Hi Ian, This is for a more broadband application. I saw a thing a while back (don't recall quite where...) that had a row of discs which could be rotated into a coil. I looking for a range of perhaps: .1 to 2 uH If it wasn't for the current, I'd try making a variometer. 73 & Good morning, Marv WC6W P.S. -- The link doesn't come up here... maybe I have the wrong version of Acrobat or something else wrong... -- wrote: Marv wrote: > >Anyone happen to know anything about the numbers involved with tuning coils by interposing copper (or silver!) vanes between the turns? > >Specifically, what order of max-min ratio is obtainable? > >This is for a low impedance application therefore, the currents run high, indicating against a roller inductor or bandswitch. A rotatable shorted turn is sometimes used as the tuning element in single-band pi- tanks for 6m, but even when the shorted turn is buried completely inside the coil, it will only change the inductance by 10-15%. There's an example in: http://www.newsvhf.com/6mstripline.pdf (5MB file. Also the filename is misleading - it's not a stripline, but a pi-tank.) A vane would operate by induced eddy currents, but it would have poorer magnetic coupling to the main coil than a 'buried' shorted turn does. I'd guess that the available decrease in inductance would be significantly less than 10%. The surprising thing is how little heat is lost in the shorted turn, even at the 1kW level. The secret seems to be to avoid making a joint in the loop. Methods that work include making the loop from a very short length of seamless copper pipe, or as a flat copper disc with a large hole in the middle. 73 from Ian GM3SEK ___________________________________________________________________ Try Juno Platinum for Free! Then, only $9.95/month! Unlimited Internet Access with 250MB of Email Storage. Visit http://www.juno.com/value to sign up today!