lambdaanttech
Newbie level 6
Hello All ,
I want to design a Franklin antenna from PCB material .
First I designed the antenna with a NEC based calculator to obtain impedance and radiation pattern .
Afterwards I scaled the model down from free space to FR4 substrate .
As far as I knew , the velocity factor is calculated as 1/sqrt εr .
The VF value was 0,466 .
After milling out the antenna I did some field strength test to compare the 2 element Franklin antenne versus a simple split dipole .
The split dipole had more gain then the Franklin ...
Measuring and calculating and remaking the antenna didn't solved my problems .
Finally I took 2 pieces of FR4 strip PCB to make a dipole and measured the resonance frequency on the VNA .
I found out that way that my calculated VF for FR4 was not correct .
Based on my measurements the VF factor of FR4 should be 0,84 - 0,86 .
Searching the net for some formulas how to calculate the VF factor from the dielectric constant didn't solved my problem .
Comparing my calculated/measured VF factor with the calculations with AppCAD confirmed that the VF factor was right , but I don't know how to calculate it .
Could someone help me about the formula to calculate the VF factor from dielectric constant for PCB's.
Thank you,
Kenny
I want to design a Franklin antenna from PCB material .
First I designed the antenna with a NEC based calculator to obtain impedance and radiation pattern .
Afterwards I scaled the model down from free space to FR4 substrate .
As far as I knew , the velocity factor is calculated as 1/sqrt εr .
The VF value was 0,466 .
After milling out the antenna I did some field strength test to compare the 2 element Franklin antenne versus a simple split dipole .
The split dipole had more gain then the Franklin ...
Measuring and calculating and remaking the antenna didn't solved my problems .
Finally I took 2 pieces of FR4 strip PCB to make a dipole and measured the resonance frequency on the VNA .
I found out that way that my calculated VF for FR4 was not correct .
Based on my measurements the VF factor of FR4 should be 0,84 - 0,86 .
Searching the net for some formulas how to calculate the VF factor from the dielectric constant didn't solved my problem .
Comparing my calculated/measured VF factor with the calculations with AppCAD confirmed that the VF factor was right , but I don't know how to calculate it .
Could someone help me about the formula to calculate the VF factor from dielectric constant for PCB's.
Thank you,
Kenny