Owen wrote:
On 17/10/10 22:42, Alejandro Lieber wrote:
Since I built my first 80meter/40meter 6aq5 + 6DQ6 transmitter with pi
output in 1972, when I want to vary the inductance of a coil in a
tunner, or loading coil in an antenna, I just short circuit some turns.
I see that this is the usual practice everywhere.

My question is why do we not just leave the turns open circuited instead
of short circuiting them.

It appears to me that in the short circuited turns, a very big current
must be circulating, adding heat losses and lowering the Q of the
circuit.



Only fairly basic AC circuit theory is needed to analyse the effect of
the shorted turns.

If you have a air cored solenoid inductor of n turns, and short m turns
at one end, you can treat that as two independent inductors of n-m and m
turns with some flux coupling factor k. The mutual inductance can be
calculated, and a T equivalent of Ln Lm-n Rn Rm-n M elements constructed
and solved. k of course depends on coil construction and n and m, a
value can be determined by measurement of the reactance of the
combination. (You might be surprised at how low k is.)

One could look at one of the standard equations for solenoid inductance
(e.g. Wheeler's) and get a feel for it. The ideal fully coupled multi
turn solenoid would have inductance proportional to Nturn^2.

Wheeler (for inches) is: L (uH) = r^2 * n^2 / (9 * r + 10 * l)
so there's the n^2 term on the top, but there's also the 10*length term
on the bottom.
For 2" diameter, 5 turns/inch, I calculated Wheeler L and for comparison
Length^2/6 (so that the number would be comparable at a length of around
12")

length turns Wheeler L uH/inch Length^2
2 10 3.45 1.7241 0.67
4 20 8.16 2.0408 2.67
6 30 13.04 2.1739 6.00
8 40 17.98 2.2472 10.67
10 50 22.94 2.2936 16.67
12 60 27.91 2.3256 24.00
14 70 32.89 2.3490 32.67
16 80 37.87 2.3669 42.67
18 90 42.86 2.3810 54.00
20 100 47.85 2.3923 66.67
22 110 52.84 2.4017 80.67
24 120 57.83 2.4096 96.00
26 130 62.83 2.4164 112.67
28 140 67.82 2.4221 130.67
30 150 72.82 2.4272 150.00


You can see that for this kind of coil, the coupling from turn to turn
must be pretty low.. The L looks closer to a linear function of length
than to the square of turns. If it were perfectly linear, it would be
as if there is NO turn to turn coupling, and is just a series
combination of single turn uncoupled inductors. If you look at the
uH/inch column you can see that once you get into the 10 inches long and
up range, it *is* almost completely linear.




Essentially, when the power lost in the shorted turns is low (due to the
combination of low k and low R), then the technique works fine.

We (hams) have some pretty inadequate word based explanations for some
of these kind of things when there are simple quantitative solutions at
hand.

An example is the traditional explanation of link coupling ratios. See
http://vk1od.net/tx/concept/lctr.htm for a quantitative explanation
using the same techniques as suggested above.