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Re: [Amps] FL-2100B input swr problem

Subject: Re: [Amps] FL-2100B input swr problem
From: Jim Brown <>
Date: Tue, 4 Jul 2017 22:56:18 -0700
List-post: <>
Here's a fundamental point that use of the word "ground" as if it's a single point and ideal is totally wrong and causes confused, muddled thinking. EMC guru Henry Ott (Bell Labs, consultant to virtually every major corporation, and author of the EMC "bible" now in its third printing) talks about "the hidden schematic hiding behind the 'ground' symbol.

He also talks about keeping track of where (exactly) the current flows, and he's talking about ALL the current in a given path. For example, wiring for both AC and DC power also often carries RF noise currents. It's absolutely necessary that every one of those paths for AC and DC power be treated as a transmission line if we are to control the RF currents in that path.

Current flows in a loop, and we must keep track of all the sources and all of the paths. Thinking about those caps -- we intend for them to carry current, so we must know both the source and the sink of that current. If we want circuit stability, or if we want to short out noise generated by a source, we must connect the capacitor to the point that is nearest to the source of that noise current.

A mistake I often see in circuitry like output networks, filters, and antenna tuners, is reliance on a chassis to carry return current. That GREATLY increases the length of the path, and thus the inductance. That changes the circuit's resonances, AND it generates a magnetic field that is proportional in strength to the area of the loop, and couples into other circuits in proportion to the area of their loop. Has anyone noticed that SMART amplifier designers use coax (or twisted pair) for wiring in the path for T/R switching? This minimizes the loop area and lowers the Z of the path -- it's only the Zo of the transmission line, in parallel with the high-Z path of the highly inductive path via the chassis, so no return current flows on the chassis and it all flows in the transmission line.

My advice to the next person tackling this repair job is to tie the replacement 
mica cap ground leads to the exact same points as the OEM caps, even if you 
think you can arrange them in a neater and easier fashion.

Exactly right -- if, hopefully, the original designer thought about all of the above. Or, if he isn't that smart, he staggered around with different layouts and stumbled into a layout that is stable. :) Either way, it's a can of worms we don't want to open unless we're prepared to "own" the resulting instability.

73, Jim K9YC

On 6/30/2017 8:33 PM, K2CB wrote:
What I found was that how and where the 10 and 15m caps are grounded makes a 
big difference in the overall tuned circuit.

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