[time-nuts] GPS Antenna Feed Line Decision

Bob kb8tq kb8tq at n1k.org
Tue Sep 5 19:08:01 EDT 2017


> On Sep 5, 2017, at 5:19 PM, Charles Steinmetz <csteinmetz at yandex.com> 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 ….


> Best regards,
> Charles
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