Hi > On Sep 5, 2017, at 5:19 PM, Charles Steinmetz <[email protected]> wrote: > > Mike wrote: > >> I tried to see if there was any difference in the signal quality reported by >> Motorola UT+ and U-Blox Neo 6M (not timing grade but has a good 1PPS) over >> 30m of 2 different cable types. >> I only had RG58(75 Ohm) and RG174(50 Ohm). > > RG58 is 50 ohm. RG59 is the "equivalent" 75-ohm cable. > > Also, there has been some discussion of internal reflections with mixed > source/cable/load impedances. Yeah, there is some, BUT: The reflections are > generated at the *interfaces* between impedances, not down the length of any > one cable. "F" connectors are 75 ohms, so if any part of the system uses > them, they will generate approximately the same reflections at the interfaces > to 50 ohm components (cable impedance and impedance of source and/or load) as > would using 75 ohm cable. > > To eliminate reflections "entirely" (in reality, "mostly"), you need to use > 50 ohm [or 75, or other] components exclusively (source and load Z, all > connectors, and cable). > > Yeah, I could pull my Tbolts apart and replace the F connectors with SMA, > BNC, TNC, N, UF, whatever. But any degradation caused by using 75 ohm cable > and connectors is completely swamped by other errors in the system. One of > my Tbolts runs with about 30 feet of RG6 quad, the other with about 150 feet > of the same, and all timing and positional solutions are indistinguishable > from a test mule I cobbled together that used only an adapter (about an inch > and a half long) between the antenna and the Tbolt. In that case, the > "cable" delay was so low that any reflections were time-displaced from the > direct signal by such a small amount that it was meaningless for all > practical purposes.
The next “bump” is the input impedance of the receiver. If you dig into it, receivers front ends rarely have good return loss numbers. There are a number of reasons for this. One is that a “mismatched” front end is usually a lower noise solution. If the front end is 103 -123J ohms, 75 ohm vs 50 ohm cable is not the big issue. To a lesser extent, the same thing may apply to the antenna preamp output …. Bob > > Best regards, > > Charles > > > _______________________________________________ > 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.
