On Fri, Jul 5, 2019 at 12:54 PM Jack Brindle via Elecraft < elecraft@mailman.qth.net> wrote:
> As commonly used, bandpass filters have one main use - to protect > receivers. That may be the local receiver, protecting it from sting out of > band signals, or a neighbor receiver, gain protecting it from strong out of > band signals. > You are only thinking of good Elecraft radios. There are plenty of crap radios that transmit harmonics or even wideband noise. > This quite well describes SO2R stations, where we are very concerned about > receiver damage. The BPF is commonly placed between the transceiver and > antenna, which for a multiple of reasons needs to be 50 ohms impedance. > One of the major jobs given to the BPF is to suppress harmonic content of > the transmitted signal, for which the antenna load will most certainly NOT > present a 50 ohm load to the BPF. There are many references for this, I > would refer to W2VJN’s publication “Managing Interstaion Interference” and > the excellent articles by K9YC (who will most likely join into the > discussion shortly). Jim has done a lot of testing with BPFs, and has some > very interesting articles comparing and discussing their use, as well as > other articles about how to avoid damaging receivers in SO2R stations with > their use, as well as the use of stubs for harmonic suppression. > > The point is, BPFs are designed for both scenarios, to pass signals > in-band, where the load impedance is 50 ohms, and reject out of band > signals, where the impedance is almost never50 ohms. Note that most BPFs do > not do much for in-band signal rejection (by design). Using them to protect > a receiver in the same band is a recipe for disaster. > > In general a BPF will present a low impedance shunt load and a high impedance series load. Even into an unmatched load, it is likely that the BPF will still have a much lower shunt impedance and a much higher series impedance than what it is working into. In the example I showed, the change out of the passband was much less than the change in the passband. Out of the passband, it still works pretty well to reject signals. The unmatched load affects the in band performance a lot. I found Elsie to be really useful to get an understanding of what filters do under various conditions. 73, Mark W7MLG ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
