"Backwards" Diplexer->Triplexer->Triband antenna ..

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WL7COL, Feb 8, 2011.

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  1. WL7COL

    WL7COL Ham Member QRZ Page

    Yet another moment of education - just for me. :)

    I have a FT-897D and a DR-235 and I'm interested in running these two radios to a single tri-band antenna (for various reasons).

    I'm a bit stumped on how to get the single-output 2m/70cm from the 897 and the output from the 1.25m on the same feedline. The stumping comes from a V/U on the same connector.

    What if I ran a diplexer 'backwards' where the output from the 897D was connected to the output connector on the diplexer. From there, the 2m input lead connects to the 2m input on the triplexer. Also from there, the 70cm input lead connects to the 70cm input on the triplexer. The DR235 connects, of course, to the 222Mhz input on the triplexer.

    Can I run a diplexer "backwards" like that and not sizzle-pop my transciever?

    Thank you.



    73 WL7COL
     
  2. W0HC

    W0HC Ham Member QRZ Page

    Can you provide some sort of diagram of what you propose?
    Also, it would help me if you describe these radios since I'm not familiar with all of them and don't want to google.
     
  3. K4PYR

    K4PYR XML Subscriber QRZ Page

    Not with a duplexer, but you could use a triplexer designed for your three bands, such as shown here: http://www.austinantenna.net/triplexer.htm. Just Google "ham radio triplexer", and you will see a few vendors that supply them at various costs.

    Good luck and 73,

    K4PYR
     
  4. WL7COL

    WL7COL Ham Member QRZ Page

    I don't have visio on this machine so I've done what I can with notepad. Since the forum doesn't respect tabs (or, more correctly, I don't know how to force it to), I've attached the diagram as a *.txt file. It is crude - sorry about that. :\

    So you won't have to bing the radios, the Yaesu FT897D is a HF/6m/2m/70cm radio. There are two RF connectors on the back: (1)HF/6m and (1)2m/70cm. The Alinco DR235 is a 1.25m FM mobile radio.
     

    Attached Files:

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That should work fine if you have a triplexer that is LP for 2m, BP for 135cm and HP for 70cm and has sufficient out of band attenuation. They are usually specified for how much attenuation they actually provide to the adjacent ports for the frequency ranges indicated.

    When all are "receiving," of course it will work fine.

    The issue is what happens when you transmit. The 2m/70cm duplexers have a lot of isolation because the frequencies are so widely spaced and usually the 2m side is a 150 MHz low pass and the 70cm side is a 380 to 400 MHz high pass. Rejection can easily be 50 dB, so if you have a 100W transmitter running the power showing at the off-band port is only 1 milliwatt. Receivers can almost always handle at least 20 milliwatts, so you'd have lots of margin there.

    But the 135cm part would need to be bandpass (combination of LP and HP), and you'd need to know how much rejection it truly offers at 148 MHz and also at 430 MHz. It may not be enough.

    Do you already have the triplexer? What are its ratings?
     
  6. WL7COL

    WL7COL Ham Member QRZ Page

    I don't have either the diplexer or triplexer yet. I am looking at:

    Diplexer: CF-416A from NCG/Comet. Detail lists 60db isolation.

    Triplexer: CFX-324B from NCG/Comet. Detail lists 40db isolation.

    Of course if these are subpar or just junk, I'm open to suggestions.
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    They're not junk. In fact for stuff that includes 222 MHz, there are very few choices.

    40 dB isolation is probably enough considering the power level. That would take a 50W transmitter down to 5 mW, which should be perfectly safe for a receiver -- if it meets their spec!
     
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