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Strange Antenna Contest - RF Cafe Forums
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wb9jtk |
Post subject: Strange Antenna Contest
Posted: Wed Apr 18, 2007 8:29 am
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General |
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Joined: Tue Dec 26, 2006 5:39 pm Posts: 58
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The annual strange antennas
contest is the last weekend in May. I am contemplating using my new roof.
I just got a steel roof on my house. The carport is aluminum. These surfaces are parallel
and about 7 inches apart, and not connected electrically. The carport is bolted to the
concrete driveway, so it is somewhat grounded at RF. It is a single story house, so
they are both pretty close to the concrete slabs. I cannot imagine the feed
point impedance, but I expect such an antenna would be very broadbanded. The
roof is approximately 10 m x 20 m. The carport is 3.5 m x 14 m. I have read
plenty about Ufer grounds for DC. However I cannot find anything on how concrete performs
at RF.
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sm7ovk
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Post subject:
Posted: Wed Apr 18, 2007 3:39 pm
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Captain |
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Joined: Thu Jan 11, 2007 12:59 pm Posts: 23
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Hi,
it'll depend very much on how wet it is. A tower with a concrete mounting
the connection can be very good to ground through the concrete if it's wet, in the summer
the resistance will increase. For lower frequencies I think you will find the same type
of relations. Dry pure concrete will be much more as an insulator - BUT your concrete
often contain iron for increasing the capabilities, don't remember what it's called
in English. They can affect a lot on higher frequencies. You can get a coupling through
the concrete.
73 Jens
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nubbage
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Post subject:
Posted: Thu Apr 19, 2007 3:29 am
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General |
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Joined: Fri Feb 17, 2006 12:07 pm Posts: 343
Location: London UK |
I have in the distant past been involved in a program to measure the dielectric properties
of industrial materials. Concrete complex permittivity, as Jens states, does depend
on the history of the slab. Under the conditions encountered in regular house building
the likely permittivity is around 9 and the conductivity sigma around 100 millisiemens,
both measured between 1 and 2000MHz. With these values, you could simulate the concrete
ground in NEC and model the antenna over this 'real' ground. Also if you Google concrete+permittivity,
you will unearth a number of papers with graphs showing values over a range of compositions,
frequencies etc. 73 G3OAD
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Posted 11/12/2012
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