If you need a variometer for QRP device, I offer instructions for making it from very easily available parts. Such a variometer is reliable up to a power of 30 W, maybe even more, but I have not tested it with a higher power.
Material breakdown for the variometer
What we will need:
- 1 empty tube of stick glue for paper with dimensions external diameter approx. 29 mm, length 105 mm. The dimensions are not too critical, I give the ones I used.
- 1 ferrite core used as a suppressor element on supply cables to monitors with an outer diameter of about 17.5 mm, an inner diameter of 10 mm and a length of about 29 mm.
- suitable conductor for coil winding. I used a wire with a diameter of 0.35 mm with PVC insulation with an outer diameter of 1.2 mm.
- insulating tape.
Variometer construction
The construction is easy to understand from the pictures. We fasten the core to the sliding nut originally used for moving the glue stick by winding a suitable layer of insulating tape. We wind the appropriate number of turns of the coil on the case. We wind the coil to the open end of the case and secure the winding with instant glue.
I do not deliver a winding significantly longer than the length of the core. Winding 20 turns of said wire with a branch on 13 turn, I got such tuning, 20 turns = 9.5 to 19 μH, 13 turns = 5.2 to 10 μH and 7 turns = 2.3 to 4 μH. The only drawback is the absence of an indication of the core position, but if we tune the coil for optimal adaptation according to the measuring device, it does not matter too much. At the end with minimum inductance, the thread will stop us, and at the opposite end, it is possible to block the thread by deforming it with heat or some other stop and not to wind it at all costs. I did not have the opportunity to measure Q, but according to the deflection of the needle on the inductance meter, I estimate it to be 130 to 180. For the right QRP, a smaller case with a smaller core would also be sufficient, but I did not have it available. I want to try a variable capacitor on a similar principle, but I have to get another case. I will inform you about the results.
Miro, OM3CKU