Re: Quarter Wavelength Frequency

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

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

--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: "Anthony" <ant17-at-optushome-dot-com.au>
 >
 > hi guys can anybody tell me how to calculate
 > secondary quarter wavelength
 > resonant frequency not just the standard resonant
 > frequency
It might serve you well to read the current idiotic
thread entitled
Tesla Coil Construction Question...

where a guy has the idea that his secondary will
resonate in Mghz instead of Khz
From
http://groups.yahoo-dot-com/group/teslafy/message/1130
The standard thinking is that for
high h/d ratio secondaries, where yours is 45/6= 7.5,
the actual resonant frequency for your secondary may
be as high as 50% above the quarter wave calculation.
So here we make the quarter wave calculation.
You have 1048 turns -at- 6 inch diameter. Each turn is
then 1.57 ft, and the wire length then is 1645 ft.
This then is a quarter of the cycle for resonance or
we need to find the time period for 1645 ft * 4 = 6580
ft which equals 1.24 mile. C is 186,000 miles per
second...
1.24 mile/186,000 miles per sec = 6.7 *10^-6 sec
the reciprocal of this value is a cycle in that time
period, or about 149,252 hz. Given the fact that your
secondary has this higher h/d ratio, you may wish to
start out with a value 50% higher then the calculation
just made, or about 224,000 hz, or 224 Khz. I noticed
you were using the Mhz term, implying mega hz or
millions of hz. This should be incorrect, the
frequency wouldnt be that high given the no.s you have
shown...
Again to argue against the Mhz thinking of that
individual...
http://groups.yahoo-dot-com/group/teslafy/message/1134
Aha, you are employing radio frequency formulas often
quoted in Mhz. My Pocket Reference manual via Thomas J
Glover shows the following formula for RF in Megahz.
LENGTH OF AN ANTENNA
Quarter Wave Antenna:

Freq in Mhz = (984)/wavelength in feet.
Again the
wavelength itself is 4 times the actual quarter wave
value of the antenna, or in this case the distance of
your TC secondary length.
1.57 ft/turn * 1048 turns = 1645.36 ft.
Four times that for wavelength figure gives 6581.44
ft.
Applying the formula gives
MGhz freq = 984/6581.44 = .1495 MGhz = 149.5 Khz

Since the advise is to use a value 50% greater then
that for starters,(since the h/d ratio is so high) to
find the closer actual resonant freq for the
secondary, this would then be 224 khz, exactly as
before made as a prediction.

Historically there has always been a certain amount of
criticism on the tesla list for those asking about the
quarter wave length figure to calculate a secondaries
resonance figure, because typically it doesnt apply
for larger H/d ratio secondaries.
 >From
Answer to the quarterwavelength riddle
http://groups.yahoo-dot-com/group/teslafy/message/250
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.

The extraordinary persistance of the wire-length myth
comes from the willingness of people to accept things
on faith without making even the most basic of cross
checks. (Paul Nicholson)
See the comments and graph in
http://www.abelian.demon.co.uk/tssp/misc.html

For the idea of a natural resonant frequency of any
typical inductor, formulas may not apply, and even
scope measurements may be in question, since every
scope also has its own stated internal capacity, and
different scopes with different internal capacities
show those differences. For spirals containing massive
internal capacity, by employing flattened windings
with a plastic dielectric barrier between windings,
scopes with differing internal capacity give different
answers, and even the 1x vs 10x probe settings may
also give different answers. For a double set of Mega
Cable Radio Shack speaker wire spirals in four layers,
the differences between obtainable readings can vary
between 330,000 hz for ordinary returned winds vs
winding routes enhancing internal capacity where then
the same 25 pf internal capacity scope then records
250,000 hz. Those readings give values about 5 times
below what the the calculations employing
quarterwavelength would provide. Thus on the single
layered solenoid the resonant value may be higher then
the quarter wave value, and on coils employing
internal capacity, it may be much lower then the
quarter wave value. For a more comprehensive
mathematical proof involving how internal capacity can
be enhanced by winding routes in multilayered coils
see;
True Meaning of Bifilar/ 2-d Bifilar Calculations
http://groups.yahoo-dot-com/group/teslafy/message/582
"Thus by this data then the 1-d bifilar has 31% more
internal capacity as measured by these samplings then
does its adjacent layering method, and further then
the 2-d bifilar then has 590% more internal capacity
than its 1-d method!"
These URL's can be accessed by signing on to yahoo
group at teslafy at
http://groups.yahoo-dot-com/group/teslafy/
Sincerely HDN












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