27 August 2007

Air core coils. An update

I'm willing to try a different output filter topology on my 4m TX, so now I need a 113 nanoH coil. I would also like to fit the transmitter into a metallic box to see if RF leakage during stand-by becomes more acceptable.

113 nH is 5 turns on 8mm diameter (drill bit), about 10.5mm long.

I decided to try with a thicker wire, say 1mm diameter. Even if I wound it using some good finger pressure, it did spring back. But they are only 5 turns, so once the support was removed I tightened the coil with a circular finger/hand movement.

Now I have a self-supporting 8x11mm coil whose inductance can be adjusted by stretching/compressing it.

So the coil wire diameter vs. coil diameter plays an important role on the need for a permanent coil former. How to find out? I haven't come up with a rule, sorry! Try and see, and YMMV.