raypsi wrote:
On Aug 27, 3:58 pm, K7ITM wrote:
On Aug 26, 12:05 pm, Tim Shoppa wrote:



On Aug 26, 2:26 pm, K7ITM wrote:
On Aug 26, 10:46 am, Tim Shoppa wrote:
...
The brass-screw-in-a-solenoid is something I just tried in my
reproduction of the MMR-40 and I am very impressed with its stability
and mechanical simplicity as a PTO. A similar but not identical
mechanical design is used in the WA6OTP PTO, I haven't tried it yet.
Tim.
Though not actually PTO if it's a brass screw, right?
Permeability of brass is slightly less than air, which is why the PTO
goes up in frequency as I screw the brass screw in.
It's not an iron-powder slug (which would go down in frequency as it
enters the solenoid) but it's still a PTO.
If you haven't tried them, I highly encourage you look at the PTO in
the MMR-40 and the WA6OTP PTO. Completely 100% homebrewable and I'm
very happy with the results.
Tim.
Well, I beg to differ with you about the (main) reason the frequency
changes. Consider what happens if you have two coils magnetically
coupled and you monitor the inductance of one while you apply a short
across the other one. Then consider what you'll see on the inductance
meter as you change the coupling between the measured coil and the
shorted one. The references I've been able to find so far suggest the
permeability of brass is slightly higher than that of air, but I
suppose it's a function of the composition of the brass; in any event,
I'd bet that the shorted-turn effect is very much larger than the
permeability effect with respect to changing the inductance of the
oscillator coil.

Cheers,
Tom

I agree, except Air has no permeability, you can't magnetize air. You
can't magnetize brass. What you have is a proximity detector. aka
metal detector.

73
n8zu
Brass slug acts as a shorted turn, absorbing rf and decreasing
effective inductance. You can tune a circuit with a link coupling coil
of a few turns connected to a low ohm value pot!