[TowerTalk] Are All Low-Pass Filters Alike?

[Peter Chadwick]

One problem is that filters are generally specified for use in matched
systems - a whatever ohms resistive source and a whatever ohms resistive
load. Occasionally,
you might design filters to go between different source and load impedances.
In the transmitter low passs filter case, the load at harmonic frequenceis
is rarely (except on the case of multiband trap beams or log periodics or
other wideband antennas) the design load imepdance. This affects the
attenutaion, although not necessarily very much. A rule of thumb is that
load impedance affects mainly the 'in band' ripple, while source impedance
has less effect - although this is only true on a very global, first level
approach. The result of this is that no matter what the filter did in the
lab on a  'notwork' analyser, in practice, the harmonic attenuation you get
may vary. This is why people developed absorbtive filters with high and low
pass cascaded. So the first questions is to ask 'Do you need a low pass
filter, or is it just a 'feel good factor'? If you have TVI, AND it's caused
by transmitter harmonics, then you do.  If not, you don't. So you need a
fair bit of anal;ysis first to make sure the problem is transmitter
harmonics - if it isn't. the low pass filter will do nothing for you. But
you might like to have one (like me) on the basis that it's nice to know you
done something to reduce spurious signals.

73

Peter G3RZP