transmission line measurements 2

[BobM2]

I'm incorporating a Linx RF module onto a circuit board. How to get its RF input to a remote antenna is my challange. A this point I'd like to connect a couple of small gage stranded wires between the board and a bulkhead SMA connector- one wire soldered to the central post of the connector and the other attached to a ring terminal assembled on the body of the connector. The wires would be about 1 inch long. The RF frequency is 918 Mhz. I'd like to test the assembly for standing waves or impedance. What would be the proper way to do this?

 

[Higgler]

To maintain 50 ohm impedance (assuming that's what you're using), if your circuit board has a ground plane then connecting your IC to the coaxial cable in "microstrip", you'd find the proper width of a connecting line by using the Rogers corp website here

http://www.rogers-corp.com/mwu/mwi_java/mwij_vp.html

example, Microstrip 50 ohm in dielectric 4.5 and circuit board thickness 0.031 is 0.0569 inches at 1 ghz. Changing dielectric constant to 2.94 for the same board thickness and frequency widens the strip from 0.0569 to 0.0774 inches.

Soldering a wire as you noted is probably fine.

If your circuit card doesn't have a ground plane, that can be a slight problem just attaching two wires to an IC at those frequencies.

Testing the assembly for VSWR requires an vector network analyzer (8753 or 8720 from Agilent, etc.) or if you have a coupler with good isolation performance you could use that to see how much energy reflects off the circuit. Too much detail to describe how to do that shortly.

kch

 

[01VIPER]

There are a couple of other free transmission line calculators available.
1. - TXLINE 2003 from AWR

http://www.mwoffice.com

(you'll need to register to enter the download section.)
It does Microstrip, Stripline, Co-planar Waveguide, CPW Ground, Round Coxial, Slotline, Coupled Microstrip and coupled Stripline.

2. - AppCAD from Agilent

http://we.home.agilent.com

Search for AppCAD, scroll to Design and Analysis tools,
download setup302.exe
It does Microstrip, CPW Ground, Stripline, Parallel Line,
Wire over Ground, Coax Round and Square, Trough Line and Slab Line.