urgent advice needed for repairing antenna cable inside my phone - Page 1
Forum section: electronics / repair

author: bozza
author_rank: Contributor
timestamp: January 15, 2023, 12:46:03 am
content: As I was rebuilding my phone into a new frame an antenna connector snapped and lifted off the pcb.There are two pcbs in my phone. The main pcb which sits at the top and a smaller secondary pcb which contains the speaker, headphone jack, usb-C port and vibration motor at the bottom of the phone. I suspect the second pcb also has the function of a secondary antenna. The top and bottom pcbs are connected through a flex cable and the antenna cable. The antenna cable runs down the side of the metallic phone frame. The frame itself does appear to have continuity with ground.To my surprise all the radios seem to function without that antenna cable. (Wifi , bluetooth, cellular internet, FM radio and voice calls seem to function normally). I feel like my phone reception is slightly worse though.Here are some ways I thought about fixing this.The proper way to fix it: somehow figure out the size of that connector and order the replacement part. Order a replacement cable. Wait 2 weeks for the items to arrive. Run a new trace and use a rework station to solder the part back on.That's nice but I would like to fix my phone without waiting two weeks so I can close it up without having to reopen it again.I could either cut the antenna cable at one end and just solder the wires straight to the pcb using the smallest soldering iron I have.This makes me slightly uneasy as I would be breaking the antenna cable even more. I wouldn't reuse this cable either way though since the connector is already slightly bent.I could also try to solder two generic wires onto both connectors instead. (One wire for ground and the other for the signal wire). Would this actually work? I know that the antenna cable has a wire core which is shielded with ground. Can I just run ground and signal side by side? Does that make sense? I don't know much about antenna design.The final option would be to try to salvage the broken connector and just solder that back onto the pcb. This is probably my least favourite option as the pcb connector and the connector on the cable appear to be at their EOL.Which of these three methods would be the best?

author: Swake
author_rank: Frequent Contributor
timestamp: January 15, 2023, 09:25:27 am
content: Decision is all yours....We don't know your skill setWe don't know your experienceWe don't know what tools you haveWe don't even know what phone this isIf it was me doing the repair and based only on your pictures:1/ The PCB traces are f*cked but the PCB connector is still fitted on the connector of the cable. No need to order a new one, just pry those 2 apart.2/ Clean the PCB traces so you'll be able to solder a patch later on.3/ Glue the PCB connector in place with UV glue (or epoxy glue but it needs 24h to cure well enough or super glue but that will not be as strong). Make sure to keep the connections free from glue so you can solder something on it.4/ Solder it back on, eventually using very short and small patch wire.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: January 16, 2023, 12:21:53 am
content: Imho, isn't it a non generic cable but a coaxial one ?Imho first place is correct; wait for the most suitable replacement part

author: bozza
author_rank: Contributor
timestamp: January 16, 2023, 04:55:27 pm
content: Thank you both for the fast replies.I have gone ahead and soldered two separate wires. The signal wire was the smallest I could find in my collection (36AWG which is just above 0.1mm). The ground wire is the slightly thicker one (32AWG silicone sleeved).My phone reception is now excellent so I am assuming that the new "antenna" works as it should.High five!

author: bozza
author_rank: Contributor
timestamp: January 16, 2023, 05:27:48 pm
content: Quote from: abdulbadii on January 16, 2023, 12:21:53 amYes the original cable is a coaxial cable. I don't know how using two separate wires will affect the performance.At the moment I am getting excellent reception with the two separate wires. I have also seen someone on YouTube using two separate wires instead of the original one.Perhaps someone who has studied antenna design is capable of answering this question.

author: Mario87
author_rank: Frequent Contributor
timestamp: January 17, 2023, 08:23:32 am
content: To be honest I would have just re-used the original connector and repaired the damaged trace, an easy 10 minute job with the right skills and tools.As for running the 2 wire side by side, it may seem to have worked fine, but the point of coaxial cable is that the GND is your shielding to minimise both emissions from the antenna and increase immunity against external interference.You might find with your work around that you lose signal more easily than before or interfere with nearby devices more readily or a mix of both.

author: bozza
author_rank: Contributor
timestamp: January 19, 2023, 05:11:04 am
content: Quote from: Mario87 on January 17, 2023, 08:23:32 amIn my particular case it was the right choice not to reuse the coaxial wire. The phone is actually functioning much better now than before.I have rebuilt this phone a bunch of times already and the connectors on these coaxial wires are only rated to be plugged in and unplugged a few times! ( I can't remember the exact amount anymore but the spec sheets list these connectors breaking after around 5 uses).The coaxial cable must have already been damaged beyond repair. Even when it appeared to be fine it wasn't making full contact with the connector.I am now getting a much faster cellular internet speed and my reception is better than it has ever been! Go figure!

author: TheMG
author_rank: Frequent Contributor
timestamp: January 19, 2023, 05:53:10 am
content: Pure luck basically.Coaxial cable is not just about the shield. It has controlled impedance characteristics, as does the antenna, connectors, and even the PCB traces. Impedance mismatches cause signal reflections (SWR) along the way which can reduce the amount of signal that actually gets to a receiver, and if the antenna is used for transmission, can also potentially damage the transmitter.In this case since the phone was still working with it completely disconnected, most likely it is a receive diversity antenna.I bet with your "fix" what is actually happening is the wire itself is kind of acting as an antenna, you just got lucky that this ended up improving rather than impeding performance.

author: bozza
author_rank: Contributor
timestamp: January 19, 2023, 09:48:01 am
content: This comedy sketch comes to mind

author: wraper
author_rank: Unknown
timestamp: January 19, 2023, 09:54:11 am
content: Two separate wires is garbage. It's certainly some high frequency signal which requires coaxial cable with certain impedance. Solder a thin wire strand to a central pin of the connector and then solder connector back to PCB on ground terminals. Then solder wire strand to the broken trace.

author: bozza
author_rank: Contributor
timestamp: January 19, 2023, 10:09:45 am
content: Quote from: wraper on January 19, 2023, 09:54:11 amover-engineerverb [ T ] (also overengineer)UK/ˌəʊ.vər.en.dʒɪˈnɪər/ US/ˌoʊ.vɚ.en.dʒɪˈnɪr/to create, design, or build something to be more complicated or perform more actions than is necessary or helpful:Her strategy is to over-engineer systems so that they will operate well even if conditions are more challenging than expected.Technicians often overengineer a product, introducing numerous features that customers do not need.https://dictionary.cambridge.org/dictionary/english/over-engineer

author: wraper
author_rank: Unknown
timestamp: January 19, 2023, 03:47:29 pm
content: What over-engineer? I said how to repair by placing the connector back so it works as intended. What you made is totally not ok for high frequency signals. It's better than no antenna at all, but it's garbage.

author: drhex
author_rank: Regular Contributor
timestamp: January 19, 2023, 06:46:52 pm
content: No offense @bozza but do you think the manufacturer would add a coax with connectors for way more money if two cables would do the same?