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Author Topic: Using a Roof Rack as an Antenna?  (Read 3181 times)
K4SSS
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Posts: 5




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« on: November 23, 2011, 09:29:27 AM »

Hi all.

Wondering if any of you have ever considered or actually used your roof rack as the actual antenna?  I have a Nissan Xterra that uses 2 inch in diameter longitudinal rails along each side, and the 2 in diameter crossmembers are fully movable from front to back, so I got to thinking that I might be able to make use of them in some sort of yagi design?  Any ideas?  Pros and cons?  I'd be interested in hearing anything that you might be able to offer me about this idea, as this is completely new ground for me...

Best regards -- Bob  K4SSS
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K0BG
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« Reply #1 on: November 29, 2011, 03:44:59 PM »

The bottom line is, it doesn't work very well.

The biggest issue with factory installed roof racks, is the spacing between the rack, and the roof. In short, it isn't enough! And, the only viable way is to use the rack as a loop. In the best of cases, the loop is no longer than 24 feet, and usually less. This means that at any HF frequency, the loop is going to have a very low impedance. Read that as low radiation resistance, and hence efficiency.

You can match anything, even an antenna with an input impedance of a few tenths of an ohm. However, most of the input power dissipates as heat in the matching network. You be much better off using a decent screwdriver antenna, and any roof rack, no matter its overall length, or height.
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KC5CQW
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Posts: 98




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« Reply #2 on: December 05, 2011, 05:54:16 PM »

Nah, I use mine for occasional luggage and flood lights for playing in the woods.

Having said that, it's a good idea but it just won't work. Good on you for giving some thought to different antenna possibilities.
You can use the rack as a foundation to mount taller insulating supports to run a wire loop.
This is referenced in the (free to download from SGC) SSB Users Guide.
I believe they cite a minimum 18" spacing from the roof.

You could use some painted 1/4" fiberglass tent poles for stand-offs.
It might look a little silly. But then, so does my 102" whip with cap hat.

The Xterra is a good looking vehicle and a well mounted (and thought out) HF vertical antenna install will not take away from its looks.
I'm planning an install into a Jeep Cherokee Sport and if anything, I think an HF antenna makes it look tougher!




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W6RMK
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Posts: 669




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« Reply #3 on: December 05, 2011, 08:22:11 PM »

There is an Australian manufacturer (Q-Mac) that makes a roof rack antenna, but it's basically a standard compact loop, with the rack forming the bottom part of the loop (there's another piece of tubing that goes up over the top for the other side of the loop.

There's also some interesting work where the windshield frame is used as a loop antenna.

Anything can work. They're all about the same physical size.  The difference is in convenience, and, of course, RF exposure.

Look for papers by Austin or Belrose.
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G4ZOW
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Posts: 44




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« Reply #4 on: January 26, 2012, 04:11:08 AM »

In a past life I used to work a broadcast engineer lady out on the US west coast who built a 40m DDRR antenna on top of a converted school bus.

Her signal rocked but I don't think she was running barefoot  Shocked

Scale this one down to 10m from the AM band: http://www.offshore-radio.de/fleet/kingdavid.htm

David G4ZOW
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KB4QAA
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« Reply #5 on: January 27, 2012, 10:30:34 AM »

There is at least one manufacturer of 'stealth' antennas for governmental agencies that makes such an antenna.  No idea of the performance.  Certainly it is a less than ideal configuration/location.
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K0JEG
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Posts: 679




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« Reply #6 on: January 28, 2012, 11:09:46 AM »

http://www.stealth.ae/plugins/custompages/detinf.php?id=372&id_categories=136

If I had an SUV I'd consider it (or homebrewing something like it).
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