The reason that such cable mismatch isnt a problem is that the VSWR of the Thunderbolt input is relatively high.
Bruce Christian Vogel wrote: > Hi Steve, > > >> these frequencies. I was thinking outside the square and see that >> dual/quad screened RG6 is cheap and plentiful now but of course it's >> 75Ohm and there would be a big fat impedance mismatch using this. I >> thought about looking at fitting impedance matching baluns at each end >> but that is not cheap and there are losses involved with this approach >> anyway. >> > > the manual of the Trimble Thunderbolt GPSDO recommends doing exactly > that. 75Ohm cable obviously is cheaper, due to it being ubiquitously > used for TV and video installations and has lower loss due to the > higher impedance (impedance/resistance ratio). They (Trimble) claim > that reflections will not introduce any problems. > > One can argue that, to cause interference effects, a wave has to > travel back from the GPSO to the antenna and back to the GPDO (where > it interferes with the direct signal). This will be attenuated by the > SWR twice, and dampened by the cable loss twice. > > The issue at hand has been discussed at length one (two?) year(s) ago > and included a link to an article that analytically analyzed the > effect of multipath (of which the reflections are a specific case) on > GPS accurady, if I remember correctly. > > Chris > > --- Quote from the Trimble manual (ThunderboltBook2003.pdf), Page "3-5" --- > Note ? RG-59 is a 75 ohm coaxial cable. The ThunderBolt and the Bullet > antenna are > compatible with either 50-ohm or 75-ohm cable. Compared to most 50 ohm > cable, 75 > ohm cable provides superior transmissibility for the 1.5 GHz GPS signal and a > better quality cable for the price. Mismatched impedance is not a problem. > > Note ? The input impedance of the ThunderBolt RF input & its antenna > is 50 ohms. > > > > > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
