CenterClick NTP200 and NTP250 Documentation - Antenna

Troubleshooting Antenna Issues

GPS reception issues can be one of the most difficult problems to resolve. The guide below has some steps you can take to help determine the root cause.

Tools available in the NTP2xx appliances

We've added information in our appliances to help debug.

• The GPS Details page including:
  • Current used count
  • Overall SNR reading
  • SNR of each satellite
• NTP/GPS Graphs
  • Overall SNR graph
  • Satellite usage graph

Changes to antenna position/reception are generally observable within 5-10 seconds, however some items such as initial aquisition or trends, can only be determeined with longer-term testing (1-6+ hours). In some cases it may take several tests to try to improve reception

Example 1:

Lets look at this example, an appliance that has reception but it comes and goes. Symptoms include occasional 'no-gps' errors and stratum >1 on occasion, however most of the time there is gps lock and satellites used is >6.

Let's first look at the easiest indicator for reception quality, the immediate overall SNR:

In this example, a SNR of 20.5 is marginal, however if I refresh the page a minute later:

we can see it's now 17.8 which is poor.

The SNR displayed here is the real-time average of the 6 satellites with the highest SNR and is a good reading of the signal strength. A reading of less than 20 is poor, 20-25 is fair, 25-30 is good, and greater than 30 is great.

For toubleshooting purposes, you should ignore the 'seen' satellite count as only the 'used' count is generally useful. And having said that there isn't much gained by a larger number of 'used' satellites as long as it is more than 6-8. The SNR is much more important than the number of 'used' satellites.

The overall SNR is a good readout of the real-time reception, however it can be quite noisy and doesn't give a good picture of the longer-term quality. For that we must look at the graphs for the same device. The NTP/GPS Graphs page provides a graph of the same SNR value, but over time:

We can see that most of the time, it's centered around 18 +-3, but occasionally drops out for a few hours as well. Also on this page we can see the counts over time:

We can see that while the used count on our two stapshots above were 10 and 12, in actuality, most of the time it stays within the 5-10 range and there are lots of dropouts.

In conclusion, if were were to encounter the appliance above we'd conclude that it is unlikely there is a hardware issue, and the cause is simply poor reception.

Example 2:

Here's another example, this appliance has a slightly better reception than example 1, and doesn't have issues.

The SNR is averaging around 21 over time vs the 18 in example 1 and those extra 3dB make a difference. There are zero dropouts in the 'used' count and we would conclude that while reception is near the limit of usability it is stable and the appliance is fully functional.

Example 3:

In this example, this appliance has worse symptoms than example 1 or 2. GPS shows no-fix and zero 'used' satellites frequently, but not all the time.

There is a period of several days were no signal was good, but it occasionally comes into operation for a few hours before dropping off every so often. Also, it is important to note that when there are used satellites the SNR is still low. In this example, it is unclear if the antenna is bad or just very poor reception.

Also note that the 'seen' count is quite useless as it never goes down during the outages.

Is my hardware bad?

Antenna SMA connector short circuit

The most common antenna defect is a poorly made connection joint between the cable and SMA connector. This will result in a near or full short circuit between the pin and sheild of the SMA connector but may only do so under some physical stress (i.e. only when screwed down hard, bent in one direction, etc...).

We test all antennas we ship against NTP2xx appliances and can usually elimiante these types of defects, however, on occasion this type of fault can be seen if the fault got worse due to flex/bending of the cable/connector junction.

You can usually test for this fault using a multimeter:

1. Disconnect the antenna.
2. Use a multimeter and check the resistance on the cable end of the antenna between the SMA pin and SMA sheild. The antenna module should read >100 KOhm.
3. Use the multimeter tip to push the pin a bit to one side and then the other without touching the shield to see if it short circuits.

WARNING: Do not intentionally push on the the SMA pin in the direction towards the cable. This can result in the pin actually sliding into the connector making it slightly too short (see below).

In some cases a short circuit defect only occurs when the SMA is screwed down and the pin has pressure on it. Here is an example of a short circuit that only occured once the SMA connector was screwed down. We are using a bare SMA edge connector to attach the multimeter.

There are some symptoms of this type of fault while under operation:

• The GPS data (SNR / satellites) show the same as if no antenna was connected
• The SMA connector is hot
• The temperature of the appliance is elivated, by 10-30F

SMA connector pin alignment or length

Sometime mechanical damage just happens. We've seen a few occurances of the SMA connector being damaged, usually on the antenna end but occasionally the NTP2xx as well.

The pin can get bent resulting it not aligning with the slot on the connector. This usually results in it being very difficult to screw down the sheild and in the process it deforms the appliance-side SMA connector preventing another pin from ever connecting properly. This type of damage is easially observable after the fact.

