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Author Topic: effects of roofrack and cb antenna on vhf tx?  (Read 680 times)
KE5MJS
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Posts: 26




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« on: December 24, 2007, 04:41:16 PM »

Hey guys!  I have a question.  I want to permanently mount my radio antennas to the top of my jeep cherokee.  I would have three antennas, a short one up front for the scanner, next would be a larsen dual-band, and last a wilson cb antenna.  Keep in mind this is a cherokee, so I dont have a lot of acreage to work with.  I'm also gonna be putting either an aluminum or steel "basket" type roof rack up on top.  This is later though.  My question is:  Will the antennas and (eventually) roof rack effect the larsen antenna at all (tx and rx)?  What do you guys recommend?  Thanks in advance!

Javier
ke5mjs
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KG4RUL
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« Reply #1 on: December 24, 2007, 05:56:33 PM »

It won't make enough difference to even worry about.
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W3LK
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« Reply #2 on: December 24, 2007, 07:47:58 PM »

It all depends upon where and how the antennas are mounted in relationship to the "basket".

Some more details are necessary before anyone can make an informative answer.

73,

Lon - W3LK
Naugatuck, Connecticut
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KE5MJS
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« Reply #3 on: December 25, 2007, 11:17:46 AM »

The Larse will be mounted just infront of the basket.  At least 6 inches away.  What are the effects of mounting like that?  SHould I just not put the rack on?
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W3LK
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« Reply #4 on: December 25, 2007, 12:17:20 PM »

I am not sure what a "basket" is, but if it is tall in size, you need to keep it more than six inches away from the Larsen. The problem you will have is detuning. It MAY have little real effect, but a substantial amount of metal close to antennas, especially transmitting antennas,  is not an ideal situation.

If this is to be a permanent installation with the basket fastened into the roof and electrically grounded to it, it might be possible to mount the antenna on a bracket on top of the front edge of the basket.

As for the scanner antenna, it will have little effect. You might have the same detuning problem with the CB antenna, but not to the degree with the dual-band. It al depends upon where, in relation to the basket, you are mounting the CB antenna.

Is there a web site you can refer to that shows what you are planning to install?

73,

Lon - W3LK
Naugatuck, Connecticut
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KE5MJS
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Posts: 26




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« Reply #5 on: December 25, 2007, 07:59:34 PM »

http://www.jeepforum.com/forum/showthread.php?t=394937

That link takes you to pics of the roof racks.  Keep in mind that the one I plan to put will be about 4 or 5 feet long.  And will be mounted all the way to the back.  

But if its gonna be that much of an effect on transmitting, I might not put it.  I rather be able to talk!
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W3LK
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« Reply #6 on: December 26, 2007, 07:49:02 AM »

After looking at the photos, I see your problem.

If you really need the roof rack I would mount the dual band on one front fender and the CB antenna on the other. While not ideal, it would be better than putting them right next to the rack.

The scanner antenna I would mount on one of the front corners of the rack, since transmitting considerations don't matter.

If you electrically bond the rack to the frame of the Jeep you could mount the antennas on the rack itself, but then you run into the problem of the antennas getting in the way of anything overhanging the rack. Personally, I'd go with the front fenders.

I hope this has been of help.

73,

Lon - W3LK
Naugatuck, Connecticut
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KE5MJS
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« Reply #7 on: December 26, 2007, 11:09:48 AM »

It has been a great help! I currently have the cb mag-mounted on the roof and the dual-band on the front fender.  The whole purpose for getting the roof rack was to mount off-road lights.  But I guess i'll Just get a dedicated light bar for those.  Time to do some photoshoping and see how it will look with those 3 permanently...  Thanks again!

Javi
ke5mjs
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N1LO
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« Reply #8 on: December 28, 2007, 11:19:10 AM »

Hello Javi,

I have seen a roof rack on a minivan completely upset the 2m antenna. As it turned out, the crossover arm was aluminum, and close to a 1/2 wave on 2m (it was around 38-40 in.). The sides of the rack that held it were insulators, leaving the arm free to resonate. We moved the antenna to the rear of the roof and the problem went away.

 As long as the antenna is not buried down inside the rack structure, and there are no ungrounded pieces like the situation above, it should be ok. The problem could also occur if you had a piece grounded on one end, floating on the other, and it was around 18-20 in long.

--...MARK_N1LO...--
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KE5MJS
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« Reply #9 on: December 30, 2007, 05:55:37 PM »

Ok then, no roof rack.  So the other two antennas shouldn't have a MAJOR effect on the 2/70 antenna?
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