Hi Gordon, I am referring to a 1/4 wavelength transmission line. This length of line has the property of transforming the impedance from one end to another. (The square root of the product of the input and output impedances equals the line impedance) So if this line is AC short circuited at the far end (say by 3 x 4p7 caps to ground) then the reflected impedance at the "open" end is very high. Effectively you are feeding the DC via a high AC impedance - which is what you are trying to achieve with an inductor. The same effect is used by the 1/4 wave 70.7 ohm matching lines used in the design. In the pwer feed case the acutal line impedance is not critical, so for ease of design I'd use the same line dimensions as the matching sections. Typically you would feed the DC through a convienient inductor/ cap (100uH/10nF ?) filter and then through a ferrite bead to the decoupling caps and then to the 1/4 wave line. This minimises signals fed in both directions. Since you are operating in a 50ohm system, anything with an impedance much greater than a couple of hundred ohms will appear more or less open circuit (ac wise), while any component with an impedance less than ~5ohms or so will behave somewhat like a short. Ferrite beads do look resistive at vhf frequencies, but you are only requiring AC impedance, not reactance so they do the job just as well (If they added inductance it could introduce additional resonances so this is a plus). I'm not sure about how good they are / what grades to use at 1575MHz but I imagine they should be OK, although totally resistive at that frequency. The problem with operating well above self resonance is that the behaviour is not too well defined. Inductors and capacitors can have several self resonant areas (Depending on size & construction & adjacent metalwork) so if you happen to hit one you could get very good - or very poor performance. Or temperature variations etc. It is simpler to keep below the first resonance unless you have very well defined components. Richard (Christchurch NZ) Gordon Williams To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE:] RF Questions about GPS Antenna Splitter Sent by: pic microcontrolle r discussion list 20/05/04 14:02 Please respond to pic microcontrolle r discussion list Hi Richard, Could you describe the idea behind a 1/4 wave line inductor? I haven't heard of it before. My understanding is that the ferrite bead idea starts to appear as a frequency based resistor rather than an inductor at frequencies above several 100 MHz. It appears that the equivalent resistance will be several 100 ohms at GPS frequencies. I'm not sure that is really what I need ... Do I need to worry about self resonance if I am well above the resonance frequency? Thanks for your other suggestions. Regards, Gordon Williams Ottawa, Canada ----- Original Message ----- From: To: Sent: Tuesday, May 18, 2004 5:38 PM Subject: Re: [EE:] RF Questions about GPS Antenna Splitter > I would use a 1/4 wave line rather than winding an inductor as anything you > wind is will have self resonances below 1.5GHz. A ferrite bead may be a > suitable alternative. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body