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?

Cheers,
Tom

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.

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

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

On Aug 28, 7:29*am, raypsi wrote:
On Aug 27, 3:58 pm, K7ITM wrote:
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.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

The "little" is key for the PTO designs I mention, it allows rather
fine tuning with very simple mechanicals. If I think of it as shorting
turns as it moves in, that is completely out of whack in my head with
the observed changes, because shorting all the turns causes a big
change in inductance, not a small one.

Tim N3QE

On Thu, 28 Aug 2008, Tim Shoppa wrote:

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

Which of course is why you see iron core and ferrite slugs in high
inductance coils, but in small inductance, like you'd see at VHF,
the slugs were brass.

Or, that tuning wand with some sort of ferrite at one end and brass
at the other, so you could tell whether the tuned circuit was too high
or too low.

At that level, brass and ferrite were seen as the same thing, a means
of varying the inductance. The choice was made by how much variation
you needed, and which way, rather than what action was taking place.

Michael VE2BVW

On Aug 28, 6:25*am, Tim Shoppa wrote:
On Aug 28, 7:29*am, raypsi wrote:

On Aug 27, 3:58 pm, K7ITM wrote:
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.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

The "little" is key for the PTO designs I mention, it allows rather
fine tuning with very simple mechanicals. If I think of it as shorting
turns as it moves in, that is completely out of whack in my head with
the observed changes, because shorting all the turns causes a big
change in inductance, not a small one.

Tim N3QE

Hi Tim,

OK, a 'speriment for you, and another way of looking at it that will
yield the same result:

Make two "slugs," one solid and one hollow. If it's permeability
that's doing it, the hollow one (with thin shell, of course) will have
considerably less effect. If it's the "shorted turn" effect, both
will be about the same.

Consider that there is no time-varying magnetic field inside a shell
made out of good conductor--really zero if it's a superconductor, but
dropping to practically zero after a few skin depths. So you are
removing a volume of magnetic field when you put a conducting slug in
the coil's field. With less field, there's less energy stored, which
implies lower inductance. The same thing happens when you put a coil
inside a shield-can: the inductance is reduced. A points that seems
to be not well known: the unloaded Q of a helical resonator is less
than the Q of the same coil that's not inside a shield. There are
graphs available to determine the lowering of inductance by a
cylindrical shield around a coil.

Cheers,
Tom

On Aug 28, 9:25 am, Tim Shoppa wrote:


I agree, except Air has no permeability, you can't magnetize air. You
can't magnetize brass.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

The definition I found for:
Permeability (electromagnetism), is the degree of magnetization of a
material in response to a magnetic field

In this case 1 means no permeability.

73

n9zu

On Aug 28, 4:31 pm, K7ITM wrote:
On Aug 28, 6:25 am, Tim Shoppa wrote:



On Aug 28, 7:29 am, raypsi wrote:

On Aug 27, 3:58 pm, K7ITM wrote:
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.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

The "little" is key for the PTO designs I mention, it allows rather
fine tuning with very simple mechanicals. If I think of it as shorting
turns as it moves in, that is completely out of whack in my head with
the observed changes, because shorting all the turns causes a big
change in inductance, not a small one.

Tim N3QE

Hi Tim,

OK, a 'speriment for you, and another way of looking at it that will
yield the same result:

Make two "slugs," one solid and one hollow. If it's permeability
that's doing it, the hollow one (with thin shell, of course) will have
considerably less effect. If it's the "shorted turn" effect, both
will be about the same.

Consider that there is no time-varying magnetic field inside a shell
made out of good conductor--really zero if it's a superconductor, but
dropping to practically zero after a few skin depths. So you are
removing a volume of magnetic field when you put a conducting slug in
the coil's field. With less field, there's less energy stored, which
implies lower inductance. The same thing happens when you put a coil
inside a shield-can: the inductance is reduced. A points that seems
to be not well known: the unloaded Q of a helical resonator is less
than the Q of the same coil that's not inside a shield. There are
graphs available to determine the lowering of inductance by a
cylindrical shield around a coil.

Cheers,
Tom
I agree Tom
It's not permeability that changes the inductance it's the mutual
inductance that is changing the inductance.
Any metal brought near a coil acts like another coil. If that coil is
out of phase. Then the inductance is lowered.
So with brass it's not the permeability it's the inductance of the
brass slug and the mutual inductance of the 2 coils that varies the
total inductance, of the main coil that the brass slug is in.

Towit a brass slugged adjustable coil is just a variometer.

73
n8zu

On Aug 29, 4:08*am, raypsi wrote:
On Aug 28, 4:31 pm, K7ITM wrote:

On Aug 28, 6:25 am, Tim Shoppa wrote:

On Aug 28, 7:29 am, raypsi wrote:

On Aug 27, 3:58 pm, K7ITM wrote:
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.

I think you're confusing permeability with susceptibility. Even vacuum
has non-zero permeability.

I'm still considering Tom's points. What I know from decades of
experience, is if I put a brass slug into a solenoid, the inductance
goes down by a little bit. "Little" is relative to the effect of a
ferrite slug, which makes inductance go up by a lot in comparison.

The "little" is key for the PTO designs I mention, it allows rather
fine tuning with very simple mechanicals. If I think of it as shorting
turns as it moves in, that is completely out of whack in my head with
the observed changes, because shorting all the turns causes a big
change in inductance, not a small one.

Tim N3QE

Hi Tim,

OK, a 'speriment for you, and another way of looking at it that will
yield the same result:

Make two "slugs," one solid and one hollow. *If it's permeability
that's doing it, the hollow one (with thin shell, of course) will have
considerably less effect. *If it's the "shorted turn" effect, both
will be about the same.

Consider that there is no time-varying magnetic field inside a shell
made out of good conductor--really zero if it's a superconductor, but
dropping to practically zero after a few skin depths. *So you are
removing a volume of magnetic field when you put a conducting slug in
the coil's field. *With less field, there's less energy stored, which
implies lower inductance. *The same thing happens when you put a coil
inside a shield-can: *the inductance is reduced. *A points that seems
to be not well known: *the unloaded Q of a helical resonator is less
than the Q of the same coil that's not inside a shield. *There are
graphs available to determine the lowering of inductance by a
cylindrical shield around a coil.

Cheers,
Tom

I agree Tom
It's not permeability that changes the inductance it's the mutual
inductance that is changing the inductance.
Any metal brought near a coil acts like another coil. If that coil is
out of phase. Then the inductance is lowered.
So with brass it's not the permeability it's the inductance of the
brass slug and the mutual inductance of the 2 coils that varies the
total inductance, of the main coil that the brass slug is in.

Towit a brass slugged adjustable coil is just a variometer.

73
n8zu

Kind of a variometer with external connections to only one of the
coils and the other one shorted out, I suppose. ;-) If you have an
actual variometer with two coils you can put in series, you can get
significantly more inductance variation than with a brass (or copper)
slug. In the case of series coils, you can both add and subtract:
you get the sum of the self-inductances of the two coils, plus OR
minus the sum of the mutual inductances. If the coils are tightly
coupled (not particularly easy to make, mechanically, for one coil
rotatable inside another...), the mutual inductances are nearly equal
to the self inductances--so for example if k=0.8 max and the coil self-
inductances are 1uH each, you can vary between 0.4uH and 3.6uH total,
a 9:1 variation. If you could manage to squeeze k(max) up to 0.9
through some heroic effort, you could go from 0.2uH to 3.8uH or 19:1
variation, enough to tune a tank over a 4:1 frequency ratio. Q isn't
very good with the coils opposing, though, since you still have the
same copper losses as when the coils are aiding.

Cheers,
Tom