Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Building a Diplexer form COTS parts

Building a Diplexer form COTS parts

Building a Diplexer form COTS parts

I am working on some UHF SATCOM test equipment. We currently have a Trilithic FD1000 Diplexer that works to split/combine the TX and RX, so that radios that have a single RF port can be hooked to radios that have separate RX and TX ports. Unfortunately this FD1000 is pretty expensive and we would like to perform a similar function, perhaps with some parts procured from mini-circuits. The TX band is approximately 292-318 MHz, the RX band is 243 - 264 MHz. There must be a way to do this with some splitters and filters, is that correct?

Please note that I am not an RF engineer, but know enough to muddle along.

RE: Building a Diplexer form COTS parts

A Diplexer as used for UHF Satcom is basically "just" some filters to filter the uplink and downlink bands from each other to provide the necessary isolation. Often an LNA is built-in as well.

The problem is that the isolation spec for your application is very likely going to be difficult (perhaps impossible) to achieve using a collection of off-the-shelf components. Such diplexers are often of quite intricate design and may be of an unexpected architecture (perhaps not L-C). They might be (for example) silver-plated to help achieve the required 'Q'.

You'd have to work out your own isolation requirement based on the transmit power and adjacent band noise of the transmitter, including any Passive Intermodulation (PIM), etc. As well as many other factors, even cable leakage.

If it's just for testing, another approach is to use two antennas, sufficiently separated.

There are commercial diplexers using 'cans', but they're usually designed for the V/UHF Land Mobile Repeater or V/UHF Ham bands. Also, they're quite narrow, perhaps too narrow for your application.

NOTE: Everything mentioned above is publically available.

Your posts on this topic suggest that this is a professional gig, and that you're aware of all the applicable approvals to access the military satellites. Not just somebody that's going to transmit without all the paperwork in place. Right? Sorry for asking, but it's important.

RE: Building a Diplexer form COTS parts

Thanks for the information VE1BLL. Yes, I work for a major defense contractor, and understand the major hoops to jump through to get OTA. This setup would not have any OTA capabilities. This would be for equipment checkout testing only with direct radio to radio connections through a simple Downconverter made from a mixer and signal generator to simulate a satellite. I am working wit like 1dBm of power.

Sounds like it might just be easier then to bite the bullet and buy the correct COTS Diplexer then?

RE: Building a Diplexer form COTS parts

It depends on your system architecture, and the purpose of your testing.

Starting at the beginning, the purpose of the diplexer can be two-fold.

One is that the receive path will likely require a LNA installed close to the antenna, and therefore the transmit RF needs to be routed around the LNA. So the terminal is likely to have separate Tx-Out and RX-In connectors, both of which ultimately (probably) need to be connected to a single antenna (for most applications). So the diplexer is what allows all that 2-into-1, Tx around the LNA, to happen.

Second is that some terminals may be full-duplex, where they can receive and transmit at exactly the same time. So there's no possibility of using a TR switch. Both need to be connected at all times, using a duplexer.

Depending on the purpose of your test, and how your gear is arranged, one might be able to provide two paths, with the (two) sat-sim frequency shifters wired as required. You presumably would need two sat-sims, if you can't reverse the frequency on one terminal.

Perhaps other wiring plans may be possible, like using the concept of combining the two Tx, and the two Rx, with power dividers.

It really depends on what is the purpose of the test.

RE: Building a Diplexer form COTS parts

If you're in a shielded room where you're not going to cause anyone any interference, then some of this could be done over the air. Put your sat-sim on a stool ("in orbit") with two antennas. And then use separate TX and RX whips on both terminals.

Some people put their manifold little rubber duck whip antennas inside a seal metal can (3.8L paint can), with bulkhead BNC connectors to get through the lid. Your RF spectrum is thus inside a sealed metal can.

This presumes that your frequency shifting sat-sim is designed such that it's not going to oscillate when the Input can see the Output. As it's a frequency shift, it shouldn't unless there's a point in the spectrum where you get positive feedback. A Spec Analyzer to monitor what's going on would be a good idea.

You might need series attenuators as required.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close