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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