Re: NEC-LIST: Hi-Q inductors

"Frank Donovan" <donovanf_at_erols.com> writes:
> Eric,
>
> The only issue I take with your approach is your use of
> PVC in high power RF applications. PVC supports fire
> if ignited by any source, such as an arc from any
> component in a high power RF amplifier.

PVC isn't particularly flammable compared to some other things. And
presumably one would design such that you're not going to arc or overheat?
Decent clearance distances, decent thermal margins?

I would worry more about thermal deformation which will cause a performance
degradation, long before it catches fire. PVC softens noticeably at fairly
low temperatures (50-60 C).

>
> PVC is a particularly bad choice for any use in a high
> power amplifier. I recommend that you rewind your
> coil on a form that cannot support fire, such as
> ceramic or porcelean.

There are lots of fire resistant composites (FR4?) which are lighter and
stronger than ceramic or porcelain.

>
> Another example of bad RF engineering is the use of
> combustible materials on antennas, particularly at
> high power. There have been many cases where
> cheap plastic insulators and traps with plastic housings
> or coil forms have ignited. Can you imagine flaming
> plastic dropping onto a wood roof or into a tinder
> dry forest during a drought?
>

Many cases? Can you cite a few? Used within their design limits?

Of course, if you melt a 100W rated antenna by running a kilowatt through
it, it's your problem, not that of a faulty design.

Not that one should design carelessly, but one should also put enough margin
into the system so that credible faults don't result in fire.

> There's more to good RF engineering than high Q...
>

Jim Lux

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