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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 
 
General
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Joined: Tue Dec 26, 2006 5:39 pm
Posts: 58
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.


 
   
 
sm7ovk
 Post subject:
Posted: Wed Apr 18, 2007 3:39 pm 
 
Captain
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Joined: Thu Jan 11, 2007 12:59 pm
Posts: 23
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


 
   
 
nubbage
 Post subject:
Posted: Thu Apr 19, 2007 3:29 am 
 
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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