what is the best substrate for a microstrip power divider ?

[guytoub]

hello!

i made a braodband ( 1-3 Ghz) wilkinson power divider with FRF4 substrate and it works very well.
but when i try to make another one (2Ghz - 18Ghz) with FRF4 Er = 4.5 or Duroid Er= 2.57 i've got problem with S21 parameters .
between 6Ghz and 18Ghz i've got a S21 who decrease between -8dB an -30 dB?

can somebody tell me why? it's the substrate is the cause of my problem
which topology is the best ? straight lines or curves?

thanks a lot

PS: don't care of my english i'm a beginner

 

[vfone]

Re: what is the best substrate for a microstrip power divide

Definitely the reason of poor S21 at those frequencies is the FR4 material.
For higher frequencies you can use one of these materials from Rogers:
http://www.rogerscorporation.com/mwu/pdf/prodselect.pdf
Curves lines usually are used in broadband splitters to minimize the total physical length.

 

[guytoub]

Re: what is the best substrate for a microstrip power divide

thanks vfone for your explanation.

i use curves lines but it's difficult to join them together and respect the λ/4

another question more.

can you have explanation about the relation between frequecies and substrate?
for example which substrate you'll use for a divider at f0= 5Ghz?

 

[vfone]

Re: what is the best substrate for a microstrip power divide

The loss tangent of the material is frequency dependant
Here you can find formulas for Er and Dielectric Losses:
http://qucs.sourceforge.net/tech/node31.html

 

[hitmo]

Re: what is the best substrate for a microstrip power divide

a good material at high freq is RT 5880 or materials from Taconic like RF-35. RT 5880 has low tand=0.00019 n is very stable at high freq works very well in Ku band.

 

[dowjones]

it is not only the loss tangent. there are more effects associated to the substrate material which are important at high frequencies: surface waves, which are generated by leakage and are linked to the substrate thickness and epsilon.

I suppose you use SMA connectors soldered to the MIC traces. These connections are also important.

You also mention that it is a wilkinson splitter: do you know the parasitics of the resistor?

regards

 

[guytoub]

Re: what is the best substrate for a microstrip power divide

hello!




to answer to dowjones: i use cms resitors but i don't know their parasitics .

i use SMA connector soldered to the µstrip.

thanks