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

[J. Forster]

On the disk, several things come to mind:

An electrostatic shield.
A shorted turn to increase the losses.
A magnetic field barrier to reduce the coil coupling.

I don't have such an IF to play with. Can you send me some rough
dimensions for this sketch:

   ///////     ||    ////////
-----------------------------  --
          XXXXXXXXXX            5
-----------------------------  --
   ///////     ||    ////////
   <--1--><-2--><-3-><--4-->

If you can get me some dimensions, I'll enquire.

As to finding the optimum, I'd try two (or better 3) of spacings, plot the
results, and pick what you want.

Best,

-John

===================







> 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
>
>