For Multi-Multi or SO2R operations, quality bandpass filters are needed.
Not only produced harmonics have to be rejected, but any below frequencies also, to minimize distortions produced by receiver itself in presence of strong outband signals.
Suppose you already have a 1KW QRO producing 2’nd or 3’rd harmonics at -50dB over fundamental. Unwanted signal is +60dBm – 50dB = +10dBm … way too much, so where is the beef ?
The beef is where another cheap 80 dB high power low pass filter will retrofit power amplifiers. Yet , outband RX 2 notch filters will squeeze out another precious decibels.
3 Comments
Kevin · May 7, 2024 at 2:44 am
Hi Gabriel
Interesting stuff with these filters. I have my station setup for SO2R with 2 x IC7610. I have some space here with seperate RX antennas about 100m away from the TX antennas. I just have filters on the RX side only, 5th order Chebyshev and these switch automatically with a band decoder. This results in virtually no interference, but I’m interested in your idea with band pass followed by low pass. I think this works better for you than the notch idea ? I might try this also on transmit side too with my other TX antennas that are in the same area. 73’s Kevin VK6LW
yo8rxp · May 7, 2024 at 2:34 pm
Hi Kevin
I seems the Notch filter gives great rejection for lower outbands but is quite unstable and narrow bandwidth.
My icom 7300 give me headaches when dealing with “below” outband signals over S9+30 therefore somehow, if listening in 7 Mhz and transmit on 3.5, I have to reduce this signal even stronger not for harmonics but to prevent transceiver itself not to produce second harmonic. Low pass is used to reduce even better transmit harmonics, nothing special here while IC 7300 has good spurs levels.
I did an extra 40 dB rejection using coax notch filters but they are too bulky. Bottom line, will take some time until I will develop cheap yet powerful Multi-Multi operation filters.
73′ and good luck !
Mihai · August 10, 2024 at 10:29 am
Salut Gabi!
Daca unele din designuri sunt publice ma intereseaza si pe mine. nu pentru SO2R, ci in general pentru FD. in momentul asta am doar variantele de 1KW de la HyEndFed pentru 20 si 40m.