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What affects the antenna radiation pattern? - RF Cafe Forums

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Mr.Whatever
 Post subject: What affects the antenna radiation pattern?
Posted: Fri Jan 19, 2007 5:22 pm 
 
Captain
User avatar

Joined: Fri Apr 07, 2006 6:10 pm
Posts: 18
Good day, folks!

If I had a dipole antenna used for a radio FM receiver, will mounting it on a metallic stand affect its antenna pattern? What if I use a PVC or wooden pole?

Another thing, from an antenna book, it states that if I lower the antenna closer to ground the antenna impedance decreases. Does this apply to all antennas? I'm assuming that changing my antenna height from ground will have an affect on its pattern. Is that right?

Thanks!


 
   
 
nubbage
 Post subject:
Posted: Mon Jan 22, 2007 6:01 am 
 
General
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Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi
If the dipole has its axis vertical therefore parallel to the pole, it might couple to a metal pole and distort the pattern and the match, depending on where it is relative to the pole, and how many wavelengths it is away from the pole.
If it is less than one wavelength from the pole, and the pole passes right past the antenna, then it would be better to use fibre-glass tube for the pole.

If the axis of the dipole is in line with the pole, in effect an extension of it, then use fibre-glass for a mechanical connection to the metal pole.

PVC has an unfortunate tendency to creep out of shape over time, and degrades under ultra-violet from the sun, going brittle after a year or so. So-called u-PVC is better than the ordinary stuff, but fibre-glass is even better. Wood is OK if you impregnate it with a polyester wood-preserver.

If the antenna is horizontal and the pole is vertical, there should be little or no interaction between them.
Lowering the antenna to within about one wavelength or so above ground has the effect of coupling it to another "phantom" antenna, being the electro-magnetic image of it in the ground mirror. Hence you have 2 antennas in parallel to put it crudely. It effects not only the impedance but also the pattern.


 
   
 
Mr.Whatever
 Post subject:
Posted: Tue Jan 23, 2007 11:02 am 
 
Captain
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Joined: Fri Apr 07, 2006 6:10 pm
Posts: 18
Thanks for the explanation! Are there any resources that I can see this explanation mathematically? I have Balanis' Antenna Theory book, but I'd have to go thru it again page by page. I haven't touched it in a while.






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