[ARC5] BC-455 IFT - photo attached

Wed Oct 29 18:48:38 EDT 2014

  Even if you had perfect magnetic coupling between the coils, the voltage induced into the 2mH coil
would be in series with over 25kOhms of inductive reactance. If the stray capacitance of the wiring
to the grid and the capacitance of the grid of the first IF to ground and screen totaled 10pF that would
be around 5.6kOhm capacitive reactance  to ground. You would have loads of signal loss via this path, especially if the magnetic
coupling is not unity.  Or look at it from a impedance ratio step up. The 2mH coil has lots of turns compared to the primary. 
  At unity coupling the turns ratio would be about 11 and impedance step-up ratio would be that squared or about 125.
So the impedance seen by the grid of first IF amplifier would be 125 times that of the plate of the mixer. The plate impedance of the
mixer is about half megOhm. That times 125 would be around 12.5 MegOhms as seen by the grid of the first IF amp.
  Not very effective. They effectively have a tuned parallel plate tank circuit directly coupled (as well as possible)  into the
grid of the first IF amplifier. We have not even included the interwinding capacitance of the secondary in this.
  However, as it is capacitively coupled to the 2mH coil the interwinding capacitance becomes a non issue since it
becomes part of the capacitance across  L6 and is tuned out along with the rest of the stray capacitance 
when you peak the tuned circuit.

73
Bill wa4lav

________________________________________
From: ARC5 [arc5-bounces at mailman.qth.net] on behalf of Dennis Monticelli [dennis.monticelli at gmail.com]
Sent: Wednesday, October 29, 2014 5:48 PM
To: Ian Wilson
Cc: ARC-5 Maillist
Subject: Re: [ARC5] BC-455 IFT - photo attached

No doubt there is significant winding capacitance but I don't think that
will affect the coupling coefficient we are trying to evaluate.

Besides the measurement of the net inductance at 2830 was inclusive of the
winding capacitance.  If fact, there was no way to separate it with simple
measurement techniques.

Dennis AE6C

On Wed, Oct 29, 2014 at 2:39 PM, Ian Wilson <ianmwilson73 at gmail.com> wrote:

> The untuned winding will have fairly hefty self-capacitance (recall that
> it's a little over 2mH, and is not split into smaller units in the way that
> RFCs often are).
>
> I would be surprised if adding 20pF across the untuned winding had
> much effect on the primary (but I have been wrong/surprised before).
>
> 73, ian K3IMW
>
> On Wed, Oct 29, 2014 at 2:33 PM, Dennis Monticelli <
> dennis.monticelli at gmail.com> wrote:
>
>> Ken,
>>
>> The pic is very helpful.
>>
>> The pie coil is not a toroid. It's wound axially not radially.  So it will
>> indeed couple to the single layer but not very well.  If there was a
>> iron-powder core running through the form, it would couple much better.
>>
>> One way to determine the degree to which the tuning on one side would
>> interact with the tuning on the other side is simply to solder a fixed
>> capacitor across the untuned side of this loosely coupled transformer you
>> pulled from the radio and then re-dip the assembly to see how far it moved
>> off resonance.  Try soldering about 20pF across the untuned coil to find
>> out how many kHz it moves.  Electrode plus wiring capacitance is probably
>> closer to a 10pF load, but a larger cap makes it easier to measure the
>> delta.
>>
>> Dennis AE6C
>>
>>
>>
>> On Wed, Oct 29, 2014 at 1:19 PM, Kenneth G. Gordon <
>> kgordon2006 at frontier.com
>> > wrote:
>>
>> > OK. Here is a photo of the IFT with the cover removed. It probably won't
>> > show up on the forum, but Dennis will see it, and if anyone else would
>> > like to see it, let me know.
>> >
>> > There is about a 3/16" space between the pie-wound coil and the single-
>> > layer coil, both wound on the same form. As I said, it looks pretty
>> loose
>> > to me. In addition, that pie-wound coil looks like a toroid to me,
>> > although it isn't wound on a toroid core. Even so, I would think that
>> > coupling to that form of coil would be somewhat problematic.
>> >
>> > I agree with Mike: a high-impedance broad-band input to the grid, AKA RF
>> > choke. :-)
>> >
>> > Ken W7EKB
>> >
>> >
>> >
>> >
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