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

Didier Juges didier at cox.net
Sun Jan 28 19:41:57 EST 2007


Dr Bruce Griffiths wrote:
> Didier Juges wrote:
>   
>> Dr Bruce Griffiths wrote:
>>   
>>     
>>> Didier Juges wrote:
>>>   
>>>     
>>>       
>>>> Dr Bruce Griffiths wrote:
>>>>   
>>>>     
>>>>       
>>>>         
>>>>> Chris
>>>>>   
>>>>>     
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>>>>>>> Christopher Hoover wrote:
>>>>>>>   
>>>>>>>     
>>>>>>>       
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>>>>>>>>> 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.
>>>>>>>>>     
>>>>>>>>>       
>>>>>>>>>         
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>>>>>>>>>                 
>>>>>>>>>                   
>>>>>>>> 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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>>>>>>>>
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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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>>>>   
>>>>     
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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
>>>   
>>>     
>>>       
>> Bruce,
>>
>> You have done it again. Now, I have more ideas for interesting 
>> experimentation and still not more time :-)
>>
>> I have 3 coax runs going from my ham shack to the top of my tower to 
>> feed the HF (14 to 30 MHz) and two VHF antennas (6m and 2m, or 50 MHz 
>> and 144 MHz). One cable (HF) is regular RG-213, another (6m) is RG-214 
>> (essentially like RG-213 with double shield), the last one (2m) is 
>> Ultra-flex air dielectric (more like 9913). All 3 runs are close in 
>> length, about 135 feet. I have fed them with the 1 PPS signal and looked 
>> at reflections with the storage scope (that's how I know the length). It 
>> is interesting to see the big reflection when the signal gets to the 
>> antennas. Of course, the antennas are a poor match for the 1 PPS signal 
>> (fortunately), so they are essentially a short circuit.
>>
>> I need to take pictures, now that it is relatively cold here (everything 
>> being relative) and take the same pictures in the summer and look for 
>> differences between the 3 coax lines. Out of the 135 feet, 60 feet are 
>> in the air going up the tower, 50 feet are in the ground (in a 4" PVC 
>> pipe), the rest is in the garage and the attic, so temperature is not 
>> well controlled or constant, but it should all vary in the same direction.
>>
>> Didier
>>
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>>   
>>     
> Didier
>
> One potential method for measuring the "average" temperature of a cable 
> is to measure the dc (or low frequency) resistance of the inner conductor.
> If this is copper then it has a tempco of about +0.4% /K.
> For copper clad steel and other inner conductor materials you will need 
> to measure the resistance tempco, however they should have a similar 
> tempco, except of course in the unlikely event that an alloy such as 
> constantan, manganin, karma etc were used as in some delay line cables.
>
> 135 feet of 9913 has an inner conductor dc resistance of about 0.12 ohms 
> at 25C with a tempco of about 480 microhm/K.
> You will need to use a 4 terminal measurement method to achieve the 
> required accuracy.
>
> The one way propagation delay of 135feet of 9913 is about 162ns with a 
> tempco of around 8ps/K.
> Measuring the variation of the cable propagation delay with temperature 
> will be somewhat tricky.
>
> RG213 has a tinned stranded copper inner conductor, RG214 has a silver 
> plated copper inner conductor.
>
> Bruce
>   
Bruce,

The 2m antenna is equipped with a mast mounted preamplifier, so it is 
not a short at DC. To measure the resistance, I need to take the tower 
down, which is not very hard (it is a folding tower), but I need a 
little bit of time to do it, with no wind and no rain, no today  :-) 
Today has been cold, windy and rainy.

The other antennas are pretty much a short at DC, even though I need to 
measure the resistance at the feed point one time to be sure. Of course, 
the connectors may also degrade over time and may have increased contact 
resistance. I have used the DC resistance method in the past when I 
wanted to check if the connectors were in need of cleaning or replacement.

Interestingly, I just did it again, and the 6m antenna measures 0.82 ohm 
while the HF antenna measures 0.42 ohm. That's strange because I do not 
remember such a difference. I need to check my notes from the last time 
I did it. I would not expect much DC resistance in either of the 
antennas themselves, even though the HF beam is rated 1.5 kW and 
therefore uses much heavier gage hardware. Maybe there is something 
wrong with the 6m feed line or a connector?

The RG-214 used on the 6m antenna also has silver plated braids, which 
are MUCH easier to solder than the bare copper braid of the RG-213.

I did not go through the numbers, but obviously, measuring 8ps/K 
variation will require other equipment. The difference between average 
daily temperature in winter and summer (if you pick your days) may be at 
most 30 degrees C, so that's 240 pS. I could measure that with the 5370 
with a suitable signal generator.

I just checked, the Extra-flex cable I use on the 2m antenna has 84% 
velocity factor, like the 9913, versus 66% for the RG213/214.

Based on your calculation, I must have about 185' of feedline on the 2m 
antenna, which would be surprising, unless I have a big loop in the 
attic :-)

The same calculation gives me about 159 feet of RG-213 for the HF 
antenna. That sounds about right compared to the 0.42 ohm DC resistance.

The HF antenna is lower on the tower and the feedpoint is in the middle. 
The 2m antenna is long and the feedpoint is at the end of course, and 
it's at the top of the tower, so there is more cable there, probably 15 
feet. There must be another 10 feet coiled somewhere.

Still more questions than answers.

The good news is that for timing applications, it's going to be a while 
before I worry about being 240pS off from UTC :-) I am sure that for 
VLBA applications, every pS counts, but I am not at that point yet...

Didier KO4BB



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