Re: bang power ? (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 29 Sep 2007 12:32:58 -0700
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: bang power ? (fwd)

Hi Phil,

I just had to chime in:

>This would be one argument for the classic use of  variable off-axis 
>inductance for tuning - it doesn't affect "k". 
>  
>
Yep, off-axis tuning mainly changes L. Small change to M.

>Also a good reason for using a "good number" of  turns on the primary - 
>along with less frequency tuning  sensitivity, it would have less effect on 
>coupling.
>
Yes. Longer windings (higher turns) will decrease the ratio of change 
for L and increase the ability to fine tune a coil. Short small length 
primary's can be very sensitive to a change in frequency with only a 
small change on the primary tap. Larger turn coils are far less sensitive.

>Which relates to why I  was looking at Dr. Rzeszotarski's paper. I was 
>checking to see if I was within  recommended "surge impedance" guidelines 
>(although it seems this rule of thumb  has either been ignored or disproved since 
>then).
>
Dr. Rzeszotarski's MANDK is an excellent program! I don't think surge 
impedance guidelines have been disproved at all. It is difficult to 
quantify what is good and bad.

> 
>Tuning the primary by moving the tap along the  *inside* turns would give 
>finer frequency tuning with a correspondingly smaller  change in "k", correct?
>  
>
This is why I wanted to chime in. Just the opposite will occur regarding 
k. Although the change in L is smaller when tuning from the inside 
versus the outside, the change in mutual inductance and thus k will be 
far greater than tuning from the outside. The proximity of the inside 
turn has a large effect on k as compared to the outside turn. It's a bad 
idea in my opinion to tune from the inside. For example:

Given a 4" x 20" sec with 900 turns of 24 awg and a primary which has a 
6" id and 16" od for 10 turns: L is 30.06uH and k is at 0.128. The coil 
would need a 0.00371uF cap size (476kHz). Say you had a .004uF on hand 
and wanted to tune the coil to this cap size.  Well, you need your 
primary to get down to 2.798uH. If you tune from the inside keeping the 
OD unchanged at 16", you will end up with a 1.74 turn primary with an id 
of 14.26". K will be .072 (huge change from the original 0.128). Now if 
you tuned the coil from the outside, you will end up with 3.067 turns 
with an od of 9.068". K will be 0.132 (small change from the original 
0.128). This is kind of an extreme example, by I wanted to show the 
difference.

Tuning the primary by moving the tap along the "inside" turns would give 
finer frequency tuning with a correspondingly "larger" change in "k".

Take care,
Bart