Clay Autery cautery@montac.com said:

Crazy thought department.  Can you also run a parallel run of RG-6 and run
some tests to see if you can measure the difference?

--
These are my opinions.  I hate spam.

Hi

There is pretty much no experiment you could run that would show a difference
between the two. With a normal GPS, the “front end” of the radio is in the antenna. The
filtering and RF amplification there determine a lot of things. The cable is just a
chunk of wire in the middle of the system.

Bob

Agreed.
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) . I used the signal quality graphics of the respective manufacturers utilities winoncore12 and u-center. There was very little visible difference in the levels and reported resolved positions were as in the same ball park ( I never get exact replication after surveys ).
Neither was there any significant difference in the 1PPS signal. I only have a 2 channel scope so had to measure each seperateley against a standard (PRS10). The receivers 1PPS quantization swamps the difference in the cable delay (which I was able to differentiate when measured separately).
It make sense as the GPS signals are very weak and the receivers are good at getting data out, so just throwing in a bit of extra attenuation and noise doesn’t phase them at all.

I was thinking of doing a test with just a twisted pair….

"The power of accurate observation is commonly called cynicism by those who have not got it. »
George Bernard Shaw

Mike wrote:

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.

Best regards,

Charles

Hi

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