Hi Guys,
I came to this thread after replacing a bad USB / Charging port unit with a cheap after market part from ebay. I have done this twice on my old and much abused S2. I obtained a port of the same Rev as the last one which worked (Rev2.3), but this one left me with very low received signal although the USB connection and charging port work fine. I am not inexperienced in this kind of thing and obviously checked my work and the connection of the micro coaxial cable to the small socket on the USB board. After fitting and checking I had a VERY weak signal about -113dbmW when my wife's S2 was showing about -72dbmW at the same time in the same position on the same network. Hopeless.
Anyway - I saw that neat little testing RF port and made a small antenna of the correct measured length and got a much better signal by doing that (in comparison to my defunct internal antenna). A couple of points though:
Be CAREFUL what you poke in this hole as it has a very delicate coaxial switch inside it which can if abused, permanently disconnect the small coaxial cable to the inbuilt antenna.
You MUST make sure that apart from the part of the fine wire antenna needed to connect in the small RF socket at the centre that the wire you use is insulated from the shell of the little RF port which is at ground potential. It would be very easy to short out the signal by allowing connection between the centre and the outer shell of the connector.
You MUST cut the wire to the correct length for the RF signal frequency you are using. Antennas need to be resonant at the frequency in use, and not some random length.
My GSM service is at 1800MHZ and my 3g service is at 2100 MHZ. Since I only use the phone for GSM calls not data, I cut the length of my antenna for 1800MHZ which means I needed 36mm OUT OF THE HOLE, so I made my wire 39mm total which includes the bit that goes into the connector. The wire is at most 0.3mm diameter, 5 amp fuse wire. DON'T FORCE fat wires into that hole unless you want to destroy the coaxial switch inside it.
I removed the now useless white coaxial cable which connects the USB board and the RF board and put it aside in a safe place in case I want to restore normal operation later.
I then covered the wire except for the lower 2.5mm by sandwiching it in insulating tape and trimmed off the surplus width so I had a strip of plastic covered wire with 36mm covered and about 2.5mm bare. It is important to have a narrow strip (about 2mm) because I need to lose that wire into the groove that used to accommodate the white coax wire to the internal antenna (which is now useless because of my bad USB/charging port).
I also used a jewellers loup so I could see well, to carefully cover the ground parts of the shell of the rf socket with scraps of insulating tape. When I bend over the wire to run down the little channel for the coax cable, there is a risk that it might short circuit the tiny RF port.
Now, having inserted the tiny wire antenna, check the signal. I had two galaxy s2 phones side by side on the same carrier and I got more or less the signal strength on both of them at the same time. When satisfied that the signal is OK, carefully bend the little wire antenna over and tuck it into the channel that used to hold the little white RF cable that you already disconnected. Then replace the back shell of the phone and the screws which hold it on, being careful about the placement of the power and volume buttons which can sometimes fall out as you are putting the phone back together. I found it best to have the back facing upwards on the desk and place the main body of the phone onto it, face up. Insert the top of the screen first and checking the volume and power buttons remain in position, snap it together.
WARNING:
If attempting for some reason to replace the internal antenna either because you have a bad usb/charger board like me, or because you broke the little RF socket by meddling. YOU MUST work out the correct length of antenna for your carrier and service. I have gsm 1800MHZ from EE in the UK. YOU though may have gsm 900 mhz, in which case you will need a longer antenna. you can work out the length of a half wave antenna by using the following formula:
143/F in MHZ where F is the frequency of the carrier's service to you. This gives the half wavelength in meters.
BUT - WE NEED A QUARTER WAVE ANTENNA TO COME OUT OF THE CENTRE OF THAT RF SOCKET SO ->
143/f (IN MHZ) Gives the length of a half wave at the frequency. Divide that by 2. This gives the 1/4 wave length in meters. Multiply by 100 to get it in centimetres and that is the length of the wire OUTSIDE the hole.
For the record GSM 900 needs 79.4mm antenna length which you can probably lose down that channel for the white RF cable WHICH MUST BE REMOVED. You can't have a radiating antenna right alongside another wire or significant metal part without ruining performance.
This solution for my OLD phone probably does not provide an ideal impedance match and I would not do it for a good, new phone, but to save my almost five year old battered Galaxy S2 from the trash bin, it works and gives almost original performance on the service the antenna is designed for - 1800MHZ. It works on 2100MHZ data too but probably not as well.
It ain't perfect, but it works. If you want to go for a Rolls-Royce job, you could experiment with antenna length using the signal strength read out in the status menu by making the initial wire a bit too long and snipping back a couple of mm at a time to see where you get the strongest performance. You will probably end up trimming too much off before you work out where the sweet spot is, but just keep records and make another one of the ideal length. Also, bear in mind that the signal strength from the cell tower goes up and down depending on factors we have no control over so only do this trimming thing if you have another phone to compare side by side and can be sure where the best spot is.
The original antenna and matching circuit is very broad band which lets the same antenna work on 900MHZ, 1800MHZ and 2100MHZ. This work around for a broken phone will not be broadband like that, but if you can restrict yourself to one frequency, it will get you along as long as you need on an old damaged phone. I just walked around my city trying the signal this morning (I'm retired
) and there was only one spot in a real hole where I dropped down to unusable signal levels. I took a spare phone on the same network and that could have made a call in that spot, but it was right down in signal too with only -105dbmW of received power.
Good luck
