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Registered Member #1517
Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304
Hi Guys,
I am working on a project with a radio link that I bought from sparkfun. These to be exact: Receiver Transmitter I need to get the signal for the antenna outside of the metal enclosure and then outside of a chassis to a small antenna like this one: Antenna I am trying to find some lessons, or maybe some advice on how to go about designing the PCB to hook the radio links to external antennas. I am aware of the need for impedance matching, this is mostly where I am struggling. I just need some practical advice, and I'll be filling in the theory later.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Chico! Go Wild Cats!
Keeping RF traces short are the biggest priority.
If you need to connect from a known working circuit, to a external antenna as you have linked; then just get an SMA male coax thingy, a SMA coax cable 3 inch long, and whatever you need to match the cable to the antenna.
No big deal, if I understand your question, you could even get your SMA pieces here at our local Ripoff-Shack, I mean Radio Shack.
Registered Member #3040
Joined: Tue Jul 27 2010, 03:15PM
Location: South of London. UK
Posts: 237
personally what I do is :-
SMA socket on case. SHORT rigid wire from back of socket to transmitter/receiver board with (as Patrick says) as short as possible track to the ANT pin on the module. Multicore with power/data to main micro board which can be mounted wherever you like.
That way you avoid the need for long leads/coax on the RF side. Something like (these are my development modules) but with chassis mounted sockets instead.
You'll need extra mechanical fixing for the RF board to the case, don't just rely on it hanging off the back of the SMA socket!
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
The pics that Martin shows are what I had envision, so do that. then run a little SMA coax to/through your case to the final antenna location.
Also, cell phones, WiFi repeaters, WiFi cards and such are usually good examples of RF layout at the board level, but there are some big corporation screw ups out there too.
@ Martin: Do you limmies call coax "multicore" ? other wise I dont know what that means!
Registered Member #3040
Joined: Tue Jul 27 2010, 03:15PM
Location: South of London. UK
Posts: 237
Patrick wrote ...
...... so do that. then run a little SMA coax to/through your case to the final antenna location.
The whole idea of my method is you use no coax at all. The RF board is mounted DIRECTLY to a chassis mount SMA socket via a few mm of rigid tinned copper wire then you run data and power wires back to your main board.
Registered Member #3040
Joined: Tue Jul 27 2010, 03:15PM
Location: South of London. UK
Posts: 237
Ah right I missed the chassis bit of "outside of the metal enclosure and then outside of a chassis" I assumed something like this bolted to the case and the antenna screwed onto it but it would be no different to use an extension coax running to the antenna instead either way the RF pcb/socket mounting method is the same. Anyway the basic rule is as short as possible link from PCB to SMA socket then only use coax from the socket onwards and bear in mid that the longer the RF cable run the more signal loss hence why I like to place the RF board right next to the antenna.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi all
I recently laid out a PCB for a 433MHz RF module at work. Just put a board mount SMA connector right next to the module's antenna pin, and connect it with the shortest track possible. Also make sure there's a short, wide connection (ideally groundplane) between the SMA connector shell and the module's ground pin.
If a PCB track is short compared to a quarter wavelength (which is about 17cm at 433MHz) then you don't have to bother about calculating track widths for 50 ohm impedance and so on. A fattish track is still nice, though.
Another trick is to just take a piece of miniature coax (such as RG174), strip the end as short as you can, and solder it straight to the module's antenna and ground pins, which are usually next to each other. Then the board design doesn't matter, you can use perfboard if you want, or duct tape or whatever. The RF is coming out of the module and going straight into the coax.
As others have said, try to keep the coax length to a minimum too, but it's only 433MHz, a few inches of coax won't kill it.
Registered Member #1517
Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304
Thanks for the help guys. I go to CSUC. Last semester here... it's been a long time coming.
I am building this module to go in several different robots so I am trying to make sure that it can be wired up in several different ways.
I will never know if it will go inside another chassis or not. Or how far the coax will need to run to get out. I hope to include extra so the lids of these things can be removed...
You said a few inches of coax, can I do up to .5m? .8m?
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RF PCB: Hooking up an SMA/BNC
