The number of turns required to wind a coil will depend on the size and type of the
toroid cores you are using. The type 2 (red) cores are often used because they are
more common, but the type 15 (red/white) cores are somewhat better at 160m, as
they have 2.5 times the permeability and so require fewer turns for the same inductance.
You can find all the information you need to wind your coils at the Amidon website:www.amidoncorp.com/specs/
Let's take an example: looking at the tables for type 15 material here:http://www.amidoncorp.com/product_images/specifications/1-07.pdf
the AL value for a T-130-15 core (the 130 tells you it is 1.3" in diameter) is 250uH/100 turns.
Inductance goes up by the square of the number of turns, so we can calculate the required
number of turns for a 60 uH coil as follows:
ratio of desired inductance to AL value = 60 / 250 = 0.24
number of turns required = square root ( 0.24 ) * 100 turns = 49 turns.
That's probably too many to put on a core and still use fat enough wire, so let's try stacking
4 cores. That multiplies the AL value by 4 (approximately). Now we have a value of
1000uH / 100 turns. Repeating the calculations:
ratio of desired inductance to AL value = 60 / 1000 = 0.06
number of turns required = square root ( 0.06 ) * 100 = 25 turns.
That's getting more practical, though it gives you a relatively large change in inductance
when you change the number of turns by 1. You might want to make a "main coil" with
4 cores that has just enough inductance to resonate the antenna at the top end of the
band (or a bit above that), and then use a second coil on a single core to give smaller
inductance steps for fine tuning, putting both coils in series. You can then place taps
around the smaller coil to resonate the antenna in different parts of the band. You can
also experiment with connecting the bottom of the coil to ground and tapping your coax
a couple turns up the coil from there to provide matching to 50 ohms.
There are similar charts available for the type 2 material, which is available in larger sizes
(up to 5.2" diameter) than the type 15 (maximum size 1.57"). In spite of the larger size,
the AL value doesn't get much larger than that of the single T-130-15 core, so you'll
still need about the same number of turns, but it is easier to fit them on the larger core.