High gain antenna Cellular

[Unordung]

Hi folks,
I work for a company which designs, and services displays used in the transit system here in Australia.
The displays are used to provide information to the customers of the transit system such as timetables etc.

In a few sites we are experiencing significant connectivity issues and I'm trying to find a potential solution.

It seems some sites have a horrible connection and only stay online for short periods. Im wondering if there is a high gain antenna / amplifier solution we could try
to increase our chances of getting the data.

[Kleinstein]

An antenna with additional amplifier is likely not work as the cellular phones / networking cards tend to use the same antenna for send and receive. The modern input stages are often aready quite good, so hard to get better for an exteranal amplifier.

There should be high gain, which means directional antennas.  There may actual be legal limits to this as they increase the effective power in that direction. One may have to reduce the sending power to stay inside the limits. At least for the GPRS standard there should be some headroom, as early car based phones did use higher power (AFAIR 8 W compared to 2 W) than handhelds. So a 6 dB antenna gain should be OK.

Beside the signal stregth, there is however also a distance limit in the cellular standards, just from the delay.

[iMo]

The cellular operators may help you with this. They can change parameters of their BTSes remotely such your signal integrity improves.

[E Kafeman]

Local nearfield noise, dirty power source, high temperature?Metallic enclosures placed in the sun can get very hot during summer.
Current antenna, what type? Monopole with magnet foot, internal PCB antenna or external antenna, omnidirectional?What type of communication?
GPRS, nb-IoT...Any special frequency bands used at problematic sites?
Faulty own equipment or faulty antenna feeding (cable, connectors) is excluded? 
Weak signal permanently or from time to time? is RSSI, received signal strength, logged or noted? Corresponding values for other types of communication can be RSRQ or snr.
Any strong transmitter placed close to same location?
Local celltower out of free channels? Can happen if a lot of people is at same location, all using the phone.

To improve situation in a effective way must the problem be defined or else will you only spend time and money to often little use..

A directive antenna such as a Yagi-Uda pointing at celltower will increase RSSI and suppress noise from other directions compared to an omnidirectional antenna.
An Yagi-Uda antenna do also suppress unwanted frequencies and by then reduce interfering noise as it is more narrowband then a simpler monopole/dipole.
Sometimes is a directional antenna unwanted as any direction is unknown and then is focus more on an omnidirectional antenna with high efficiency.
Is it possible to temporary place a cellphone hidden in your control-box at same problematic location and let it log RSSI for same frequency and operator?

[nali]

Having spent over a decade in the same field I feel your pain. Heck, I may even know the company you work for...

In addition to what others have said:

Antenna specs: Plenty of snake oil, especially when it comes to "gain"! Also check the radiation pattern, and if the supplier can't provide one find one that does - which at least shows some form of competence rather than a reseller of chinesium junk.

Coax: Many antennas come with the cheapest, crappiest RG174 which is lossy as hell. If you do have to run coax for any length make sure it's decent stuff.

Get a network analyser, they're pretty cheap now. Do a proper site survey. Find out where the active cell is, which bands it uses and select & site your antenna accordingly.

Your display equipment should have some sort of heartbeat/watchdog and be able to hang up / redial if it loses connection. Unfortunately it's not an uncommon problem for static equipment on a mobile network in that the network "forgets" the client is registered - and at the same time the client modem doesn't know it's not registered so doesn't attempt to re-register. In fact most displays I've seen implement a power switch to actually power cycle the modem which sorts out most issues.

Finally, Mobile Data: You may need to change your SIM to a multi-carrier type if you have such providors where you are. But that's generally a strategic decision as it affects infrastructure (i.e. network routing), and more expensive.

[Halcyon]

There are a number of solutions. But as always, "it depends".

Are you on the fringe of reception or are you well and truly outside of the reception area? (Have a look at the reception maps your carrier provides).

Which carrier/cellular band/frequencies are you using?

High gain cellular antennas are common and fairly cheap. Rojone are a local manufacturer and have a bunch of different antennas for cellular applications: https://www.rojone.com.au/category/a-antennas/base-station-systems/

Otherwise you could look at something like the Cel-Fi repeater products which when paired with either a high gain or directional antenna pointing toward the nearest base station is a way to repeat cellular signals where you need them: https://www.cel-fi.com/products/. You don't need a licence to operate them and they have been approved for use by all three carriers in Australia (you'll just need to register your unit). I've used the Cel-Fi GO G31's before which support 3G and 4G otherwise there is a newer model that works with the 5G bands.