Richard Clark wrote:
On Mon, 4 Jun 2007 02:43:18 +0100, "Mike Kaliski"
wrote:


Take a deep breath before transmitting with all the controls set to 11...


Hi Mike,

Is the warrantee void within 100 yards of a 50KW AM antenna? Or
howabout those half megawatt LF stations that litter the continent?


I'd venture that the concern is not so much radiated field (although, in
the nearfield it drops off pretty quick, so a 100W transmitter on a car
might well create larger field strengths than that 50kW AM station.

It's probably more the potential for conducted RF via the radio's power
leads, etc. Imagine that if the chassis of the radio is RF hot,
relative to the car body (oh no, that never happens, no waygrin), so
now you have significant RF current flowing in the vehicle wiring harness.

From a straight field standpoint, it's easy to exceed the ANSI RF field
Maximum Permissible Exposure(MPE) with a 2m transmitter and 50W on a
car. Combination of the limit being lowest, close proximity to the
radiator, etc. (I seem to recall the limit is reached at about 1 meter
away from a groundplane/whip antenna at 50W, albeit at 100% duty cycle)

But then, you have done your RF safety analysis, right? (Even though
there's a categorical exemption from the requirement for routine
evaluations for amateur mobile and portable operations, that doesn't
relieve you from the requirement for safety).


In comparison, you're not going to be driving somewhere the field
exceeds the limits, because they'll have a big fence keeping you beyond
the distance for uncontrolled exposure limits.


As far as the original question about through glass...

Glass isn't the lowest loss dielectric to make a capacitor of (heck,
even at the 100kHz used in tesla coils, it's lossy enough to crack from
the thermal stresses). The glass is the dominant source of loss in using
FR-4/G-10 PC boards at microwave frequencies, for instance. However,
even a spectacularly bad loss tangent of 0.01 is only going to result in
a 1% loss. (wood has a loss tangent of 0.03 or thereabouts at 100MHz,
depending on moisture level)

A bigger issue would be wires/conductive films for defogging.

Jim