I was surprised to find K1TTT repeating the same mantra about high impedance
of W3LPL filters in his free e-book.
I did some quick simulations with AADE and I have to correct myself first -
parraleling 10 m and 160 m BPF causes 7 db loss as I overlooked low 50 Ohms
working impedance. Return loss unfotunately drops to poor 5 dB. K1TTT case
of 160/80/40 m BPF lumping does only neglidgeable 1 dB loss. However, I'd
preffer triplex RX output in his M/M environment.
QS1R is digital RX with A/D direct conversion of the whole 0-30 MHz
spectrum. CW Skimmer then processes 6 contest sub-bands, each 200 kHz wide
with 500 Hz bandwidth, 2400 channels total. Our own TX is causing trouble
on only one of them and should be notched. ICE protection with high speed
clipping diodes makes RX matters only worse. My CW Skimmer RX antenna is
simple omnidirectional 6 ft vertical with 220 uH grounding for 1,8 MHz
resonance and JFET source follower. It is at maximal distance from
horizontal TX Yagis and dipoles as near field drops with the third degree
of distance.
http://www.sm5bsz.com/dynrange/intermod.htm figure 8 is a rare picture of
digital RX colapsing after saturation. I quote him: "Note that a 2 dB
change, from 1 dB below saturation to 1 dB above saturation causes the third
order intermodulation product to increase by 61 dB." His latest posting
http://www.sm5bsz.com/lir/sim1/sim1.htm ilustrates nicely colocating QRM
problems.
I guess this subject might be discussed in more details at "Contest
University" :-)
HNY & 73 de Mario, M/S56A
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