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.)

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.

BTW, the solenoid inside an aluminium tube is a case of an inductor
surrounded by a single shorted turn... but if R in that turn is very
low, then little heat is generated in the tube.

We also sometimes use a movable shorted turn to adjust an inductor... a
brass or preferably silver plated brass slug was often used in VHF / UHF
tuned circuits.

Owen