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.
SO2R filters
121 dB attenuation
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Low cost
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Easy to build
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Alignement is a nightmare !
D.I.Y. or Buy ! The choice is yours only !
Popular
- HIgh power lowpass filters
This filter has to be upgraded for SO2R operations
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.
- Low power bandpass filters
- 5 Poles Bandpass filter+ 3 poles low pass filter.
- About 0.5 dB insertion losses.
- About 121 dB rejection at second harmonics.
- 100 W continuous power.
- Low heat dissipation.
- SWR 1:1 – 1:1.4 for CW segment only.
- T94 size toroids.
- Quality silver mica capacitors.
- Easy to build.
- Not so easy to tune.
- About 200 Euro total cost for 6 filters.
D.I.Y. or buy, that is the question !
01 February 2024 status update :
Due to high SWR, I removed the notch filter placed into high impedance rail and add 2 new ones at the low pass filter. Now I can barely see any SWR on transmit.
31 January status update: new F/2 notch filter added
Example : For 20 meters band, F/2 = 7 Mhz
If transmitting in 40m and receive in 20m, transceiver second harmonic is at minus 120 dBm.
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What happens with 40m carrier, aint it to big ? 50dBm (100w) - 80dB (20m filter) = -30dBm
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This 30 dBm is too strong for Icom IC7300, and transceiver itself will start producing second harmonic due to intermodulations, internally !
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Another 40 dB notch filter was added to knock off any remaining 40 meters signal to a safe zone below S9 inband, so listening in 20m and transmiting in 40m, total second harmonic should be heard at S1, maybe S2 level.
73's and good luck !
4 February 2024 update:
I have injected a Rigexpert AA30 with more than 0dBm level into a vertical antenna. Another Half Square antenna was fed to tranceiver via 14 Mhz bandpass / notch filter. Watch out the video below and make the math . 73's de Gabriel, YO8RXP
3 thoughts on “SO2R bandpass filters”
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
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 !
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.