Hi AJ,
What am I missing?
Let's try.
RF heating?
I just did the maths. With ~30µH and no stray effects you should be
getting a bit more than 1A of RF current in that choke. The RF
resistance of the wire at 14MHz is around 4Ω. So, maybe 6W of heating
from RF, in addition to some smaller heating from the DC. Not enough to
blow up a ceramic insulator, I would say. So it's not this.
You said you "looked for any series". I understand you mean that you
checked for series resonances and didn't find any in the band. But
that's not the whole story. Did you actually measure the impedance of
that choke (magnitude and phase) at 14MHz? If not, then try to do so.
I would not be surprised to find such a choke having its main resonance
far below 14MHz. Which means that at 14MHz it would have a capacitive
reactance, and possibly a rather low one. In that case the RF current in
it would be much larger than 1A, and the heating from RF current would
be correspondingly higher (I²R). That could well make the insulator
burst from thermal stress.
I assume the ceramic you are using is fine for RF work. If not, it could
be so lossy that it heats up from the inside. You can detect this by the
choke measuring a reasonable inductance but a low Q. Or on an impedance
meter, a reactance in the area of a few kiloohm in series with a
resistance of more than 10Ω or so. And when sliding the coil off the
insulator, the Q gets much better, or the series resistance getting much
lower. If that's happening, look for a more suitable insulator.
By the way, a longish closewound coil like that one can hardly have a Q
much higher than 200. Probably less. At 3kV and 1A in it, the reactive
power in that choke is around 3kW, assuming it's not higher from
parasitic capacitance. At a Q of 200 that means 15W of RF dissipation,
and that's pretty much a best-case assumption! If the actual impedance
is lower due to capacitance, the dissipation will be larger. And if the
Q is lower than 200, the dissipation will be higher too.
You very likely need a better choke.
Can you measure the choke's temperature after a two minutes transmitting
or so? Either with a non-contact IR thermometer, or else VERY CAREFULLY
by quickly shutting down the amp after transmitting, making ABSOLUTELY
sure all high voltage has been discharged, and then opening the lid and
touching the choke with your finger. If it smokes and smells like
barbecue, and you find yourself in dire need of practising your
vocabulary, you know you have too high loss in that choke. But do this
finger test ONLY if you have a completely safe discharge system there.
In the heat of the battle it's just too easy to do something stupid, and
if you take your time to triple check that the high voltage has drained
off, the choke will have cooled off too.
The recipe to survival of the choke is very likely a better choke. One
that has lower RF loss, if possible, and which is large enough to safely
dissipate that loss. A sharp airflow around the choke would help too.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|