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I'm not sure if this is the proper StackExchange site, but you guys know RF and I don't know who else would know better.

I take the commuter rail to work every day. I have 60-70% cell phone coverage. I can regain 500 hours of my life per year if I could maintain a reliable data connection all the way in.

I've thought of switching carriers to get a hotspot device that supports external antennas, and carrying a 8dBi antenna in my backpack to place on the train table. Then I thought perhaps I can do the same with my cell phone and save that hassle.

I looked into the teardown of my device - the LG G5. It has two connections which would become perfectly accessible with some carefully drilled holes in the back cover. See bottom right:

LG Motherboard

Would gaining access to these ports, locating an adapter, and hooking up an antenna be effective and/or even possible in this scenario?

I could get a soft case with holes drilled in the right places, hot glueing the adapter cable to it for strain relief.

I also love the idea of being able to have a 2 foot antenna on a magnet on the top of my car next time I go camping or for a remote roadtrip.

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I would not do this, unless you are prepared to damage your phone.

There are multiple antennas in a mobile device, examples could be 850 MHz, 1700 MHz, 2100 MHz, 2600 MHz for GSM/3G/4G/LTE.

Some of those antenna's may be dual-or-multi-band, or MIMO — or other technologies. Simply "connecting an antenna" to an unknown port on a board, may result in damaging the circuit. Besides that, the ports you are mentioning may be there for diagnostics for the manufacturing process of the board/circuit.

Some antennas would be for WiFi and GPS as well.

I would not modify your mobile device, unless you have intimate knowledge of the circuitry.

What you could do is look into a small 12V/battery-powered 3G booster. There are various shops which would sell them; however you will have to carry the booster, the battery, the donor antenna, and the distribution antenna with you on your journey.

I have a booster from http://www.myamplifiers.com, for base install, and it did work for me. But as that is a home install it is a different piece of equipment. There are amplifiers made for mobile installations which run on a 12V supply, but I have no further experience with those.

Furthermore, in some countries/areas the use of a booster is illegal; you may want to look into this.

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From general radio-equipment principles, there's no reason you can't do this. However, there are many caveats, and it may be impractical without technical information you can't have. Here's the points you should consider:

  • It may be illegal to use a device with a modified antenna. This is true of some radio services in the US; I don't know if it's true of cellular radio and you haven't specified a location.

  • Those connectors may not be antenna ports, but serve some diagnostic purpose, or be for Bluetooth or WiFi antennas rather than cellular antennas. You need to find this information.

  • An antenna used for transmitting (as cell phones do) must be impedance-matched to the transmitter. If not, there may be damage to the transmitter. Some degree of mismatch is tolerated (and it is possible that the existing antenna is severely mismatched due to being small) but unless you have the datasheet for the transmitter chip you don't know what its limits are, or even what the optimum impedance is.

  • If one of those ports is an antenna port and is not normally occupied by an antenna connector, then it must be the case that you will be connecting your antenna in parallel with the built-in antenna. Connecting two antennas in parallel reduces their impedance (see previous point) and combines their radiation pattern (which is probably not significant if your new antenna is substantially better, but is still worth noting).

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  • $\begingroup$ Modifying a type-approved radio transmitter and then using it (except on the amateur bands for amateur radio purposes with an amateur radio license) is highly likely illegal, because you would then be using non-type-approved equipment on frequencies where you are not licensed to transmit using equipment not type approved for such use. $\endgroup$ – a CVn May 9 '16 at 21:26
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Remember that the antenna shape factor will interact with the operating frequency of your device. Changing the shape and/or the length may influence the energy and frequency radiated from your device. If you have Spectrum Analyser you may be able to optimise the output power and tweak the antenna shape to yield the MAX output power, or improve on RF power efficiency if you wish.

Have you tired to use bi-directional RF amplifier to add on the RF gain and probably change the receiver sensitivity?

Remember that increasing the range or coverage may yield to increasing the data error, if using data communication. So you may get sluggish communication. Unless you have the schematic I wouldn't recommend to tinker with it, unless you don't mind losing it.

A word of caution: if you walk with modified RF equipment on you and big brother sees you he may be mad - it did happen to many people though.

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