For a single frequency use like GPS, the impedance should be close to the target. It is true for scanners and such, 50 ohms is quite nominal. (This notion of DC to daylight and maintaining 50 ohms is fantasy. ) But for a GPS, you know exactly the application.
The loading could effect the antenna pattern if it were not for the preamp. -----Original Message----- From: Jim Lux <[email protected]> Sender: [email protected] Date: Wed, 23 Jan 2013 06:07:49 To: <[email protected]> Reply-To: Discussion of precise time and frequency measurement <[email protected]> Subject: Re: [time-nuts] "Better" gps antennas than a Symmetricom 58532A On 1/23/13 12:41 AM, Hal Murray wrote: > > [email protected] said: >> This URL goes into some of the issues involved in using 75 ohm coax in a gps >> system. I do acknowledge that several GPS manufacturers have promoted the >> use of 75 ohm coax so some of the conclusions might be arguable.. > > If the coax is short, the loss due to mismatch dominates. If the coax is > long, the attenuation per X meters will dominate. And it's not clear that there's actually "loss" due to mismatch. Most antennas/preamps/receivers don't have exactly 50 ohm impedances. 75/50 is only 1.5:1, and there's an awful lot of antennas and receivers out there that only claim 2:1 VSWR. The usual spec for the antenna is 1.5:1, so for all you know the Antenna and Receiver is already 75 ohms, and RG6 will give you a better match than 50 ohm LMR400. probably a bigger issue *might* be that reflections will create synthetic multipath from the signal reflecting back and forth in the cable. > > There should be a crossover length What is it for 50 ohm RG-58 and 75 ohm > RG-6? > > > If you are designing your own antenna, you have the option of making one that > matches 75 ohm rather than 50. > > > > _______________________________________________ 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.
