| Author |
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672
| | Posted on Sunday, December 07, 2003 - 7:50 pm: | 
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can any one tell me how a metal roof may effect beams or any antenna? |
mikefromms
| | Posted on Sunday, December 07, 2003 - 10:34 pm: |
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If you are less than 20' feet from the metal roof it can run your swr up.
mikefromms |
Alsworld
| | Posted on Monday, December 08, 2003 - 12:08 am: |
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I ran this question by Marconi about a year and a half ago and he told me that each setup will react differently. It may be the ultimate groundplane, but as Mike said, it may cause much reflection as well. Only way to test that particular setup is to try it out. Each home/antenna setup is entirely different.
The house I plan on retiring in has a metal roof so this question sparks an interest in me as well. It's a tough answer over the internet. I'm listening still....
Alsworld |
Pig040
| | Posted on Monday, December 08, 2003 - 10:00 am: |
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My groundplane antenna is about 10 feet over a 30ft by 60 ft metal barn, I have a great swr, and it beats the heck out of having it where it was before. And before it was higher than it is now. The roof seemed to really help this particular antenna. |
672
| | Posted on Monday, December 08, 2003 - 12:17 pm: |
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thanks for the advice . I'll have to just give it a try and see what happends.I was thinking of puting the I2000 in a tree behind my shack.but I would have too run about 150ft of coax and I heard running to much coax could be a bad thing.not to mention setting the swr would be an all day adventure(hehe).can anyone suggest the best type of coax they all look the same to me just a little bigger or smaller. they all look like they have the same % of shielding and or 50 ohm's. ps thanks for all the help in advance |
Alsworld
| | Posted on Monday, December 08, 2003 - 9:05 pm: |
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The Times Microwave site (search under Times LMR) has a great calculator comparing coax.
I personally run a 125ft solid section of DavisRF Bury Flex coax. Double shielded low loss coax with an excellent outer jacket. Their site also has a chart and spells out the losses associated with different coax.
Alsworld
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Mr_Rf
| | Posted on Monday, December 08, 2003 - 9:51 pm: |
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FYI all, the term used when objects near an antenna effect the SWR is called "Mutual Impedence Coupling." This coupling generally occurs when objects are in the "Near Field." Near Field is generally defined as 1/2 wavelength distance in free space (492 / 27.2MHz = 18 ft.). This distance can increase slightly when beam antennas with gain are close to the metallic roof.
However, any metallic abject such as a tin roof within the "near field" will cause gain or loss in a signal that is generally proportional to the shape of the roof.
For instance, In the case of an ground plane antenna mounted beside a building with a tin roof, the longer the distance of roof is in a specific direction...the more gain that will be exhibited in those directions, while the directions going away from the building where no roof is present will exhibit little or no gain and sometimes even less gain than the antenna is spec'd for.
Another good example of this is on vehicles. Anennas mounted near one end of a vehicle, say a 102" whip on the back bumper will radiate (get out) best in the direction of the front of the car, while an antenna mounted in the center of the roof will radiate in all directions well with slightly better results off the front and rear directions of the vehicle.
Hope this helps. |
672
| | Posted on Tuesday, December 09, 2003 - 4:25 am: |
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it sure does!! thanks to every body! |
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