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Subject: Re: Shorted Turns in RF Tank Circuits

Date: 04/23/00 at 9:56 AM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: Re: Shorted Turns in RF Tank Circuits posted by John on 04/23/00 at 1:00 AM:

: : 1. The shorted turns do note overheat because there is no real change in resistance, it is still the resistance of the short length of wir.: 2. Switching the taps on the inductor will not change the overall inductance of the wire. Shorting out the turns actually reduces: the number of turns, or unwinds them without the physical act of unwinding. The number of turns is critical for inductance, changing: the tap point does not change the inductors value.I think the concern is the coupling between the operational turns and the shorted out turns. The answer to that is, that being air core, the effect is negligible.We routinely use RF equipment that generates up to 150 KW. This equipment is tuned by shorting out the appropriate number of turns in the tank coil. All it does is to change the effective inductance of the tank. It should be mentioned that there is an output transformer in the tank also, which is effectively in series with the tank coil, and the output is inductive as well. So therefore the tank coil is not the only inductance in the tank circuit. But I will say that there are no appreciable extra losses developed in the tank coil as a result of its being shorted for part of its length.It must be that the fact that it is an air core inductor prevents it from acting as a shorted out autotransformer, which is what I think the main objection was.Regards,John

Hi, John.

Your description about the shorted turns is correct. It is essentially the idea I wanted to convey to Mike Gray. I started making a sketch of it, but I never got around to finishing. However, even if you've been successful at 150 kW, I still have to disagree with the basic idea of shorting out unwanted turns.

When you short the unwanted turns, even though the coupling coefficient between the shorted and non-shorted turns is small, it is not zero. Therefore, there will be a circulating current induced in the shorted turns which serves no functional purpose, but which does cause some heating loss with whatever resistance may be in that area.

It may be that a good coil design, nice and big with a layer of silver plating, may make this permissible, but I disagree with the basic philosophy of permitting that circulating current to exist at all.

Thank you for elucidating the basic effect and also for your experience with the practicality. Clearly, it can be and has been done.

John Dunn - President
Ambertec, Inc.

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