Hi Steve, I find it hard to believe myself, but the numbers don't lie. A 1.5:1 SWR junction gives 0.177dB transmission loss and a -13.98 dB reflection. If you have a true 50 ohm antenna and receiver and use 75 ohm LOSS-LESS cable, the transmission loss will be only 0.35dB and the doubly reflected wave will be 28dB down. Any line loss makes things rapidly better still. 15m of TV foam-type RG-6/U will have ~3.7dB cable loss or ~4dB total loss, and a reflection 35dB down. BTW, return loss of the cable itself is rated at -15dB @ 1500MHz! And, as has been pointed out, the SWR of the receiver and antenna will not be 1:1 either.
Good luck, David At 08:47 AM 4/7/2009, you wrote: >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. _______________________________________________ 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.