Alignment issues can also cause the SMA pin on the antenna side being pushed back into the connector towards the cable. This makes it too short to later contact with the appliance in future attempts. The pin of the SMA connector should protude about 2.5mm out of the base.

GPS module or power supply damage

The GPS module on the appliance can be damaged, you can check the basics with a multimeter:

1. Disconnect the antenna
2. Power off the appliance
3. Check the resistance on the appliance's antenna connector, sheild to hole. It should be >1 KOhm.
4. Power on the appliance
5. Check the voltage on the appliance's antenna connector, sheild to hole. It should be 3.0-3.3V DC.

Improving reception

Location

We have found that in most cases, people do not have issues with using the included antenna indoors. however it is best to:

• Placing items on top of or within 3" of the sides of the antenna (this includes coils of the antenna cable itself)
• Placing the antenna or device near noisy electrical equipment such as unshielded power supplies, cheap appliances or open-frame hacker/iot/dev kits.
• Placing the antenna in a location with large HVAC ducting above (which may be hidden in suspended ceilings)
• Placing the antenna pointed to the side more than 45 degrees off vertical.
• Placing the antenna below ground level near a wall such as a corner of a basement.
• Placing the antenna inside building that have large amounts of concrete, metal, or brick especially in ceilings or the roof.

• Have at least a portion of a field of view free of obstructions (for example being near a window or side of a building)
• When possible having as wide a field of view with little obstruction
• If necessary, using an outdoor antenna will always produce better reception

Ground Planes

The Antenna's effectiveness will be improved if attached to the top of a metal sheet or plate that is parallel to the ground of at least about 5" in size. The antenna puck includes a magnetic base.

Antenna Cables and Extensions

The included antenna has a 3m/10ft RG174 cable. RG174 is small an flexible, but not excellent for long distances as it has relatively high loss per foot compared to other options. While it can work for GPS at longer than 10ft lengths, it is not recommended that you extend RG174 beyond 10ft unless the expected signal gain by doing so would counteract the added loss due to cable length.

For longer runs, it is recommended to use RG58 cable instead of RG174. Both are 50 Ohm Impedance and commonly found with SMA connectors for GPS and cellular applications. RG58 and similar types of coax cables marketed as 'low loss' or 'ultra low loss' can be used.

NOTE: Avoid cables designed for WiFi as they likely have RP-SMA instead of SMA connectors.

Regardless of what cable type you use, a shorter antenna cable is highly desirable as losses are cumulative per added foot. The NTP250 features PoE for powering itself over its Ethernet connection so that the NTP250 itself can be located in otherwise difficult to install locations. Ethernet does not suffer significant losses even out to 200+ feet and it is preferable to relocate BOTH the NTP250 and its antenna to a better location than to extend the antenna cable. The NTP200 can also be relocated but a local power source would be needed wherever it is located.

Alternative Antennas

There are a variety of antennas available for purchase, the following hard requirements are needed when picking an antenna:

• Be a powered or 'active' antenna, typically these will be listed as having around 28dB gain, and/or a built-in LNA/SAW.
• Accept 3.3V power over the antenna cable, for example an antenna may accept a voltage range of 3V-5V or 2.5V-5V or 2.5V-3.6V which all would be acceptable, however an antenna requiring 12V would not.
• Be tuned for 'GPS L1' frequency (1.575Ghz), however it may also list GLONASS/Galileo/BAIDU/etc.. all of which are in the 1.55-1.62Ghz range. A WiFi or Cellular antenna will not work.
• Have a SMA male connector (or adapter)
• Use 50 Ohm Impedance cable (usually RG174 or RG58)

The following products have been selected after review of their data-sheet or specs, they appear to meet the criteria above, however unless specified below they have not been verified to work with the NTP200/NTP250. If you obtain one, we'd like to hear about your results.

Antenna	Mounting Type	Waterproof	Voltage	Cable Type	Cable Length	Tested
	Surface screws or threaded pole	Yes	3V-5V	RG58	23ft	Mixed results. Works for some but not others, vendor may have switched components.
	Through hole screw mount	IP69K	1.5V-3.6V	LMR100	10ft	unknown
	Through hole screw mount	IP67	2.2V-5V	RG174	20inch	unknown
	Through hole screw mount	IP67	2.2V-5V	RG174	13ft	unknown
	Magnetic Base	Unknown	3V-5V	RG174	16ft	unknown
	Magnetic Base	Yes	3V-16V	RG58?	16ft	Verified to work - WARNING: needs SMA adaptor

Cable	Cable Type	Cable Length
	RG58	10ft
	RG58	33ft
	RG58	49ft