I find this software plus 3/4ths inch wide 1/4th inch thick brass strips from McMaster-Carr make it easy... Kick in 0.25 more dB of loss if you use brass instead of OFHC copper rods.. Its very error tolerant, especially when you add in a few adjustment screws.
http://www.vk3um.com/interdigital%20filter.htmlFrom one of the on-line calculators, ie WA4DSY:
Design data for 3 section interdigital bandpass filter.
Bandpass ripple = 1.000 DB
Center frequency = 1420.000MHZ
Cutoff frequency = 1416.500 and 1423.500 MHZ
Ripple BW = 7.000 MHZ
3 DB BW = 7.664 MHZ
Fractional BW = 4.930 MHZ
Filter Q = 185.280
Est QU = 1966.202
Loss Based on this QU = 2.257 DB
Delay at band center = 114.624 NS
Freq DB loss
* 1410.0 32.7
* 1410.5 31.3
* 1411.0 29.7
* 1411.5 28.1
* 1412.0 26.4
* 1412.5 24.5
* 1413.0 22.5
* 1413.5 20.2
* 1414.0 17.7
* 1414.5 14.9
* 1415.0 11.8
* 1415.5 8.2
* 1416.0 4.3
* 1416.5 1.0
* 1417.0 1.0
* 1417.5 1.0
* 1418.0 1.0
* 1418.5 1.0
* 1419.0 1.0
* 1419.5 1.0
* 1420.0 1.0
* 1420.5 1.0
* 1421.0 1.0
* 1421.5 1.0
* 1422.0 1.0
* 1422.5 1.0
* 1423.0 1.0
* 1423.5 1.0
* 1424.0 4.3
* 1424.5 8.2
* 1425.0 11.8
* 1425.5 14.9
* 1426.0 17.7
* 1426.5 20.2
* 1427.0 22.5
* 1427.5 24.5
* 1428.0 26.4
* 1428.5 28.1
* 1429.0 29.7
* 1429.5 31.3
Quarter wavelength = 2.078 inches
Length of interior elements = 1.867 inches
Length of end elements = 1.870 inches
Ground plane space = 0.750 inches
Rod diameter = 0.250 inches
End plates are 0.500 inches from center of end rod.
Tap external lines up 0.046 inches from shorted end.
Line impedances: End rod: 78.536, other: 80.355 Ext. lines: 50.000 Ohms
Dimensions (inches)
El. No. End to C C to C G[k] Q/Coup
0
1 0.500 1.498 2.024 0.644
2 1.998 1.498 0.994 0.644
3 3.496 0.000 2.024 2.216
4 3.996
Box inside dimensions: 2.078H x 3.996W x 0.750T inches
You have to, of course, optimize it before you build it, the above is an example of 3 iterations using hardware store rod sizes.
If you have nothing to align it with, I find High Sierra's harmonic generators kick ass, and have good value for the price, especially the one with an external frequency input:
http://www.hsmicrowave.com/CMG.htmlSteve