[Galaxy S II General] evilv:
Hi Guys,

Old thread but there are probably still people using the Galaxy S2 and encountering this problem. I came to it 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. The second time I got a fake or not correct version USB port. The USB and charging port bit worked fine, but the RF signal transits the port to get to the internal GSM 3g antenna in the base of the phone. It is VERY COMMON for these ebay parts to not connect properly to the phone internal antenna. Loads of people have this problem if you google the issue of low signal after replacing the USB / charging port. This is because there is no connection through some of these ebay ports to the antenna, or they get the wrong part. After fitting the board 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. Hopeless.

Anyway - I saw that neat little testing RF port and made a small antenna of the correct measured length and got at least a 10db signal gain by doing that (in comparison to my defunct internal antenna). A couple of points though:

You MUST make sure that the wire you use in the centre of the hole is insulated from the shell of the little RF port.

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 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 need 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.

I had my phone completely open and I removed the tiny white coaxial cable from 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 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 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. I saw one photo above where someone had inserted bare wire twisted and that looked like it probably short circuited the 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 quarter wave whip antenna (which is what we have here) by using the following formula:

143/F all divided by 2 , where F is the frequency of the carrier's service to you. his gives an answer in meters.

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 it 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 intitial wire a bit too long and snipping back a couple of mm at a time to see where you get the strongest performance. bare 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 if you have another phone to compare side by side.

Good luck

EvilV