It's critical to understand that a filter is a passive network
that interacts with the impedance of the source and the impedance
of the load. When we design filters, we make the optimistic
assumption that they will be terminated by a purely resistive
impedance at all frequencies. That assumption ONLY is true on the
test bench, when we drive the filter from 50 ohms and load it with
a 50 ohm analyzer. It is a BIG LIE in the real world, so filters
in the real world don't come anywhere close to test bench
performance. W3LPL recently pointed this out in a post to a ham
list -- maybe even this one!
Example: Our transceiver may be a 50 ohm source and our power amp
may be a 50 ohm load at the operating frequency (but probably
not). If, by luck, it is, the response through the filter at the
operating frequency will be as predicted. Most transceivers, amp
inputs, and antennas are not anything close to 50 ohms at the
frequency of harmonics, so the performance of the filter won't be
anything close to what it measured on the test bench.
Now, if we're building a filter that's only in the signal path on
receive, we can isolate its input and output with resistive pads
that cause both source and load impedances to approach 50 ohms.
Now the filter will act pretty much like it does on the test
bench. But we can't transmit through those pads -- they'll fry
with the first dit!
73,
Jim Brown K9YC
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