. It also saved me a few bucks not having to buy the metal hole saw. Although I do need to send back 1 bracket and cable I do not need. Then I need to go back to return the shorter antenna and accessories for that. Haaa. I guess my lack of patience has gotten the best of me...