On 10/20/10 2:04 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.

After taking a good look at the loading coil, its apparent that there
isn't much choice. The bottom of the coil is attached to the lower mast,
and a four pronged plate that the tap wire is attached to at the same
junction. So unless no tap is used, some portion will be shorted/
bypassed or the like.

And given that GLA systems is no longer in business, my loading coil
just got a lot more valuable, so I'll have to experiment on a new coil.
I have the Acrylic top and bottom pieces, next will be getting the rods
and make the wire cheannels.

- 73 de Mike N3LI -