[Amps] Experience with Heart Pacemakers?

[Manfred Mornhinweg]

Hi all,

I think it should be pointed out that even if you use perfectly shielded coax 
cable, perfect connectors perfectly assembled, and a perfect ground connection 
to all your equipment, you can still have a huge amount of radiation from the 
feedline, and even from the grounding wire.

It is highly useful to eliminate this problem, not only for the safety of people 
using pacemakers, but also simply to reduce interference to computers, TVs, and 
all sorts of other things in the home, and to reduce noise pickup in RX. Because 
a feedline that radiates TX power, also picks up RX signals, and the portion of 
feedline that's inside the home usually runs close to devices that create huge 
amounts of RF noise, such as fluorescent lamps.

Feedline radiation from a coax cable happens when the line carries a common mode 
current, or when the shield isn't at ground potential. It should never do that. 
But if one end of the coax shield is grounded (in the shack), and the other end 
is directly connected to a balanced antenna, like a dipole, Yagi, Quad, Delta, 
and so on, then the feedline will conduct a significant common mode current, and 
its shield will be at a significant voltage relative to ground, except very 
close to ground itself, and at half waves from there.

The prime solution for this problem is using an effective, good quality balun 
between the feedline and the antenna, or else using an antenna that is very 
unbalanced and has one end grounded, such as a vertical antenna mounted above a 
system of 100 buried radial wires.

As Ian wrote, common mode chokes can and should be used to suppress any common 
mode currents on feedlines. A 1:1 current balun is nothing else than a common 
mode choke. The most important spot where one of these devices needs to be 
placed is where the coax connects to a balanced antenna, but additional common 
mode chokes at other places can never hurt (as long as they are good 
quality...), and will reduce common mode currents arising, for example, from the 
feedline not being perpendicular to the antenna for a long enough distance.

It's interesting to note that 100% coverage of coax cable shield is not required 
for low feedline radiation. As long as the size of the holes is very small 
compared to the wavelength, the electric field stays confined between the 
conductors. And as long as the two conductors are truly concentric, the magnetic 
field still cancels outside the cable, even if instead of a continuous shield 
there is only an outer conductor formed by a few wires distributed evenly around 
the center conductor.

Extending this concept a little further, we arrive at open wire feedlines, which 
also operate essentially without feedline radiation, as long as the wire 
separation is very small compared to the wavelength, there is no common mode 
current, the sum of the voltages on the two wires is zero relative to ground, 
and the line doesn't run close to conductive objects. Common mode chokes can be 
used with open wire feeders just as well as with coax, and to force the voltages 
  on both conductors to be equal and opposite, a center-tapped autotransformer can
be used, with the center tap grounded.

I don't use a pacemaker (yet... knock on wood!), but I know several hams who do. 
Unfortunately there are some doctors who tell their patients, after implanting 
them a pacemaker, "so, now you will have to give up your radio hobby, because it 
could kill you". One of my ham friends was VERY scared by that, and truly 
considered giving up the hobby! I tried to get specifications about interference 
resistance from the manufacturer of that pacemaker, with no luck. Simply there 
was no reply. So we experimented. I had my friend check his pulse, while 
exposing him to increasing levels of RF fields, on different bands. The idea was 
to stop if the pacemaker missed a beat. In that case, my friend's pulse would 
have gone down to a very low rate, which is why he needed that pacemaker to 
speed things up. It didn't happen, even with field strengths more typical of the 
near field of an antenna at high power, far higher than any level normal in a 
shack. So he is luckily still a ham.

My advice to pacemaker users is this: Avoid very high field situations, 
specially at VHF and UHF, because in that ranges the electrodes of pacemakers 
are most effective as antennas. Microwave power isn't a big problem, because 
it's reflected and attenuated by the body, and HF isn't much of a problem 
either, because simply the pacemaker wires are too short to pick up much signal.

So, avoid cellphones close to the pacemaker, avoid VHF and UHF handies near it, 
  and just for added safety, when using high power make sure that the antennas 
are far enough from the shack, and reduce feedline radiation so that there won't 
be a high RF field at the operating position. And my most important advice: Be 
careful not to get very close to a car with a VHF mobile installation, when 
there is any chance that someone inside the car will transmit. Many mobile 
radios put out 50 watts and more, and it would be easy to accidentally get 
within the near field of a mobile antenna radiating this power! That causes the 
highest field strength I have measured so far, among all the situations a normal 
person might be exposed to, and is in the more dangerous frequency range. Inside 
the car, instead, the field strength is much lower.

If a ham using a pacemaker runs a VHF or UHF mobile installation, my advice is 
to place the antenna in the middle of the roof, of course making sure the car 
has a metal roof! Not plastic, and not a glass "sunroof"! The coax should be run 
along the metal of the car, and not closer to the driver and passengers than 
necessary. Radiation from the coax is normally very low, but could get high if a 
connection corrodes.

All that said, consider this last point: Plain simple non-ham "normal" people 
can easily be cooked a moment by very intense VHF or UHF radiation from a car 
driving past them, or parked while they walk by. Pacemaker manufacturers must 
consider this, and design their products to resist this interference. It's also 
very easy to do, because the frequencies pacemakers need to sense are extremely 
low, far below the audio range, and that makes RFI filtering very simple. So 
it's pretty safe to assume that pacemakers will resist pretty much anything a 
ham can throw at them!

Manfred



========================
Visit my hobby homepage!
http://ludens.cl
========================