[hpsdr] PennyWhistle T1 construction and measurement

[Graham]

Andy:

Inductance of turns on a common core go as the number of turns squared, not
proportional to the number of turns.

So I would expect the inductance of the secondary end to end to be about 4
times the inductance of one side.

That particular transformer falls apart, performance wise, below 1.8 MHz,
so measuring at 10 kHz is not meaningful.

The impedances quoted are for a loaded transformer, so if you have some
sort of antenna analyzer, like
an MFJ-259, if you put a 50 Ohm load across the secondary, you should see
about 50 Ohms looking into the
primary. But, only between about 1.8 MHz and 54 Mhz. Below those
frequencies, there is insufficient
inductance for the transformer to work at or above 50 Ohms.  Above those
frequencies, parasitics dominate.

If you think you have assembled the transformer correctly, I would go ahead
and continue assembling
the kit.  If you have wound something incorrectly, the amplifier may not
work properly, but unlikely you
will destroy something.

Continue with the build, and troubleshoot afterwards if necessary.

--- Graham / KE9H

==

On Sun, Oct 12, 2014 at 1:03 PM, Andrew Howell <andy at gamubaru.com> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> After sitting on my workbench for far too long, I finally decided to
> assemble my PennyWhistle.
>
> I have some very basic questions about T1 construction. I made the bifilar
> secondary and connected it up as shown in the diagram in the instructions.
>
> The instructions say is should be 50 ohms impedance on the primary, 50
> ohms on across the secondary, and 25 ohms impedance on between the
> secondary leads and the center-tap.
>
> I have an LCR meter, but I don't think that is going to help me, as it
> measures Z at a max of 10khz.
>
> I do measure 12.9 uH on the primary, and 12.8 uH on the secondary. The two
> sides of the secondary to center-tap are each 4.6 uH.  Those two secondary
> to center-tap inductances seem low. I would have expected 6.4uH, ie half
> the 12.8uH of the secondary.
>
> So, are these the values I should expect? Why don't the secondary to
> center-tap inductances add up to the inductance of the whole secondary
> leads?
>
> I expect I'm about to learn something :)
>
> 73, Andy - KF5JLJ
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