Air Coil Calculation
This page will explain how to calculate coils and will explain how you can make your own coils.
I have made coils with different number of turns and measured the inductance.
Formula :
Where:
L = inductance in micro henrys
D = coil diameter in inches (from wire center to wire center)
l = coil length in inches ( 1 inches = 25.4 mm)
n = number of turns
There are lots of Internetpages explaining how to make coils, but there are few that will give you exact values. I have been experimenting with different diameters,
length and wirediameter and found one good combination.
To make the coils, I have used a two drills with a diameter of 7.2mm and 5.8mm. The diameter is chosen to give wide range of inductance and the coils will get good
Qvalues. The wire for the coils has a diameter of 0.6mm. The diameter of the wire is not so critical. The tables below shows the results.
INDUCTANCE TABLE (diameter 7.2 mm, 0.6mm wire)

Number of turns 
Inductance (nH)
(Compact coil) 
Qvalue 13MHz
(Compact coil) 
Inductance (nH)
(Airspace coil) 
Qvalue 13MHz
(Airspace coil) 
3 
77 
407 
66 
440 
4 
122 
325 
102 
560 
5 
177 
340 
 
 
6 
240 
440 
206 
550 
7 
306 
509 
290 
690 
8 
379 
607 
319 
1300 
9 
470 
1500 
422 
>1500 
10 
582 
>1000 
515 
>1000 
11 
644 
>1000 
 
>1000 
12 
656 
>1000 
545 
>1000 
13 
745 
>1000 
612 
>1000 
14 
789 
>1000 
658 
>1000 
INDUCTANCE TABLE (diameter 5.8 mm, 0.6mm wire)

Number of turns 
Inductance (nH)
(Compact coil) 
Qvalue 13MHz
(Compact coil) 
Inductance (nH)
(Airspace coil) 
Qvalue 13MHz
(Airspace coil) 
4 
92 
540 
79 
 
5 
131 
370 
120 
530 
6 
175 
340 
155 
500 
7 
220 
300 
184 
640 
8 
272 
370 
234 
560 
9 
315 
470 
267 
770 
10 
363 
650 
313 
1270 
The coil can be compact with no space between the winding or you can pull it out, to make some space between the windings. My measurements shows that you will get the best
Qvalue if you pull out the winding so the airspace is 0.6mm between the turns in the winding. The inductance will drop a little, se the table above.
* Compact = There is no space between the windings in the coil!
* Air space= There is an airspace between the winding with the same length as the diameter of the wire you are using (0.6mm in this case).
73s
VU2FD 