Building a Diplexer form COTS parts

[thomaskuhn]

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.

 

[VEBill]

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.

 

[thomaskuhn]

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?

 

[VEBill]

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.

 

[VEBill]

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.