[SOLVED] problem with txline and manufacture of microstrips

[winges]

Hi everyone!

i have developed some microstrips, coupled mslines and cpwground with the software txline.
my values for the parameter are:
(here example for microstrip)

Dielectric: RT/Durdoid 5880 (i have choose it for FR4 with dielectric constant 4.3)
Conductor: copper

Impedance: 50 Ohm
Frequenz: 2.87 GHz
Height (dielektrikum): 0.5mm
Thickness (copper): 18 um
electrical length: 90deg

and the txline gave me physical length l=14.48mm and the width= 0.95mm

so...i have do this for some types of antennas and additionaly i have varied the parameter that i have some samples of antenna and draw everyone with eagle and gave it to the PCB manufacturing.

they looks really good )), but...i measure the S11 (reflection) of the antenna, the frequenz is not 2.87 GHz but in range of 7-10 GHz.

Have some one ideas, what i did wrong?
Should i consider something else?


Kind regards
winges

 

[strahd_von_zarovich]

Hi everyone!

i have developed some microstrips, coupled mslines and cpwground with the software txline.
my values for the parameter are:
(here example for microstrip)

Dielectric: RT/Durdoid 5880 (i have choose it for FR4 with dielectric constant 4.3)
Conductor: copper

Impedance: 50 Ohm
Frequenz: 2.87 GHz
Height (dielektrikum): 0.5mm
Thickness (copper): 18 um
electrical length: 90deg

and the txline gave me physical length l=14.48mm and the width= 0.95mm

so...i have do this for some types of antennas and additionaly i have varied the parameter that i have some samples of antenna and draw everyone with eagle and gave it to the PCB manufacturing.

they looks really good )), but...i measure the S11 (reflection) of the antenna, the frequenz is not 2.87 GHz but in range of 7-10 GHz.

Have some one ideas, what i did wrong?
Should i consider something else?


Kind regards
winges

The length which txline gives you is meaningles . You can use the length to find electrical length in TXline. FR4 is not a good dielectric for high frequencies, and probably it's dielectric constant may differ from 4.3 at that frequency. Also it's loss tangent is high.

 

[vfone]

S11 of the antenna and the transmission line impedance, they are different things.

Measure the impedance of the transmission line alone, and later measure the input impedance of the antenna without the feeding transmission line (if is possible).

Measure the real Er of your FR4 PCB, because small differences will give impedance variations.

A simple method for measuring the Er is the Parallel Plates method:
**broken link removed**

 

[winges]

oh okay thanks!
@vfone: i used an anritsu analyzer and can calibrate the transmission line, so that i can measure only the reflection from the antenna.

it gives me the impression, that a closer value to the "actually dielectric constant" can not push the frequency about 7GHz (respectively by tune the values in txline and look at the changes)

 

[winges]

i solved the problem, i switched from microstrip to patch antenna, because the length from the antenna, were you "responsible" for the 2.87GHz addicted by the width of the antenna