> > I wanted to improve that also. I called and spoke with
Bob Locher, W9KNI,
> > at Butternut. He said the voltage wasn't the only issue
and that the N750
> > doorknobs would become less stable at higher power
because of their
> > temperature rating. He suggested not only increasing
the voltage, but using
> > NPO-rated doorknobs.
NP zero (not NPO) indictates a negative positive drift of
approximately zero PPM per degree.
You will probably only get an NP0 in low value HV ceramic
transmitting caps. Most transmitting caps turn out worse
for drift than specs call for when they are actually
measured. Don't ask me why, but that's the way it is.
> The best way to attack the problem is to search hamfest
fleamarkets for
> those old fashioned bakelight cased mica transmitting
caps.
Good idea. Mica's are normally good compared to ceramics for
drift. The problem also relates to Q. It's the ESR (series
R) of the cap that causes the heating. Butternut uses a
series resonant circuit and that aggrivates current through
the "trap" and frequency change with capacitance drift. A
regular trap would have less current and less drift.
Multi-layer micas have lower ESR and better TC.
Unfortunately they often have less voltage rating.
Paralleling two ceramic caps cuts current in half, doubles
surface area. The drift with temp also is less with lower
value caps. Under ~150pF you can even find NP0 caps. They
take a drift jump around 250pF 500pF, 1000pF, and so on
depending on vendor.
73 Tom
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