Large H/D ratio Secondaries

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Harvey Norris <harvich-at-yahoo-dot-com> 

Just finished my first sensible TC secondary attempt.
Like to build more of these after this first one is
tested... Years ago I built a large sonotube model
using just some 270 winds of ordinary household
insulated 14 gauge wire, that secondary only made 2-3
inch arcing, at 1.5/1 H/D ratio. Then the quarter wave
deviance was still obvious, near 210 Khz vs 166 Khz...
Of course the finer gauge wire secondaries deliver
more of the effect of arcing so my second try is
sensible, about 925 winds of 23 gauge wire on 4.5
diameter schedule 40 PVC pipe, just over 22 inches
h value so the H/D ratio is then 4.88

A look at Paul Nicholson's excellant chart at
http://www.abelian.demon.co.uk/tssp/misc.html shows
that we might expect the highest deviance values of
quarter wavelength values at the higher H/D ratios. I
noticed on his chart that the midrange H/D ratio is
between two and six, so this a large range, but I did
not base my primary frequency designation according to
that chart, but just went with the quarterwavelength
"approximation used as the first estimation of the
probable resonant frequency" was how I based my
primary tuning circuit on, which is the ~quarter
wavelength of my coil being about 225,000 hz. Loh and
behold my first firing produced hardly nothing on the
secondary, even with Oudin connection to primary.

What I had assumed was that when one typically adds a
globe to the coil as a top load, this also reduces the
resonant frequency, so then in contrast we might still
use the quarter wave method and still at least get
some kind of decent arcing. But according to Paul's
good advise for large H/D ratios;

The 1/4 wave resonant frequency of a wire, when wound
into a solenoid, is typically more than 50% higher
than that of the straight line value.

Of Course I didnt use Medhurst changes either, but now
I see how a tesla coil project can get a little bit
challenging, and it just aint a simple thing to
predict a resonant frequency.

According to this thing I should retry the coil for
337,000 hz and expect the secondary to then respond.

Another question I have is how close to the demand
value of capacitive draw is best for operation. My
first guess called for 25 nf, using 20uH primary but
the supply was only able to pass its full current at
5.3 nf at rated value.(not considering resonant
effects) Having 4 of these NST's in parallel would
meet the demand of the supply. It has been stated
1.5/1  is okay for LTR operation. Doesnt this mean
that the inductance of the primary design would be
dependent on the amount of supply being made by the
estimated transformers in parallel? For example at
20uH primary,using 25 nf capacity; 4 transformers
would be needed to meet that capacitice reactance draw
supply but higher inductances of primaries,in turn
mean lower capacitive values being employed for the
resonances, where then the demand comes closer to the
potential amount of supply so for any pariticlar
design, the amount of transformers in parallel should
be structured to be 150% of the estimated draw,
according to that LTR premise.

In fact on the first firing based here, the demand is
four fold far more then the supply, but a narrow arc
gap is used to neccesitate arcing at about 100 volts
variac to primary. No significant secondary effects
were noted at that estimated 225,000 hz employed as
primary combination. A retry at higher frequencies
seems sensible.

HDN