antenna feed points?? help?

I was measuring at the feed point on a printed circuit board antenna and the impedance was supposed to be seeing about 200 ohms for 2.4GHz application. But when I measured it was about 20 ohms (this is from a previous engineer here). I am new to antenna and when I measured the antenna without the feed point, it measures about 180 ohms but as stated above, the with the feed point it is 20 ohms, any insight??

Hi rubdawg,

feed point of 200 seem high for a 2.4Ghz applications. It is not clear to me how you can measure the antenna without a feed? Maybe you will like to elaborate on your measurement method. May need some error correction on the network analyser if you take the feed out.

Element7k

rubdawg said:

I was measuring at the feed point on a printed circuit board antenna and the impedance was supposed to be seeing about 200 ohms for 2.4GHz application. But when I measured it was about 20 ohms (this is from a previous engineer here). I am new to antenna and when I measured the antenna without the feed point, it measures about 180 ohms but as stated above, the with the feed point it is 20 ohms, any insight??

Click to expand...

Basically I cut off the antenna from the main board and put a SMA connector and measured the antenna, it is a F antenna so I used the Ground line for the sma connector and measured using NA.

I then used a bare board (with antenna connect(a printed antenna)) and put a probe into a trace leading into the antenna. I will draw a simple diagram later to give you a better visualization. I appreciate you taking time out to help me, i am new to antenna
I have worked on transmitter and receivers for mobile phones but now in new job working on Zigbee (802.15.4)

This is used in the 2.4GHz range.

Hi rubdawg,

I will wait for you visual aid. However, just a note, as I know many F antenna are tuned with ground plane the size of the main PCB board. So if you "cut it off", the antenna characteristic may change.

Element7k

rubdawg said:

Basically I cut off the antenna from the main board and put a SMA connector and measured the antenna, it is a F antenna so I used the Ground line for the sma connector and measured using NA.

Click to expand...

the PCB antenna impedence change when the ground change,because the ground is a part of the whole antenna

the length of the PCB trace could have an impedence transformation effect. how long is your trace? you need to cal out the trace to make a correct measurement at your reference imput impedance. if not you are measuring at the wrong reference for comparison.