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