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Duplexer 400Mhz - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: Duplexer 400Mhz
Unread postPosted: Thu Mar 25, 2004 5:00 am

i have to disign a rf duplexer about 400Mhz for a full duplex communication at 1Mb/s.
I havn't find a duplexer with the different manufatcurers
How can i make it myself ?
Thank u
sory for my poor english (i am french)


Post subject: Duplexer 400Mhz
Unread postPosted: Tue Mar 30, 2004 8:22 pm

Welcome to Forum,

You can make it if you put all the efforts into it! Following are some tips for you:

First off, you will need to select your Tx frequency and then Rx frequency. After, you will need to be certain you have a split frequency (SF) between Tx & Rx and the greater separation the better isolation (isolation is very important in Duplexer applications and full radio operation) between Tx & Rx. For instance, let’s say you have 400MHz on Tx branch and 500MHz on Rx branch and the frequency separation (split) is 100MHz (i.e. Tx-Rx = Split Frequency or SF). As you can see in my example above, the band is very narrow in frequency and the split of course, it is narrow as well. Therefore, the cavity Duplexer has high Q and it will be into one Omnidirectional antenna (full duplex operation). In addition, you must select VSWR, Return Loss and maximum Tx RF power level. You could make the cavity Duplexer with Aluminum or Brass. It depends on how much money you want to spend and how easy for you to machine the metals. Chebychev will do, but Butterworth is better (no ripple). Again, it will depend on your electrical and mechanical specifications and ultimate Duplexer performance. The selectivity, rejection, Tx & Rx isolation and Q performance of the cavity will depend on the following factors: Size or number of the cavities and Fc (center of frequency). Make certain the I.L. (insertion loss) is low (3dB is too much and you will loose half of your Tx power) as possible and with low ripple as possible as well (i.e. <0.5 dB), but remember that for Digital Communications high ripple has a great negative effect and will damage BER (bit error rate) on Receiver and also high insertion loss will destroy the NF (noise figure) on Rx side. Read the specifications of your radio receiver manufacturer to make sure you don’t apply too much insertion loss on your Duplexer Rx branch. Coaxial cables to transfer RF signal from one cavity to another cavity are very critical. Therefore, size, length and velocity must be taking into consideration. Don’t forget, in any filter design, we must have tradeoffs! With that approach in mind, you must select and do the right measurement. There are several steps to go through until this is done. I mean, from design it on the paper with sophisticated hand calculator (an old fashion and less accurate and not reliable for voice or data digital communications) or by software (with software filter design tool the results are reliable and accurate), machining your cavities (make them from scratch) to tuning process. Remember, no filter (like any other electronic device) is good for use until tested and met the required specifications.

I thought this might help you to start. If you need help in order to accomplish this project, please let me know because I will be keen to help you.


John Pereira

Posted  11/12/2012

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