On 10/20/2010 03:04 PM, Owen Duffy wrote:
Mike wrote in news:i9n3jq$ds62$1
@tr22n12.aset.psu.edu:

...
So to return to my real world example, an air core solenoid used as a
tuning coil for a bugcatcher antenna, would I be wanting to short the
unused portions of the coil, or leave them unshorted?

Seems that unshorted would be bad.

I wrote some notes based on a simple model of an air cored single layer
solenoid, they are at
http://www.vk1od.net/tx/concept/TappedCoil/index.htm .

The model suggests that shorting the unused turns is a poorer solution
when the flux coupling factor is relatively high, and a very small number
of turns are shorted. Poorer both because of loss and the granularity of
L adjustment.

In that situation, the voltage induced in open unused turns is not very
high, whereas it can be extreme in cases where most of the turns are
unused.

So, a combination of methods may be optimimum, depending on the flux
coupling factor, voltage withstand, granularity of variation of L, etc.

Owen

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns add.


--
Alejandro Lieber LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map foF2:
http://1fcr.com.ar

Alejandro Lieber wrote
in :

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns
add.

Hello Alejandro,

I think the graphs in the article demonstrate that there is a simple
analytical solution, giving the appropriate parameters.

There are issues about the applicability of the model to the real world,
but the model does show trends about what is desirable and undesirable
practice. Wheeler's formula for inductance has its shortcomings
(Wheeler's formula doesn't take into account wire diameter for instance),
but it is (IMHO) sufficiently accurate to expose some of the
'interesting' effects that prompted your initial question.

The model is based on basic electricity and magnetism, stuff that isn't
so appeallng to new age hams.

Owen

Owen Duffy wrote:
Alejandro Lieber wrote
in :

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns
add.

Hello Alejandro,

I think the graphs in the article demonstrate that there is a simple
analytical solution, giving the appropriate parameters.

There are issues about the applicability of the model to the real world,
but the model does show trends about what is desirable and undesirable
practice. Wheeler's formula for inductance has its shortcomings
(Wheeler's formula doesn't take into account wire diameter for instance),
but it is (IMHO) sufficiently accurate to expose some of the
'interesting' effects that prompted your initial question.


I think that for the purposes of looking at the effect of shorting/not
shorting turns, Wheeler is more than adequate. the key is the fact that
it encapsulates the difference between the Nturns^2 (zero length
solenoid with all turns coincident, fully coupled) and Nturns (zero
coupling) for dimensions that are "practical" for ham use.

And the other interesting thing is that the graph of inductance vs
length for coils of interest here is that it looks pretty linear (and
the fact that the coil stock vendors refer to "uh/inch" kind of confirms
that)

Jim Lux wrote in
:

I think that for the purposes of looking at the effect of shorting/not
shorting turns, Wheeler is more than adequate. the key is the fact
that it encapsulates the difference between the Nturns^2 (zero length
solenoid with all turns coincident, fully coupled) and Nturns (zero
coupling) for dimensions that are "practical" for ham use.

And the other interesting thing is that the graph of inductance vs
length for coils of interest here is that it looks pretty linear (and
the fact that the coil stock vendors refer to "uh/inch" kind of
confirms that)

An interesting bit of trivia for the models I created is that the flux
coupling coefficient doesn't vary much with turns for a given diameter
and coil pitch.

When you make relatively large diameter coils of fine pitch, k is higher,
and that creates the conditions for higher loss in shorted turns.

The implication for long loose coils is that k is low, mutual inductance
is low, inductance approaches a constant L per unit length etc.

Sensibly, most air cored solenoids operate in the midrange.

Owen