What are Bandpass filters? when do I need them? Why should I use them?
I have personally spent many weekends recently working with and testing these “Band Pass Filters” for multi station operation, Band to Band they do an impressive job allowing stations to be very close and having no appreciable effect on one another. This lets a “Field Day” or club outing operate close enough to stay within a “social” distance and maintain their Club atmosphere. within the same band operating different modes like SSB vs. FT8 it is more effective to change polarization between antennas. We have used Vertical antennas for FT8 and Horizontal (dipole) antennas for SSB. there is some attenuation on the voice operation but still allows readability. We wholeheartedly believe that Band Pass filters are a very worthwhile investment for our Field Day and monthly outings.
The following “Stub” method of out of band suppression is the “Poor mans method” or the “Quick Field Fix” for multi station operation:
Simple Coax stub “Nulling” filter system K1TTT Technical Reference
Low Band Systems Band Pass Filters are used in conjunction with their multiplexers, also known as multi-band transmit combiners. Low Band Systems Diplexers, Triplexers, Quadplexers and Pentaplexers, also available from DX Engineering, provide good levels of isolation between band ports, but band pass filters are also needed for multi- transmitter operation.
Building your own Band Pass Filters: