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Re: [TowerTalk] isolation and SWR

Subject: Re: [TowerTalk] isolation and SWR
From: Jim Brown <>
Date: Mon, 10 Sep 2012 09:28:07 -0700
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On 9/10/2012 8:03 AM, Pete Smith N4ZR wrote:
Is there even an approximate relationship between these various SWR numbers and the degree of isolation afforded by the two switchboxes?

Something would have to be seriously broken for that to be the case. A more likely cause would be the stray reactances in the signal path. I've recently measured several with a vector network analyzer, and seeing a lot of strays. When I open them up, I'm finding that the manufacturers are failing to carry the return path on the circuit board, instead using the chassis as a return. Ther is, for example, no direct path between the coax connectors and the circuit board, so return current must flow from the connector via the chassis to wherever the circuit board contacts the chassis.

One box uses a two-layer board with a "ground" (return) plane on one side, which, when done right, causes return current to flow on the ground side directly under the trace. This greatly reduces crosstalk and stray inductance. But they break that trace at multiple points under the signal wiring, which completely defeats the ground trace by forcing return current to find a path around the break The result is a lot of stray inductance added to the signal path, both increasing SWR and also increases crosstalk (that is, reduces isolation). They sort of get away with it on the lower HF bands, but these boxes look increasingly nasty on the higher HF bands and some are unusable on 6M.

In addition to the stray inductance, most multi-way switch matrices will have a fair amount of stray capacitance simply by virtue of having wiring from multiple relays connected to the output bus. Thoughtful designs will use layouts that minimize the strays on the highest frequency output ports.

And probably an even dumber question. If you have two devices in line and each has (say) 30 dB isolation between a line in use and an unused one, both of which pass through them both, is the resulting isolation 27 Db? Some other number?

Probably not easily predicted.

73, Jim K9YC


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