Effect of shorting turns on a tapped air cored solenoid at RF #2

Effect of shorting turns on a tapped air cored solenoid at RF offers a simple model for estimating the effect of shorting turns on inductance (L) and Q.

A correspondent sent me a set of measurements he made of an air cored solenoid using a Q meter.

The coil was a 22t air solenoid of length 99.5mm and radius 31.56mm. Q of the whole coil (L12) was measured at 6MHz to be 475.

L and Q were estimated and measured with three different tapping points at one end of the coil.

Whilst the method described in the reference article does not attempt to estimate the effect of tapping where the unused turns are left open circuit, we might expect than when the unused section is a small part of the coil, that the effect is similar to that if the unused turns were not there.

A model as described in the reference article was constructed.

The notation is L1 is the used part of the coil, L2 is the unused part, L12 is the whole coil with no taps, Lms is measured L unused shorted, Lmo is measured L unused open, like wise for the Q subscripts.

Above, the model results There is quite good reconciliation with the predicted behaviour.

Self resonance effects of L2

The model constructed seeks to estimate the effect of L2 shorts and does not attampt to estimate the effect of L2 open circuit though as noted earlier, if L2 is a small part of L12, we might expect it is as if it just wasn’t there (ie Qo=Qmo=QL1).

We cannot say that if L2 is large enough to approach self resonance. Under those conditions significant current may flow within L2 and consume power from the field, so reducing Q.

To short or to not short?

The model and experiments show that shorting just a few unused turns may significantly reduce Q whereas shorting a larger unused section has little effect on Q.

Experience tells us that large unused sections left open may reduce Q significantly and in high power applications may give rise to extreme voltages across switches etc. Parasitic resonances may give rise to stability problems in high power amplifiers.

A combination approach may give optimal performance, leave small unused sections open, but short larger unused sections. This is done in some proprietary ATUs, though the switch mechanisms have their own issues in practice.

Another method with switched coils is to arrange the coil in sections so that coupling into shorted sections is minimised. Again this is employed in some proprietary designs.

The problem in ham applications is greater for ATUs where just a small number of turns is shorted under some conditions (eg use on the lowest bands) whereas for valve PA PI couplers, it is less common that a design short just a couple of turns.

Notwithstanding this poor fit to application, the inductor of choice for the ham ATU market is a roller inductor, and this is one of the factors that contributes to their high loss on the lowest bands.


  • Duffy, O. Oct 2011. Tapping an air cored RF inductor. VK1OD.net.
  • —. Nov 2012.Palstar AT2K. VK1OD.net (http://owenduffy.net/antenna/AT2K/index.htm).
  • Fowler, A. May 1970, Radio frequency performance of electroplated finishes. Proc IREE Australia.
  • Knight, D. Feb 2016. An introduction to the art of Solenoid Inductance Calculation With emphasis on radio-frequency applications. http://g3ynh.info/zdocs/magnetics/Solenoids.pdf (accessed 25/03/2017).
  • Wheeler, H. Dec 1982. Inductance Formulas for Circular and Square Coils. Proc. IEEE (Letters), Vol 70, No 12, Dec 1982, p1449-1450.