Actually, I suspect that they're designed for 50 ohms at least in part,
because that's how they will be measured! We try hard to present a
matched load in the passband, but, as others observed, output stages are
usually lower than 50 ohms and few antennas, even when matched by a
tuner, look anything like 50 ohms away from resonance. So all we can
really hope to gain by matching is to minimize loss in the passband.
AND -- any performance specs and/or measurements at 50 ohms in and out
are a fiction away from resonance, and with other than a 50 ohm source.
73, Jim K9YC
On 7/5/2019 11:37 AM, K9MA wrote:
Bandpass filters are generally designed for 50 Ohm source and load
resistances, mainly because the actual source and load impedances are
usually unknown. These filters will only perform as advertised with a
50 Ohm source and load. With real world impedances, you just have to
hope for the best.
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