[RFI] Defibrillator and RF Exposure Query

Jim Brown jim at audiosystemsgroup.com
Wed May 10 06:49:20 EDT 2006

On Tue, 9 May 2006 22:18:02 GMT, N6KI Dennis wrote:

>Can anyone address this issue

An important part of any mechanism for RF getting into a device is the length of 
any conductors that could function as receiving antennas within the device (or 
connected to it) as a function of the wavelength of the interfering signal.  The 
subject's body, of course, could be part of that, and it would be a better 
antenna if it were touching other large conductors (or grounded objects). 

Example: An unbalanced miniature electret mic capsule with FET preamp suspended 
above a choir almost never detects RF (I don't know of any instances). On the 
other hand, an impedance converter/line driver located 10-30 ft along the line 
from the capsule and driving a long mic cable is QUITE susceptible to RF. 
Likewise, a professional mic with poor immunity connected to a long mic cable 
and sitting on a stand across the road from a 50 kW AM transmitter won't detect 
it, but it will if someone standing on the ground grips the mic to complete a  
path for RF current through the mic. Some of these tests are described in AES 
papers on my website. 

Inverse square law, of course, will apply, so any antenna system that maximizes 
the distance between the subject and the antenna and keeps RF off of the 
feedline would be better than one that does not. In other words, I would try to 
stick with well matched resonant dipoles with good baluns and coax, and operate 
on the lower HF bands. And, of course, many hams have lots of fun running QRP. 


Jim Brown K9YC

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