Chris Cheney wrote: >> I've seen that you're right, Thunderbolt is originally supplied with 75ft >> of 75 ohm RG-59 cable, as indicated in the manual... So I've the inverse >> problem: the GPS antennas we have in the roof both have 50ohm cable :) >> So... would be good to change the impedance at the Thunderbolt antenna >> port from 75 to 50 by means of some impedance converter? (of course, not >> altering the DC supply to the antenna). >> > > If a correct match is a must-have, see > http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm on "twelfth-wave > transformers". Basically, in series: > > 75ohm----50ohm(La)----75ohm(Lb)----50ohm > > The lengths La and Lb are approx one-twelfth wavelength in the cable. > > Continuity for power feed is maintained. > > But, since the L1 free-space wavelength is about 20 cm (cable wavelength > probably 16-18 cm), one twelfth of a wavelength is rather short - quite > possibly impractically short as the connectors must be included :-) > > Chris > G3RSE > > > Forget the complication of an impedance transformer just trim the cable to the appropriate length by adding a short section of additional cable if necessary. Unless the SWR (or reflection coefficient) of the antenna and the receiver are measured using an impedance transformer may make matters worse.
Bruce _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
