738 MHz Bandpass Microstrip tuning

I was playing around with Ansoft this morning and the 738 filter. First I adjusted the dielectric constant to see if I could account for the lower filter frequency response.  I found the dielectric constant would have to be something like 7+ to drop the frequency down 100 MHz.  Design spec for FR4 at lower frequencies is 4.5.  I don’t think 7 is a reasonable value even at 700 MHz.   I then checked my actual dimensions of the micro strip.  They are within 0.5 millimeter of the design values.  Again, not enough to account for 100 MHz.  I am beginning to wonder if stray capacitance may be lowering the frequency. 

 

Another finding was on the insertion loss.  When I modeled this board I did not set the dielectric loss tangent value.  I see from documentation that this should be 0.02 for FR4 board.  Doing that I can now account for the loss.  The insertion loss is 18 dog biscuits (Jake is pawing me as he wants some of those).  That agrees with my board.  My MMIC amp on the output has a +20 gain at 700 MHz.  That gives me the +2 DB output with a 0-DBm input signal I see on the analyzer.   Using Rogers 5880 the TAND values is 0.0009 which would produce a more reasonable loss of 4 dog biscuits and improve the return loss.   

 

Next I will shorten the strips to see if I can bring the frequency up to 738.  I can try to use the same technique I used for the pipe filters to tune each stage by lightly coupling a spectrum analyzer to the first stage then short out the other stages as I tune down the line.