Probably a dumb question - tuning a mobile 2m setup?

[sdnative]

The reason I was checking SWR was to make sure the ground plane was sufficient for my 1/4 wave antenna and to make sure my connections were good as I did my own soldering.

If you are using a 1/2 wave antenna on a mag mount base, there really isn't much to adjust. Maybe check the SWR once to make sure there aren't any issues with the antenna/mount/feedline but that's about it.

[DaveK]

If the SWR reading is high how do you lower it?

With a 2m antenna, there is very little room for adjustment, mostly due to the fact that they are made well and come pre-tuned. Sometimes the whip can be raised or lowered via a set screw.

[OLLIE]

Two weekends ago I went to a ham radio and CB setup session for another offroad group (TDS). A few others and myself were asked to come in and help new and old hams to adjust their radios and teach them how to program them. It was quite an experience. I learned a lot. I was under the impression going into the meeting that 2M/70cm antennas were dialed in out of the box. After putting my analyzer on a few of these setups I realized that I was poorly mistaken, especially where Jeeps are concerned. They have a POOR ground plane. It's not just the antenna you have to think about either but the antenna wire. You have to think about things like not coiling excess wire but tucking it away in "S" type of pattern. All of these things seriously affected SWR on brand new antennas. I don't think we had one antenna that we didn't have to adjust the antenna length. If you have a good ground plane then you will have less of an issue. We had an antenna guru out there that tried to teach us all secrets to establishing a good SWR. When I get my truck back I know I'll be hooking up an analyzer to it because I know my SWR will need to be adjusted. Maybe at one of our next M&Gs we should have a radio set up day. It proved to be quite a learning experience. Maybe I could get Steve Early there (the guru) to help us out.

[Chazz Layne]

Well my CB barely gets used (I think I've used it twice in 5 years) so it's antenna gets to take a back seat (literally, back of the rack). 2m will get the premo spot smack center in the roof (rack mounted)... but that brings me to a couple more questions:

The roof is 4' by about 6.5', but it slopes down at the mid point by about 5 inches (Land Rover Discovery). Would this have a significant negative impact on ground plane?

- and -

The roof is aluminum... is this a bad thing?

[OLLIE]

Well my CB barely gets used (I think I've used it twice in 5 years) so it's antenna gets to take a back seat (literally, back of the rack). 2m will get the premo spot smack center in the roof (rack mounted)... but that brings me to a couple more questions:

The roof is 4' by about 6.5', but it slopes down at the mid point by about 5 inches (Land Rover Discovery). Would this have a significant negative impact on ground plane?

- and -

The roof is aluminum... is this a bad thing?

I would say you have a great ground plane. The ideal point is as close to the middle of your roof as possible. The slope you're talking about won't be much of an issue at all. I don't know which antenna you went with but your biggest concern will be branch potentially breaking your antenna. A mag-base is the ticket since it will just knock over when hit and not get broken.

[cruiserlarry]

Well my CB barely gets used (I think I've used it twice in 5 years) so it's antenna gets to take a back seat (literally, back of the rack). 2m will get the premo spot smack center in the roof (rack mounted)... but that brings me to a couple more questions:

The roof is 4' by about 6.5', but it slopes down at the mid point by about 5 inches (Land Rover Discovery). Would this have a significant negative impact on ground plane?

- and -

The roof is aluminum... is this a bad thing?

Well, while most metals work well as a groundplane, aluminum does not. You might have to go with a no-groundplane antenna system, or improve your groundplane in one of the following ways:

If you have an aluminum roof and cannot get a good SWR reading on your antenna, then the groundplane is probably at fault. To fix it, you can try either of these options; the first is the easiest and cheapest; the second is usually the best.

1) Run a wire from the antenna mount to the chassis of the vehicle. The wire needs to be as thick as is possible, anything over 10amp rated is suitable. Make sure it is attached to the ground part of the mount. This normally works fine however if it doesn't you may need to look at the next option.

2) Attach a metal sheet or plate underneath the mount, if the antenna is mounted in the roof then a metal plate underneath the mount is best. You can use steel or even copper for this. The bigger the plate the better, make sure it is at least 1 foot square. You will probably find that running a wire from the plate to the chassis will also help.

Good luck

[DaveK]

Chazz:

You have approached this wisely by doing your research first. There are several excellent sources for antenna installation information, other than here of course, that can be of great value to you as you get things set up.

While it is technical in nature, I have found that one of the very best sources for antenna installation and related issues is Alan Applegate's web site at: http://www.k0bg.com/. Unless you have a technical or engineering background, it can be helpful to digest Alan's information in small gulps. While the information is tops, the best part is that Alan is very giving with his time and is willing to help other amateurs on their antenna issues. I speak from personal experience.

Good luck.

[Chazz Layne]

...I don't know which antenna you went with...

Yeah, that information might just be relevant to this thread, whoops...



I went with the Diamond NR770HB and their K9000 mount so I can flip it down if trees become an issue. I'm also mounting it at the floor of the roof rack so it will nest inside the rack's "cage" to further protect the base. The rack is only 4" tall (3" from the mount point) so the antenna's base will be above the rack's rim.

Am I right in my understanding that the ground plane issue will be minimal with the NR770?

[Chazz Layne]

...Alan Applegate's web site at: http://www.k0bg.com/.

My wife is going to enjoy this one, she keeps bugging me about building our own base antenna at home.

[OLLIE]

...I don't know which antenna you went with...

Yeah, that information might just be relevant to this thread, whoops...



I went with the Diamond NR770HB and their K9000 mount so I can flip it down if trees become an issue. I'm also mounting it at the floor of the roof rack so it will nest inside the rack's "cage" to further protect the base. The rack is only 4" tall (3" from the mount point) so the antenna's base will be above the rack's rim.

Am I right in my understanding that the ground plane issue will be minimal with the NR770?

I have yet to put an SWR meter on my K9000 and 770HB so i can't answer that question yet. I'll try to bring my analyzer to BF and see what I get on it.