[time-nuts] 75Z vs 50Z for GPS receivers

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jan 28 08:50:16 EST 2007


Chris
>   
>> Christopher Hoover wrote:
>>   
>>     
>>>> Most (except for Trimble,..) GPS receivers and antennas
>>>> are designed to use 50 ohm cable.
>>>> Trimble Bullet GPS antennas have a 50 ohm output impedance.
>>>> Trimble literature however is ambiguous in that in the
>>>> Resolution T receiver datasheets talk about using RG59
>>>> to connect to the antenna.
>>>>     
>>>>       
>>>>         
>>> FWIW, the Thunderbolt manual says this on page 3-5:
>>>
>>> --------
>>> Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and
>>> the Bullet antenna are compatible with either 50-ohm or 
>>> 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable
>>> provides superior transmissibility for the 1.5 GHz GPS
>>> signal and a better quality cable for the price. Mismatched
>>> impedance is not a problem.
>>> --------
>>> --------
>>> Note - The input impedance of the ThunderBolt RF input &
>>> its antenna is 50 ohms.
>>> --------
>>>
>>>
>>> I would also add that BNC and N connectors come in both 50Z and 75Z flavors.
>>> In fact 75Z BNC connectors are pretty common, being used in professional
>>> video applications.  (Check surplus BNC patch cords carefully.)  On the
>>> other hand, 75Z N connectors are much less common, but were used in CATV
>>> plants, IIRC.
>>>
>>> -ch
>>>
>>>
>>>
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>>>   
>>>     
>>>       
For those of us who strive for the ultimate in performance with GPS 
timing receivers it is recommended that mismatched antenna cables not be 
used see:

http://www.bipm.fr/wg/CCTF/CGGTTS/Allowed/Recommendations/CGGTTS-Guidelines.pdf

Bruce



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