You are right too - if the system is perfectly balanced with Zo everywhere you get a good SWR. 0100,0100,0100But - with a non-ideal system, length adjustment is a must too. The antenna cable + the antenna carries a standing wave, that is being reflected off the active element of the antenna. If all impedances in the system are the same, you get the optimal SWR = 1. If NOT, the cable plays a role. Imagine that a resistor in a black box is at the end of a length of cable. From the outside world this length of cable will give the reflection from the resistor a phase shift since the signal must make a round trip through the length. If a 100 ohm resistor has an SWR of 2, a cable long enough to invert the signal after the round trip will make it look like a 25 ohm resistor, also with an SWR of 2 but with inversion (a cable with a multiple of 1/4 wavelength would do the trick). Since the impedance looking into this black box is a function of the SWR and the cable length, it can be seen that intentionally mismatched lines can be used to transform one impedance into another. Notice that the 1/4 wave cable inverts the impedance and preserves the SWR. This impedance inversion may be used to match two impedances at a particular frequency by connecting them with a 1/4 wave cable with an impedance equal to the geometric mean of the two impedances. A 50 ohm, 1/4 wave cable will match a 25 ohm source to a 100 ohm load : sqrt(25 x 100) = 50. Of course, it is not always easy to find the desired impedance cable! Multiples of 1/2 wavelength will give enough delay that the reflection is not inverted and the impedance will be the same as the load. Such cables may be used to transfer the load impedance to a remote location without changing its value (at one frequency). Other cable lengths will transform an impedance which differs from the cable's impedance with a reactive component. If the load is a lower impedance than the cable, a length below 1/4 wave will have an inductive component and above 1/4 (but below 1/2) wave a capacitive component. If the load is a higher impedance than the cable, the reverse is true. Above 1/2 wavelength, the reactance will alternately look capacitive and inductive in 1/4 wave multiples. This reactance will combine with the load's reactance and offers the possibility of resonating the reactive component of the load. Therefore, a cable with the "right" length and impedance can match a source and load with different resistance and reactance values. Obviously, these calculations can become quite involved and most engineers resort to a Smith chart, a computer program or perhaps the most common method, trial and error with a SWR meter or return loss bridge! In most cases, it is most desirable to match every component of a system to the chosen system impedance so that device matching is not frequency sensitive and critically dependent upon the cable lengths. But with the small 10mW 433 Mhz modules, this is often very difficult. KentTimes New Roman 7F00,0000,0000Arial> I don't understand why you are imposing this requirement. If the antenna is > designed to provide the correct impedance for the cable, then you shouldn't > care about the length. If it isn't matched correctly, then making it an odd > number of quarter wavelengths isn't going to help much, as far as I can > see, because all that will do is allow two matched conditions (one where > the antenna presents Zo(cable characteristic impedance), and the other > where Zo is the geometric mean of the antenna's impedance and the > transmitter's impedance. > > Sean > > At 07:58 AM 3/26/01 +0200, Kent Johansen wrote: > >One note of caution: beware of the stand wave ratio. (SWR). A > >quarter of a wavelength is 17 cm. The antenna cable should be an > >odd number of quarter wavelengths, corrected for the cable factor. > >It is very hard to measure SWR on 433MHz (If you don't have a > >Bird Wattmeter). The best way is to guess and correct. But a > >wrong SWR will be able to completely eat your signal. > > -- > 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