UHF aerial placement

Submitted: Tuesday, Aug 20, 2002 at 00:00
ThreadID: 1768 Views:2092 Replies:3 FollowUps:0
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Hi all,

I have a question regarding the placement of my UHF aerial. It is a 6Db non ground independent type. Whislt I understand that for best performance it should be placed in the middle of the car (ie on the roof) I was wondering if this means it actually has to touch the metal. Currently I have it mounted on the front middle of my cage type roof rack (basically just to the left of the drivers head). This is in the middle of the car but as you can imagine is not in direct contact with the large metal ground plane beneath it (ie the car itself). Have I mounted the aerial in the best place possible of have I completely misunderstood the idea all together.

Cheers
Grazza
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Reply By: Bob - Tuesday, Aug 20, 2002 at 00:00

Tuesday, Aug 20, 2002 at 00:00
It is not necessary nor desirable for the antenna to touch the metal. The proximity of the base of the antenna to the metal surface (but not touching it) provides a "ground plane" which effectively acts as a mirror, doubling the length of the antenna and making it efficient.
AnswerID: 5852

Reply By: Rob - Tuesday, Aug 20, 2002 at 00:00

Tuesday, Aug 20, 2002 at 00:00
Here are a couple of very good sites that explain it all for you

http://www.gme.net.au/land/choose_ant.html
http://www.firestik.com/Tech_Docs.htm
http://www.mobileone.com.au/tune.htm
AnswerID: 5854

Reply By: Member - Nigel - Tuesday, Aug 20, 2002 at 00:00

Tuesday, Aug 20, 2002 at 00:00
While the antenna shouldn't touch the metal of the roof, it does need to be mounted fairly close to the roof (ground plane) to be effective. The braid on the coax that terminated on the bottom of the mounting needs to have good electrical contact with the ground plane. Most roof racks are a fair way off the roof and don't have a good electrical connection to the vehicle.

That said your roof rack may provide a reasonable ground plane, so try it and see. Because your antenna is mounted on one end of the ground plane your signals won't be even (ie more towards the rear). All car mounted antenna's are a compromise except where the mounting base is directly on the roof of the car in the centre, hence the reason why ground plane independant antennas are so popular as they can be mounted anywhere.
AnswerID: 5855

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