How Do Microwave transmitters work without physical connections?

[Tech2025]

So I recently picked up 40 of 10.525 GHz Microwave RADAR transmitters:

https://www.ebay.com/itm/10-525-GHz-Doppler-radar-modules-Microsemi-X-band-transceivers-lot-of-100/113234460437?hash=item1a5d4cbf15:g:TwMAAOxywbNQ9YS0

I opened up one out of curiosity and had something like this (image attached) . I noticed some parts of the PCB aren't even connected to anything. How do the different shapes and sizes of the PCB interact with each-other to transmit microwaves?

 

[sophiecentaur]

I think the answer lies within what you wrote. There are Waves involved and they don't need a metal to metal connection. I have no idea about the details here but there appear to be lengths of conductor that lie parallel to others. There can be coupling of power between these lengths. If you look at this link about directional couplers there are some images which show similar structures for microwave couplers. There could also be coupling between adjacent layers of that circuit board.
There is no quick answer to how structures behave when em waves interact with them - the design of antennae and circuits is very non-intuitive, whether we're taking about microwaves or lower frequency radio waves. There's a lot to be said for starting with old fashioned Radio Transmitting system design where at least the active parts (amplifier etc) are easy to distinguish from the radiating parts (antennae); in microwave devices, it can be much harder to decide which is which.

 

[m4r35n357]

I noticed some parts of the PCB aren't even connected to anything. How do the different shapes and sizes of the PCB interact with each-other to transmit microwaves?

Welcome to the world of microwave electronics, where circuits are distributed over space (normal electronics models all components at the same point in space).
It is a fascinating world, far too weird and complex to explain here, but if you want some search terms, try:

• microstrip
• coplanar waveguide
  
• rat race
• quarter wave transformer
• directional coupler
• isolator (ferromagnetic)
  
• smith chart
• voltage standing wave ratio (VSWR)

 

[m4r35n357]

BTW just found some open source software that looks like it can simulate some of the items I mentioned. I have not tested it myself yet . . . not sure when I will get some spare time!

 

[davenn]

I noticed some parts of the PCB aren't even connected to anything. How do the different shapes and sizes of the PCB interact with each-other to transmit microwaves?

not sure which parts you are specifically referring to ?

but here is an annotated version of your pic that I have done that explains different parts

OK, the core of this transceiver is the DRO Dielectric Resonator Oscillator ….
this directly generates the required 10GHz frequency. The RF out goes down that stripline to that surface mount component
most likely a capacitor. Then it is split to the left to the Transmit Antenna and to the right to a mixer device … possibly a dual gate GaAs-FET

To the right of the mixer is the Receive Antenna. The incoming signal from the RX Ant. is mixed with a small amount of RF from the
DRO and produced an IF (Intermediate Frequency) that goes to the rest of the receiver circuit.

So the DRO is providing BOTH the transmit signal as well as the LO ( Local Oscillator ) signal.

You will note I have also arrowed a number of small stripline matching stubs … these are for impedance matching
so as to get maximum power transfer along the line

Those 2 little fan shaped circuit tracks to the left of the DRO puck are RF chokes

DRO's are found in every satellite LNA that sits up at the focus of the satellite dish. Some LNA's have 2 DRO's in there
when they need to operate dual band

I have taken a Satellite LNA where the DRO is operating at around 11.3 GHz and have added/glued small lumps of ceramic to the top
of the DRO puck to lower its frequency down into the amateur (ham) radio 10.3 to 10.5 GHz band


A typical DRO layout/circuit example ….

cheers
Dave

 

[davenn]

So I recently picked up 40 of 10.525 GHz Microwave RADAR transmitters:

.....

So did the responses help you to understand ?


Dave