[Prev][Next][Index][Thread]

Re: Secondary design help

Tesla List wrote:
> 
> Original Poster: "Mark Broker" <broker-at-uwplatt.edu>
> 
> I'm building my first "big" coil, and would like to make it as good as
> possible without making too many mistakes.
> 
> So far, my design consists of a 1.8kVA input (dual 15/60 NSTs) with an
> LTR-style MMC (56nF).  My primary is .50" copper tube spaced .50."  I'm
> working on a SRSG at 120 BPS.  I'd like to use a 6.27" OD, 6.07" ID x ~30"
> PVC secondary with 22AWG wire.
> 
> The only areas that I'm still trying to investigate are the secondary
> design and the topload.  First, I've read that PVC doesn't make a very good
> form due to its high RF losses.  Secondly, I'm concerned that 22 AWG is too
> small, and that I should use 20 or 18 AWG.  Finally, I'm also concerned
> that the topload is too small ("Top loads can NEVER be too big").  I've
> thrown a 6"x 24" toroid into WinTesla, with a capacitance of 32pF.  I've
> seen on one Web page that the recommended toroid size for a 6" coil is
> 8"x40" (60pF) (in conjunction with an anit-corona ring, which I may add
> after I get it built and working well).
> 
> So, my questions are:
>     1:  Will I gain much by using a PE (or similar) form over a PVC form?
>     2:  Is 22AWG wire OK on a 6" coil running at 1.8kVA?
>     3:  Is my topload of 6x24" too small for my coil?  (I DO intent to make
> a couple, but I'd like an idea of where to shoot for)
> 
> Thanks.
> 
> Mark

Hi Mark!

For the most part, it sounds like you've got a nice, solid design. A
couple of possible suggestions:

1. At the moderate power level you'll be running, 1/2" tubing is bigger
than necessary. While it won't hurt, you can use either 3/8" or even
1/4" copper tubing and save cost and reduce the physical size of your
primary with virtually no performance degradation.

2. Don't be overly concerned about using PVC for a coilform - it turns
out that RF losses in a PVC coilform will have virtually _no_ observable
impact on your coil's performance. And, PVC's high breakdown voltage,
high resistivity, availability, and low cost place it among the best
commonly available materials for a coil form. From an insulation
resistance standpoint, it's actually a better performer than
poylcarbonate or acrylic. And, you can bet that Nikola would have killed
for the stuff... :^) 

3. #22 AWG will work perfectly well for your 6" coil. If you've got it,
use it. Or, if you've got #18 or #20, you can use either of them as
well. It really won't have much of an impact on your coil's performance
compared to some of the other elements, such as having a solid,
low-resistance/low-inductance RF ground.

4. A topload CAN be too big! At least, it can be so big that your system
refuses to break out because the toroid's self-capacity is too large, or
its radius of curvature is too big. Either situation can conspire to
prevent the coil's output voltage from being sufficient for breakout.
While you should have no problems using a 6" ROC toroid, you may
experience a bit of difficulty getting unaided breakout with an 8" ROC
toroid, particularly if it's ALSO 40" in diameter. 

The corona-ring/toroid, or a couple of toroids (smaller one closest to
the top of the secondary) will permit you to try various sizes as you
dial your system in and begin pouring the coals to it. This is one of
those "trial and error" things, and it's good that you're planning on
making a couple different sized toroids. 

Good luck, and safe coilin' to you, Mark!

-- Bert --