Here is the problem:
For the capacitive element in the LC circuit we have
essentially two 660 pF caps in series.
This results in a total capacitance of 330 pf. I measured 362 pF.
To get a resonant frequency of 7.0 MHz with 362 pF we need
1.428 uH.
To get 1.428 uH on the PTO coil form we need about 21 turns
of wire.
21 turns on our coil form yields 1.440 uH and resonates with
362 pf at 6.9708 MHz
That’s pretty close to what we need, but the problem
arises when we screw in the brass tuning screw. This reduces the inductance and raises the frequency. Putting the screw
all the way in reduces the inductance to 1.138 uH resulting in a resonant
frequency of 7.8414 MHz. So with a coil
this large (that we must use if we want to tune down to 7.0 MHz) we end up with
a tuning range that is far too large. We
only need 7.0 to 7.3. In effect, this
means that we end up using only a small portion of the tuning range: We can turn the screw approximately 34 times,
but only 6 turns keep us within the range of 7 to 7.3 MHz (the 40 meter
band). There is about 50 kHz per turn of
the dial. This makes tuning
difficult. It becomes more difficult to
separate stations and tune them in. It
would be better if we could tune across the band using more turns of the dial. At least 15 turns of the dial would be
nice: That would mean about 20 kHz per
turn. But how can we do this?
Possible solution #1: Steel screw with tighter pitch on the turns.
Just using a steel screw slows the tuning rate down. In a normal PTO we increase the inductance
(and reduce the frequency) by gradually introducing a ferrous material that
increases the inductance of the coil, pushing the frequency of oscillation
down. But our brass screw is
non-ferrous. This means that putting it
into the core does not change the permeability of the coil. The permeability of brass is the same as that
of air.
What does happen, however, is that introducing the brass screw
into the coil causes currents to flow in the screw. These are called eddy currents. In effect they become shorted secondary coils.
And they have the effect of lowering the
inductance of the coil – this is why the frequency of the oscillator increases
as we screw in the brass screw.
When you use a steel screw you get both effects: As you
screw it in, eddy currents flow in the screw, reducing the inductance and
increasing the frequency of oscillation.
But you are also introducing ferrous material – this pushes in the
opposite direction, increasing induction and lowering the frequency of
oscillation. I think the eddy current
effect dominates, but the increase in permeability pushes in the opposite
direction. This means that with a steel
screw you have to use more turns to cover the same frequency range. And that is what we want.
For example, using the same coil, with screw of the same
thread pitch (the same nuts), with both screws ten turns in, one turn of the
brass screw moved the inductance .014 uH.
The same single turn of the steel screw only moved the inductance .005
uH. So just because of metallurgy, the
steel screw will lead to a lower (better) tuning rate. I used a Hillman 45479 screw
that is steel with a Zinc (anti-corrosive) coating.
But there is more: steel
screws are also available with tighter (#28) thread pitches. The Hillman 45479 uses this tighter thread pitch. This too means that more turns are needed to
move through the same tuning range. Again, that is what we want.
I found that using a steel screw with #28 thread pitch allowed
for the coverage of the 40 meter band in approximately 11 turns of the dial. That is much better than what we got with the
brass screw: About 27 kHz per turn
instead of the 50 kHz per turn that we got with brass. But it is not quite good enough. It would be better if we could use the
entire range of that PTO coil form.
Solution Two: Add
a fixed inductor in series with the PTO coil.
After some noodling, I decided to split up the
inductor: A portion of it would remain
fixed, the other portion would continue to be tunable.
I estimated that I was starting out with a coil of about
1.428 uH. So I just put a 1 uH choke in
series with the variable inductor and reduced the variable coil to about .428
uH (about 9 coil turns). This worked,
but it worked a bit too well! It would
not tune the entire 40 meter band. So I
figured I needed less fixed inductance and more variable inductance. I found an air-cored coil in my junk box and
cut it so that it measured about .650 uH.
I added turns to the variable coil, going to a total of 15 turns. This REALLY worked well and yielded the 26 or
27 turns to tune across 40 meters that you can see in the video.
TWEAKS:
Later, I tweaked it a bit more: With 15 turns of #22 wire on the variable inductor, a steel screw tuned from .791 uH (screw out) to .662 uH (screw in). I put one additional turn on the fixed inductor, making it .749 uH, or about 8 turns of #22 (wound tighter on a cardboard tube from a coat hanger than was the coil on the variable inductor). With these coils I could tune from 6.9772 to 7.386 MHz. That's a bit more than we need but this allows us to keep the tuning away from the ends of the coil where tuning is more likely to become non-linear. I am able to go from 7.0 to 7.3 MHz in 23 turns of the dial. And the tuning is quite linear: The first turn from 7.0 MHz moves the frequency 12 kHz. At the mid-point of 7.150 MHz, one turn of the dial moves the frequency 12 kHz. At the high end, going down from 7.3 MHz, one turn of the dial moved the frequency 11 kHz. That, for me, is VERY linear tuning. You probably will have to adjust the coils a bit (just squeezing the turns together or spreading them apart) to get the tuning range where you want it.
YMMV – Keep it simple!
Like they used to say in the commercials: Your Mileage May Vary. There are many ways of doing this. The objective is smooth tuning across the 40
meter band. I think that by varying the
pitch of the variable coil turns you could get a more linear tuning response (please let us know if you have any luck). You might also be able to get similar results
by changing the amount of capacitance in the feedback network (which is also the
frequency determining element in this simple Colpitts oscillator). But remember that simplicity and a low parts
count were also our objectives in this.
This mod adds only 1 part (the fixed inductor), requires the removal of
some turns from the main tuning cap, and perhaps the replacement of the brass
screw with a steel #28 screw and nuts.
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