Re: Coil form
Malcolm and all:
TL>I have built a couple of PCB plate caps using double-sided PCB with
TL>borders removed. They worked reasonably well, but it is difficult to
TL>quantify as I was just getting going on a new system (rotten tune and
TL>all that). The major failure mode was arcing around the ends. I was
TL>running this cap in engine oil (never again - it carbonizes much too
TL>easily). I've often wondered just what the merits of fibreglass are.
TL>Plenty of low powered RF circuitry is constructed on these. Even
TL>stuff in the 100's of MHz range. In fact stripline filters and
TL>transformers are etched in as part of the circuits. The losses can't
TL>be too bad (at least at low power). I might have another crack at
TL>some stage but this time in transformer oil when I have a better
TL>controlled system running.
TL>** If it's under 10 Amps it's leakage current **
I say that using fibreglass as a dielectric is reasonably sound in terms
of the loss factor of the material: you are looking at tan delta of
0.005 to 0.01 for FR4 fibreglass. For 1/16" PCB material the breakdown
voltage would be high and the values of Er would be reasonably well
defined (typically 4.5) but this assumes there are no voids in the
material. FR4 fibreglass PCB material varies a lot between manufacturers
and between batches.
Manufacturers such as Rogers, who make PTFE based PCBs, impose careful
quality control on those products intended for high frequency (GHz)
boards and the likelihood of voids is reduced as is the Er (typically
2.2) but it costs a lot more.
You can often find big sheets of double sided FR4 that have been water
damaged such that the Cu is discoloured. You could make up a load of
boards whereby the top plate is brought through to an isolated area of
the bottom plate so that the boards could be stacked. If you used some
silicon oil and thin PE sheeting, I would suggest that the edge failures
that Malcolm commented on could be minimised. It might be worth trying a
bit of resistive field grading using blotting paper soaked in copper
The main problem is that the capacitance per unit area for 1/16 FR4 PCB
is small: approx. 25nF per square metre.
A quick and easy capacitor is a big reel of URM67 or RG213 coax or
similar. Then you have a capacitance of 30pF per foot near enough so a
500m drum would result in around 0.05uF. The problem is that you are
sourcing out of 50R Zo and hence have an associated inductance of 75nH
per metre which means for a 500m reel, nearly 40uH which is a lot of
unwanted "Leakage L". You could do with having access to both ends of
the reel so that the braid can be cut back and the ends connected
together. If you have a big reel knocking about, but you don't have any
capacitors to try , this would do to get you off the ground (NPI if
there is one).
CMPQwk #1.42 UNREGISTERED EVALUATION COPY