Diplexer that will pass below 110MHz on one side and above 160MHz on the other?

JoshuaHufford

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My Father-In-Law allows me to have a receive site at his shop, which happens to be a great location for it. He asked me about putting up an FM radio antenna because reception inside a metal building is obviously poor. I was going to put one up for him but then I thought I should see how my Diamond F23 that is mounted there does at FM. I'm splitting the signal from that antenna with an 8 way divider and I'm only using 5 of the ports so I have a spare port for feeding the FM radio.

The F23 itself is quite good at receiving FM even though I have it tuned to 160MHz, but that doesn't really surprise me with the power most FM stations put out. Problem is I'm using a tuned preamp, 160-162MHz, before the power divider and it pretty much blocks all FM, which it is supposed to do. For now I put a 2 way power divider before the preamp, then feed the FM radio off one side of the divider and then the preamp off the other side. Obviously this is not the ideal setup but so far it hasn't seemed to have a huge impact on performance, but I did have to "borrow" this 2 way divider from another kit I use it in for now.

I thought if there is a Diplexer that can split the signal so I get FM from one side and stuff above 160MHz on the other that would be great, however I'm not finding anything, I realize this is a pretty close and unusual split so perhaps there isn't one made. I probably have the skills to make one if someone could tell me what components to use.

Thanks!
 

Ubbe

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Sep 8, 2006
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Stockholm, Sweden
This is a good calculator to use and you could make a low pass filter for the FM radio and select to have a series first coil with a 7th order Butterworth filter at 100Mhz. It will attenuate 6dB at 108Mhz but probably doesn't matter and at 160MHz it will be -30dB down and be high impedance that doesn't affect your coax when connected to it in parallell. RF Tools | LC Filters Design Tool

If you would like to make it more efficient you could get a FM bandstop trap filter already with all components and connectors in place and modify it to invert the design. It will have a coil and a capacitor in series to ground and a coil and a capacitor in parallell in series with the signal. It will be 3 or 4 of these two components and you cut the copper foil and solder jumpers so that the components in parallell instead goes to ground and the two components in series are instead in series with the signal path.

You can use that calculator when you look at its schematic for bandpass and bandstop filters to see how it should look and it must be a series first design to go high impedance outside of the FM broadcast band or it will shortcircuit the coax to your railroad receiver. To make it easier you could just cut out the two components in parallell to ground at the input and output of a FM trapfilter and only invert the remaining components as in a 5th order filter design. It will still pass all the FM band and are high impedance with more than a theoretical 60dB isolation at 160MHz.

/Ubbe
 

dbdunfor

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Sti-co used to make a kit to change out a FM radio antenna on a pickup, it came with a diplexer to separate the vhf and fm broadcast band. They weren't great but worked okay for covert stuff. I may have one in my stash, if you need it.
 
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