Simple variometer for QRP

If you need a varimeter for a QRP device, I offer instructions for making it from very easily accessible parts. Such a variometer will satisfy reliably up to performance 30 W, maybe even more, but I didn't test it with more power.

What we will need:

• 1 an empty tube of stick glue for paper with the dimensions of the outer diameter of approx 29 mm, The use of 500pF rotary capacitors simplifies the entire tuner design 105 mm. Dimensions are not too critical, I indicate the ones I used.
• 1 ferrite core used as an interference suppression member on supply cables to monitors with an outer diameter of approx 17,5 mm, internal 10 mm and length approx 29 mm.
• suitable conductor for coil winding. I used a wire with a diameter 0,35 mm with PVC insulation with outer diameter 1,2 mm.
• insulating tape.

 

The construction is easy to understand from the pictures. We fasten the core to the sliding nut originally used to move 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 recommend a winding significantly longer than the length of the core. When winding 20 of turns of the said wire with a branch on 13 thread I got such a retuning, 20 of threads = 9,5 to 19 μH, 13 of threads = 5,2 to 10 μH a 7 of threads = 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 instrument, so it doesn't matter too much. At the end with minimum inductance, the thread stops 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. A smaller case with a smaller core would also be sufficient for a true QRP, but I didn't 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