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

[Dr Bruce Griffiths]

Didier Juges wrote:
> Dr Bruce Griffiths wrote:
>   
>> 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
>>   
>>     
> Bruce,
>
> They recommend that reflections be 40 dB below the main signal, so if 
> you have about 15 dB loss in the cable (as I do), you only need no more 
> than 10 dB of return loss (combined at both ends) since the reflections 
> will have to travel twice more through the cable than the main signal. I 
> believe a 1.5:1 VSWR will do that (which is obtained by matching 50 and 
> 75 ohms, magically :-) Of course, if the cable is shorter and has less 
> loss, it should be better matched. So, 50 feet of RG-59 cable will meet 
> the recommendation.
>
> I have most certainly overlooked something, but I should not be too far off.
>
> Didier KO4BB
>
>
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>   
Didier

Perhaps at the subnanosecond level one may have to take into account the 
effect of cable mismatch on the phase and group delays?
It certainly would be useful if some measurements of the effect of using 
mismatched cables were made.
The Thunderbolt probably has inadequate performance to discern this.

They also recommend low cable attenuation.
However the hardest part of the spec to meet is the cable delay tempco 
which is 5 to 10 times better than standard coax.
It may be possible to meat this spec with high velocity cables (0.95c??) 
which essentially have an air dielectric.
I guess this is why the antenna cable temperature is regulated in some 
installations.

Bruce