antenna feed points?? help?

[rubdawg]

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??

 

[Element7k]

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

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??

 

[rubdawg]

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.

 

[Element7k]

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

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.

 

[springf2000]

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

 

[rfary]

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.