[time-nuts] "Better" gps antennas than a Symmetricom 58532A

[Jim Lux]

On 1/23/13 12:41 AM, Hal Murray wrote:
>
> mspencer12345 at yahoo.ca 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.
>
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