Re: Loop Antenna


David Cutter
 

Hi Chris

Adding to your note.  I did some work on systems using anechoic chambers for military gear years ago and I tried to get some correlation outside the lab, ie in the real world.  It was simply impossible, though I'd hired very expensive, calibrated kit and took as much care as I could.  I concluded that chamber measurements were strictly for those (eg military contracts, etc) that could afford it and not for the likes of us mere mortals.  For instance - safe operating distance: do the calculation but don't bother trying to measure it in the real world, you could be out by 10dB up or down due to all the reflections/absorption.

Chamber measurements are done with a sensor located at the unit under test and this sends back a signal to the power amplifier so the UUT gets the specified dose.  You then rotate the UUT to another face and put the sensor nearby and so on to get the best radiation on each face.  Testing small equipment is one thing but when it is a vehicle, like a truck or a tank or a police car, just imagine the task of where to put the sensor and the size of the chamber.  When I worked on ships I abandoned measurements and went with "can you hear me now?" 

David G3UNA



Antenna factor vs frequency: The amplifier performance is only a small part of the antenna factor. The diameter, thickness, material and shape of the loop used with the amplifier will have a far greater effect on the antenna factor. If someone wants to use the Loop Antenna Amplifier as part of an EMC test setup then they would have to have it calibrated with their own loop to generate the antenna factor. It's not something I intend to do. Antenna factor calibration in a RF anechoic chamber is an excellent way to waste money! I still have a specially calibrated antenna for 3G testing that I was forced to buy for some RF survey work 20 years ago. It cost £750 ($960) back then. As soon as it was taken out of the anechoic chamber and used in the real world the antenna pattern changed due to the reflections around it but the customer wanted to see the calibration certificate! Obviously the price was added to the cost of the job.

Regards,

Chris
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