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Passive filters and the SNA600a dipoles

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Question about my antenna setup.  Here is the hardware


- 2x SNA600a dipoles (set to block 21 length)

- 2x 10' of RG58 antenna cable

- Audio Wireless DADM226 antenna distro

- 6 receivers, all a mix of SRbs and 411a's... (block 21 only)


The question is: would there be any benefit to inserting a block 21 passive filter (such as a PF25) into the signal flow?  Would it be redundant in any way?  There are times when I have 8 channels going at once, as well as 2x IFBT4s (also block 21), so the frequency space can get a bit crowded. 



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The filter won't help for the block 21 transmitters as far as intermod goes but the filter will keep tv stations, cell phones and towers, walkies, etc., that are outside of block 21 from from causing in band inter-mods and overloads. A downside is a small (2dB) of loss through the filter and connectors. However, in today's RF environment, you rarely are in a location where the noise floor is low and the 2 dB of loss will not affect your range, where removing out of band garbage may make big improvements.


The SNA dipoles already filter out some of the out of band garbage and will improve your signal to garbage ratio. One reason dipoles work better than a 1/4 wave whip is that the whip is rarely correctly tuned to the desired frequency due to a poor or nonexistent ground plane or nearby pieces of metal or wires. Therefore the signal to garbage ratio may be low or the garbage RF may even be emphasized (!). The dipole in comparison is easily tuned to the correct frequency and does not need a ground plane. Also, the microstrip PCB usually gets the antenna a 1/4 wavelength or more away from wires and metal in most setups.


So, in sum, you already are in pretty good shape. The PF25 is just additional protection. Me, I'd pop that sucker in line and be glad I was operating in a single band where I could use it.

Best Regards,

Larry Fisher

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