When using a stub, its reflected signal combines with the incident signal at the junction. If this reflected signal were equal in amplitude to the incident signal, it would offer (theoretically) infinite rejection. Because a stub has a non-zero loss, there is never complete cancellation. If the reflection is just 1 dB down, (about 89 percent amplitude) it will only give about 20 dB of attenuation. It is pretty easy to do better than this. But I'll stick with it for this example.
If the signal is also not shifted exactly 180 degrees, as when slightly off frequency, full cancellation is not possible. If it is 10 degrees off, (900 Mhz instead of, say 810) then it is reduced to about 82 percent due to phase shift. If you had a 89 percent cancellation before, now you effectively have only about about 73 percent, and can expect a null of about 11 dB. This assumes nearly lossless coax. If you include the RC or RL phase shift due to loss, you get the total effect of imperfect Q. These numbers are close if I did the arithmetic right (grin). All this disregards the other effects on DESIRED signals. Placing a stub in parallel with the feedline parallels a reactive impedance which is numerically equal to the product of the tangent of its length in degrees and its characteristic impedance. A quarter wave 810 mhz open stub would at 405 Mhz be 45 degrees long and shunt 50 ohms of capacitive reactance with your coax. I have had some luck, in non-critical applications, with placing two tee-fittings or two splitters in a line, then adding a length of coax in parallel with the main feedline but a half wavelength (in coax) longer than the distance between the fittings. The signal is delayed 180 degrees and then summed with the original signal. If the two junctions are colocated (no distance at all, just four receptacles at one spot) this reduces to a parallel quarter-wave open stub. The farther apart the junctions are, the less upset (except for SWR in the paralleled section) you get. There are multiple notches due to this setup. When using tee's there are SWR reflections from the tee's, which might be reduced by using 93 ohm coax (100 ohms is perfect if it's available) for the notching section parts. If you use hybrids instead of tee's you avoid the matching problems. Let me know what you end up with. It's a fun experiment. Cortland ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
