Adafruit GPS and antenna cable length

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ropesca
 
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Adafruit GPS and antenna cable length

Post by ropesca »

Hi,

I'm working on a project that uses the Adafruit Ultimate GPS Breakout (ID 746) to get a high precision time from satellites (NTP Server stratum 1). Currently I have attached to the GPS "GPS Antenna - External Active Antenna - 3-5V 28dB 5 Meter SMA (ID 960), but since the cable is too short (just 5 meters long) I need to extend this cable.

So, I extended the cable to 15 meters, but the GPS isn't working well anymore. How can I extend the antenna cable without losing time precision?

I appreciate any suggestion.

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adafruit_support_mike
 
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Re: Adafruit GPS and antenna cable length

Post by adafruit_support_mike »

You're running into a fundamental limit of electronics.

Admiral Grace Hopper used to give lectures where she'd hold up an 11.8" piece of wire and say, "this is one nanosecond". Light travels at 3*10^8 meters per second, which means it can move 30cm in 10^-9 seconds.

It's physically impossible to get a signal from one end of a 15m cable to the other in less than 50 nanoseconds, and that doesn't account for physical imperfections in the cable.

Coax cable has a specification called 'VOP' which stands for 'Velocity Of Propagation'. It's a fraction between 0 and 1, and represents the speed signals can travel through the cable divided by the speed of light in empty space. High-quality coax can have a VOP up to .9, and most oscilloscope-grade coax has a VOP around .7. With 15 meters of scope-grade coax, your propagation delay will be about 71.5 nanoseconds.

Non-coax cable has capacitance distribued along its whole length, and has to be modeled in terms of a 'delay line'. As you can guess from the name, that slows the signal down even more.

Those delays are physical limits on what the cable can do. Using the best quality cable, you can approach a 56 nanosecond delay, but it's physcially impossible to reduce the delay further without shortening the cable.

waltr
 
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Re: Adafruit GPS and antenna cable length

Post by waltr »

Also, due to the C and R in the coax cable the RF signal will be attenuated and is a direct function of the cable length, ei, the longer the cable to weak the signal out.

Then there is the issue of impedance matching and impedance discontinuities that cause reflections and signal degradation.
How did you extent the cable? With the exact same cable type and a properly impedance matched connector?

A better solution may be to extend the digital/power cable and not the RF/antenna cable.

ropesca
 
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Re: Adafruit GPS and antenna cable length

Post by ropesca »

adafruit_support_mike and waltr, thank you for your reply, it really cleared me up the question.

I extended the cable with a cable similar to the original one (not exactly the same). Do you think that if I get a cable pretty much the same as the original I could extend the cable up to 20 meters without problem?

waltr, I was wondering about extending the cable between the Raspberry and Adafruit Ultimate GPS (like you said) and keeping the 5 meter antenna GPS length. Would it work better than extending antenna cable?

Thank you!

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adafruit_support_mike
 
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Re: Adafruit GPS and antenna cable length

Post by adafruit_support_mike »

At the speed of light, moving 20 meters takes 67 nanoseconds. Nothing you can do with the cable will reduce that delay, though using lower-quality cable will add more sources of delay.

To eliminate the delay, shorten the cable. Nothing else will work. Move the whole GPS breakout to the point where you've placed the antenna, then extend the power/communication cables running to that.

waltr
 
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Re: Adafruit GPS and antenna cable length

Post by waltr »

Yes, keeping the antenna cable short and extending the power/data cable would work much better.
In Ham radio when we work with UHF it is common to put a pre-amp at the antenna, even if its at the top of a 100 foot tall tower, and run an additional cable for power and control.

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