I don't have any real experience with keeping a weathertight box outside
for an extended period, though. It would be good to get some opinions
and advice from people who do.
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Some time ago I was given an enclosure for an outdoor surveillance TV camera
which had a suitable heater (50 Watts resistor) switched by a bi-metallic
temperature switch. A homebrew solution would be an opamp circuit with 2
diode type temperature sensors,measuring in-and outside temperature
,maintaining an inside temperature approx 2degr C higher than the outside
temp.
This would be adequate to prevent condensation .
Seems a bit overdone at a first glance ,but would ensure protection of
valuable RF equipment.

Perhaps someone involved with Mobile Phone Cell Systems outdoor equipment
would care to comment.

Frank GM0CSZ / KN6WH