In Reply to: Shorted Turns in RF Tank Circuits posted by Mike on 03/16/00 at 1:20 PM:
: Way back in my ham radio days I recall that a common method of switching frequency bands in a high power RF amplifier is to short out unwanted turns of the final amplifier inductor by switching a series of taps. The L/C circuit is often in a "pi" configuration. Once the required turns are shorted out (via a rotary switch), the plate capacitor must then be tuned for minimum plate current and maximum output. Well, I have two questions concerning this... 1. Why don't the shorted turns overheat, and overload the amplifier? 2. Why short out the unwanted turns, instead of simply switching taps on the inductor?Thanks!
There is something very wrong here. You do NOT want to short the turns of any coil in an RF tank circuit, whether it's a pi-network or something else! If you do that, you will have effectively created a shorted secondary in an RF transformer. You might be able to obtain some operation because the coefficient of coupling to that shorted secondary will be low, but that shorted secondary will carry a lot of unwanted current flow. Tthe correct way to change the inductive element is by selecting a particular tap on it and leaving the unwanted portion of the coil unconnected.
Resistance is anathema to an RF tank circuit, especially for a transmitter. For example, coil assemblies made by B&W were/are sometimes silver plated to keep resistance, especially with skin effect at RF frequencies, as absolutely low as possible. Tank circuit switching does not concern itself with resistance effects. Resistance effects are to be avoided as much as possible.
Take a look at some old Heathkit schematics (DX-100, DX-100B or Apache and yes, my age is showing) and see how pi-network switching was done with a series of switch selectable taps. Also, look at ARRL Handbook schematics of RF transmitting amplifiers and you'll see the same thing.
By the way, Mike, QRZ? This is WA2IBH.
John Dunn - President