[Elecraft] OT -RFI question

[Jim Brown]

On 5/25/2011 2:01 PM, FRED TOWNSEND wrote:
> Pacemakers are a lot like avionics relative to ham
> radio. The working spectrums for pacemakers, avionics, and ham radios are widely
> separated by both frequency and function. The probability of their mutual
> interaction is very small. Unfortunately the penalty for even small errors in
> pacemakers or avionics is very high. The testing is extensive but there is no
> way one can conceive of all error modes therefore testing will always be
> incomplete.

Well said.  One of the most difficult hazards to pacemakers is a cell 
phone used by the person wearing pacemaker -- worst case, it's a 1 watt 
PEP AM UHF, transmitter within inches of the pacemaker. It's difficult 
because it's AM, because of the proximity, and also because the 
wavelengths are short enough that relatively small conductors inside 
pacemaker can be effective receiving antennas, and relatively small 
signal loops can provide significant magnetic coupling of near field 
signals.Strong magnetic fields, like those generated by big transformers 
and motors, are another serious hazard.

Those hazards, and the potentially tragic results of failure, forced 
pacemaker mfrs to seriously get religion about design and mfg that 
avoids RFI.  Those hazards have all been around for at least a decade, 
long enough that any modern pacemaker from a serious mfr is almost 
certain to have excellent immunity to RFI and magnetic fields.

If you want to be extra cautious, simply be careful in your home and 
shack to avoid putting strong magnetic and RF fields around the area 
where your pacemaker is going to live.  That includes placing TX 
antennas as far away as practical, using effective common mode chokes on 
transmission lines, using power supplies with transformers that minimize 
leakage flux, and making sure that there are no wiring errors in your 
mains power (that is, AC) that set up strong magnetic fields.  These 
wiring errors are discussed in my tutorial about Power and Grounding for 
Audio and Video Systems, and in the power point about Ham Interfacing. 
Detailed advice on coax chokes is part of the RFI tutorial for hams.

  http:audiosystemsgroup.com/publish.htm

The RFI tutorial also includes suggestions for clearing the problems 
with security systems, which are well known RFI dogs.  Two common fixes 
are using twisted pair for all interconnect wiring (CAT5, for example) 
and using a small capacitor across that wiring at both ends wiring 
between the sensor and the main unit.  In some situations, a ferrite 
common mode choke may help.

73, Jim Brown K9YC