VK1SV 136 kHz page

The new variometer/tuning inductor

The ferrite variometer has been a valuable aid but since it is very lossy, I decided to build an air-core inductor/variometer to see how much more antenna current I can put up the antenna.

The resuls is a 5.5 mH fixed inductor built on a 11 cm diameter PVC pipe and a 2.3 mH to 4.1 mH variometer, built on a 16 cm diameter pipe. Both pipes were salvaged from a near-by construction site and the enameled copper wire (ECW) was a freebie from the 2010 Gippstech (yet one more reason to attend this excellent conference - freebies!). The wire has a diameter of 0.75 mm (or so). I didn't know how much ECW was in the reel, so I started with the variometer and then decided to put the rest in the fixed inductor. The wire I think was re-claimed wire, so it had a few kinks and other dents, so it's not as smooth as it could have been, it's still though quite fine for that task.

So, here is the result:

The variometer is built on the larger diameter (16 cm) pipe, to allow for enough space inside for the rotating coil. The rotating coild has an inductance of about 450 uH which is rather high, but is done on purpose to allow for a wider tuning range - I am still experimenting with antennas, and that wide range is valuable to me.

The shaft is a wooden chopstick!

The variometer has plenty of space for additional turns. This again has been done on purpose to allow future expansion, if that is deemed necessary.

The fixed inductor is 5.5 mH which, together with the variometer, is enough to tune my current, small antenna at home. The length of the coil is about 25 cm and there are over 300 turns (roughly).


When I tested the new variometer, I was surprised by the difference in antenna current I was able to achieve with it, compared to the old, ferrite loaded one. The current increased from approximately 0.3 A to slightly over 0.5 A, which is a very welcome increase. A doubling in the antenna current results in 4 times increase in the radiated power (6 dB), which is great, given that the antenna or the transmitter have not changed!