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Can this Happen? - RF Cafe Forums

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naveednoul
 Post subject: Can this Happen?
Posted: Tue Sep 27, 2005 8:56 am 
 
Captain
 

Joined: Wed Sep 21, 2005 3:40 am
Posts: 10
Location: Islamabad,Pakistan.
Every one of us knows that we transmitt most of our power signals through antenna for exmaple an F.M radio station. Is there any voltage induced in antenna when signal passes through it.
Remember Think about High Power Transmission. :?:


 
   
 
Graham
 Post subject:
Posted: Tue Sep 27, 2005 6:34 pm 
 
Colonel
User avatar

Joined: Fri Sep 02, 2005 7:25 pm
Posts: 34
Location: Hampshire UK
Antennas do have voltages induced across them by being in a space exposed to a passing electromagnetic wave. The result is currents in the antenna elements, and hence we tap in to recover some energy to be the input for our receivers.

Operating in transmit mode, we can deliver energy to antennas, and excite currents in the elements, and corresponding voltages across them. The resulting electric and magnetic fields around the antenna are local to it (near field), but in the space up to 2 wavelengths away, they do generate the radio wave electromagnetic field which travels from the antenna at the speed of light.

I once fed a modest power of about 100W to a whip antenna which was a conductor encased in fibreglass. The voltage on the end of the antenna was high enough to cause air corona breakdown at the tip, and the resin binder caught fire. If you need convincing that antennas have voltages on them, try holding up a old flourescent tube near the end of a ham HF antenna while he is transmitting, and see it light up!




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