[ARC5] Easy selectivity improvement....isn't...

[Kenneth G. Gordon]

On 6 May 2013 at 17:18, J. Forster wrote:

> There is a classic physics experiment that is relevant here:
> 
> The experiment is about peak broadening in coupled oscillators. It
> takes a 'scope and sweep generator.
> 
> Basically, you set up a couple of loop coils, a few inches in diameter
> and w/ 20 odd turns. The number is irrelevant, but they must be the
> same for each coil. You then resonate both coils, far from each other,
> with identical capacitors.
> 
> Next you set up the 'scope and sweep generator (with a series R to
> make it look sorta like a current source).
> 
> Now the experiment. You bring the second coil in, closer to the first
> coil. The peak on the 'scope will broaden, and eventually split into
> two peaks separated by a valley. This is the well-known 'rabbit-ears'
> response.
> 
> Basically, this demonstrated the effect on resonance of the mutual
> inductance term.
> 
> Applying this to IF transformers, by separting the windings, reduces
> the M term and sharpens the field.

Well, John, that is exactly the effect I am hoping for, and which ARC 
achieved after WWII by increasing the spacing between the coils. 

However, what I am afraid of is that I may have separated them TOO far and 
this will badly effect the gain, reducing it too much.

I think there is a compromise which must be watched for here between the 
selectivity obtainable vs gain...or the lack of it...through the IF transformers.

In order to adjust that spacing with the materials I have here, I am going to 
have to cut the plastic beads down a little, and measure the result, then do 
this several times.

Also, might you have any idea what that metal disc which is between the two 
coils is for?

Ken W7EKB