[ARC5] Easy selectivity improvement....isn't... (long)

[J. Forster]

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 vally. 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.

-John

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






> The article in ER magazine about moving the ceramic spacers by Norm
> Chipps discusses how ARC did it, not how "normal" hams did it. It seems
> clear to me by now that no ham I have ever heard of tried this procedure,
> and there is no wonder that it hasn't been tried....at least to my
> knowledge.
>
> For us, the process is most certainly NOT easy, by any stretch of the
> imagination.
>
> I've spent an hour or several dismantling a 1415 KHz IF transformer and
> examining it closely, shopping for and buying suitable parts, then
> reassembling it with increased spacing between the coils.
>
> The longer ceramic spacer at the bottom is threaded: 3-48 on the top and
> 4-40 on the bottom. This makes it impossible to simply move from the
> bottom to the center. That threaded "spacer" holds the entire assembly
> together, in fact.
>
> Furthermore, finding 3-48 screws longer than 1" is almost impossbile,
> although McMaster-Carr does have a 3' (foot) long piece of 3-48 all-thread
> for $1.97.
>
> The two smaller spacers between the two coils are not threaded, and the
> hole through the center of each one is larger in diameter than the long
> 3-48
> screw that goes down through the center of the assembly to hold it
> together.
> The reason for this size difference will be clear a bit further down this
> e-mail.
>
> There is also a metal disk with a hole through the center that is spaced
> midway between the two coils. The small ceramic spacers rest against
> either
> side of this metal disc, and the IF coils rest against the small ceramic
> spacers..
>
> Although I haven't yet checked to be certain, that metal disk appears to
> be
> grounded. At least it is soldered to one of the wires that connects to one
> of
> the pins on the bottom connector, and what appears to be the center-tap of
> the top coil is also soldered to this same wire.
>
> I am not sure what is the purpose of this metal disk. Perhaps someone here
> can tell us what it is for. I suspect it has something to do with
> "adjusting" the
> passband curve since it probably constitutes a shorted turn. Or maybe it
> is
> simply a shield between the two coils.
>
> Lastly, there is a fiber insulated "pipe" which surrounds the long 3-48
> screw,
> insulating it from the metal disk and from both of the ferrite cores of
> the coils
>
> So, I went to the local Michaels craft supply store today, and found a
> fairly
> large tin of plastic beads that are about the same diameter, and have
> about
> the same sized holes through their centers as the small ceramic spacers
> and
> bought this. It was $6.00 and has enough "beads" in it to "do" several
> hundred IF cans, if I ever felt that ambitious.
>
> I also stopped at our local hardware store and bought 6 ea pieces of 3"
> long
> 4-40 "all thread". Those were more expensive than the entire large can of
> beads.
>
> The IF can has a piece of fiber "square stock" about 1" long and about
> 3/16"
> square at the top of each can which holds the assembly centered in the
> can,
> and through which the long 3-48 screw goes to hold the assembly together.
>
> I carefully wrapped one layer of electrical tape around about 1 3/4" of
> the
> 4-40 all-thread, leaving about 1/4" un-covered at the "top" end and an
> inch or
> so at the bottom end.
>
> I then cleaned out the centers of the two coils' ferrite cores with a 1/8"
> drill bit
> as there was some rust and bits of glue in each one, then starting at the
> bottom and after enlarging the hole in the bottom piece of mica with my
> Dremel tool and a grinding bit, passed the insulated 4-40 all-thread screw
> up
> through the bottom coil, one of the plastic beads, one of the small
> ceramic
> spacers, through the metal disk, another of the small spacers, another
> plastic
> bead, through the top coil, and then carefully "screwed" the end of the
> 4-40
> all-thread into the fibre spacer at the top while making sure that the
> coils
> didn't rotate..
>
> This assembly JUST fits in the necessary space, and when I cut the end of
> the all-thread off, and add a 4-40 nut, flat washer, and lock washer, the
> assembly will be as solid as it was originally, and will plug right back
> into the
> receiver.
>
> I did this mechanical "exercise" before I had taken the trouble to measure
> a
> properly aligned receiver's IF passband and its shape since after
> dismantling
> the IF transformer I was not at all certain that I could get it back
> together with
> the increased spacing between the coils that would be needed to achieve
> increased selectivity.
>
> It can be done, but it certainly isn't easy.
>
> Also, I am not at all sure that my spacing might not be too much, and will
> badly effect gain through that stage. The spacing between the coils is
> about
> 1/16" greater than the length of the long ceramic spacer which was
> originally
> suggested to be used to increase the spacing between the coils.
>
> So, the next exercise will be to carefully align a receiver with a
> standard set
> of IF cans in it, then do the same with a set of "enhanced selectivity" IF
> cans
> in it, then measure passband shape and selectivity.
>
> I'll "rebuild" the other two IF cans, then will see what I get when I
> measure
> the IF passband and gain.
>
> Lastly, I am not at all sure, at this point, that the result is worth the
> effort.
>
> Kenneth G. Gordon W7EKB
>
> "Courage is being scared to death but saddling up anyway."--- John   Wayne
>
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