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Subject: Re: Shorted Turns in RF Tank Circuits

Date: 03/17/00 at 2:33 PM
Posted by: Mike
E-mail: mike@win.bright.net
Message Posted:

In Reply to: Re: Shorted Turns in RF Tank Circuits posted by John Dunn - Consultant on 03/16/00 at 11:18 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!

: : : Mike:

: : : 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.

: : : Good luck.

: : : John Dunn - President
: : : Ambertec, Inc.
: : : ambertec@ieee.org

: : Thanks for the response, John. I'll answer your final question first. My call sign was WA6YMJ. That was my call when I lived in California. I now live in
: : Wisconsin (since 1979). I let my license expire about 25 years
: : ago! I haven't done anything with ham radio since. I guess I got much more excitement out of playing with high voltage, X-rays,
: : ultrasonics, and other fun stuff. Ham radio bored me (sorry).

: : As far a shorting turns in the tank circuit, that wasn't MY design. It was a conventional practice in the tank circuit of many
: : RF amps. I will try to find a published example of this, and I'll contact you when I do. Thanks again, John!

: : Mike Gray

: Hi, Mike.

: I'd really like to see that! I've always seen switched taps for band changing, never a shorting of turns.

: By the way, it just occured to me that the old BC457 through BC459 ARC-5 transmitters used during WWII had a "roller-ductor" coil in a pi-network output tank for the pair of 1625 final amplifier tubes! The inductance value was continuously adjustable using a sliding tap! The unused portion of the coil was left unconnected.

: As far as ham radio itself goes, please don't apologize! I haven't been active myself in quite some time, but I always wanted to keep my call. At one time, it wasn't just my call because among my friends, I wasn't John, I was "IBH" if you know what I mean.

: Good luck.

: John Dunn - President
: Ambertec, Inc.
: ambertec@ieee.org

Okay, John... Here are a couple of those examples you were looking for:


Note how the band switch actually shorts out the unused windings on the inductor! One of the
schematics uses the same practice for the grid circuit. I know this technique works, I just don't
understand HOW!


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