Looking at this document "Heat Dissipation in Electrical Enclosures" and
a nice graph on heat dissipation vs total surface area for airtight
enclosures,
it would seem to me that:
1) assuming .1 db loss minimally in any balun design makes sense when
designing an enclosure. That's roughly 35 watts at 1500 watts transmit.
(I would even want to be able to tolerate twice that, designwise)
2) 35 watts would need probably 2 sq ft of surface area for heat
dissipation, to prevent the internal temperature from rising more than
100 deg F over ambient. Assuming airtight enclosure. Radiant heat transfer.
3) Balun enclosures are typically .5 sq ft of surface area or less. So
they don't dissipate the heat, leading to thermal runaway. Which is why
there's so much anecdotal information about failures.
Fixing that:
Passive air flow would seem sufficient. But it means that Baluns need to
be vented.
I've seen a lot of discussion about just how much loss there is, but it
seems to me that the real issue is correct enclosure design.
Can we agree that the real problem is that QRO Baluns require venting,
rather than airtight enclosures?
I thought the info that unpainted aluminum is bad for radiant transfer
is interesting. I may paint my DXE enclosure white and drill a hole
pattern for venting.
-kevin
ke6rad
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