Re: LF: Re: Help needed, mechanical filters

["James Moritz" <james.moritz@btopenworld.com>]

Dear Stefan, LF Group,

> So, is a microphone effect possible?
I suppose it is possible, but I don't think there are many intense accoustic 
sources of 473kHz, and in any case the filter assembly would have to be 
designed to minimise unwanted acoustic coupling between input and output, so 
would probably also give good protection from external sources. The ferrite 
transformer core in your PA is perhaps the strongest source, but I guess you 
won't use the PA and filter at the same time ;-)
> Good idea about the matching! So it is about like the input filter of 
> JA8SCD's LF converter: 
> http://icas.to/idc-136-kit/idc-136-kit-manual-eng.pdf (page 6 of 6).
That's one possibility - the "dual" of that circuit has a capacitor in 
series with the low-Z source and a shunt capacitor across the winding - see 
the handbooks.
> According to the data sheet, the insertion loss (Durchlassdämpfung) is < 2 
> dB. So if one uses a well matched 50 Ohm transmit antenna and a RX with 50 
> Ohm input, i think no additional gain is needed.
If the receiver sensitivity is reasonably good (it might not be though...), 
you could probably allow 10 or 20dB attenuation. You could reduce the effect 
of impedance variations with an attenuator pad, etc.
Cheers, Jim Moritz
73 de M0BMU