The difference between Coaxial Cable and Two Wires?

[bozza]

While I was rebuilding my android phone into a new frame the coaxial cable lifted off the pcb and pulled a pad along with it.

The coaxial cable connected the main pcb with a secondary pcb at the bottom of the phone. The secondary pcb contains the usb-c port, headphone jack, vibration motor, speaker, microphone and I suspect it also functions as a secondary antenna.

I replaced the single coaxial cable with two separate wires by soldering them directly to the pcb traces. (36AWG for signal and 32 AWG for ground). The metallic case is also grounded through which the two wires run. The signal wire is now sandwiched between the metallic frame and the black ground wire.

My phone reception is now excellent once again, but I do wonder if what I have done has any negative side effects.
Is the coaxial cable an antenna in itself or does it simply transport the signal between the two pcbs?

[james_s]

Does WiFi still work? 2.4 and 5 GHz? Coax has completely different characteristics than individual wires.

[bozza]

I was thus far only able to test wifi in the 2.4GHZ band which works fine.

I suspect the cable is for cellular internet and voice calls. After I soldered the wires onto the pcb the speed of the cellular internet increased!

This is all really just speculation and I too would like to know what that cable is actually for. Most android phones are designed in the exact same way.

[Bud]

Engineers put it there for a reason. Nobody would do it "just in case" because it is expensive to use coax in place of spagetti wires. Substitutes may work to a certain extent and effects may be subtle.

[Miti]

Why didn’t you cut the coaxial cable and solder it instead of the wires? It wouldn’t reach?

[coppercone2]

they are both transmission lines. One is shielded one is not. The impedance is not correct.

[bozza]

Why didn’t you cut the coaxial cable and solder it instead of the wires? It wouldn’t reach?

1.Convenience
2. It would have been slightly too short
3. I was concerned about the cable connector being damaged and not making a good connection with the pcb

[coppercone2]

if you are curious about what you made, research 'twin lead'. Basically its radiating alot compared to the coax in there (coax screen is made to be low leakage) and there is a reflection effecting SNR, and probobly other stuff is happening.

I kinda recommend you don't do this for something you might need to call for help on. Cell phones have a bit of power to them, and very small chips. I wonder if you are over heating something in there, along with some other things.

It's kind of like replacing a copper pipe in the basement with a piece of soaker hose that is the wrong dimensions. Kind of a  bad analogy because they also have coaxial cable that is more like a soaker hose (RF distribution used in mines), so maybe call it a aquaduct, idk. Because there are two ways to screw this up, using the wrong impedance, and also the wrong type, you did basically both.

Did you check your phones reception in a place where it used to bearly work?

this is what you did



wheras a good coaxial cable is this
(their not perfect!)


Well, if you put a proper twin lead in there (engineered), it would also not leak much (I suspect it would still be worse then a coax), but thats not what you got there.

[MathWizard]

On a breadboard, under a few MHz, like me building a radio TX/RX, if you run a few wires a few inches. If you just twist 2 wires together, and have 1 wire grounded on both ends, will that provide any useful shielding ? I guess I'll have to try myself right now.

But looking at the FFT plot of various parts of my breadboard circuit, yeah I really can see a bunch of noise get added or taken away, as I use different rows, or touch different parts or probe different spots.

Hey maybe I have an excuse to buy a big metal cookie can/box. But laying a breadboard in it while working on it, would be annoying I bet.

[T3sl4co1l]

Impossible to say. You certainly changed the impedance seen by the radio; unlikely but possible this could be destructive (transmitter mismatch).  It may result in less signal from that antenna; maybe it never chooses that antenna (assuming a switched diversity system).  The emissions pattern is certainly changed; whether that affects signal reception in permitted bands, or violates transmit levels, or causes noise in unrelated (prohibited) bands, who knows.

The next best solution would've been to cut the coax and solder it directly, assuming the connector isn't required for disassembly (i.e. can be worked around).  The best of course would be to repair the pad (can be done with a tiny dab of epoxy, replacing the torn pad and shorting over to the connecting trace) but that's quite some fine work that probably requires specialized equipment.

Tim

[bozza]

if you are curious about what you made, research 'twin lead'. Basically its radiating alot compared to the coax in there (coax screen is made to be low leakage) and there is a reflection effecting SNR, and probobly other stuff is happening.

I kinda recommend you don't do this for something you might need to call for help on. Cell phones have a bit of power to them, and very small chips. I wonder if you are over heating something in there, along with some other things.

It's kind of like replacing a copper pipe in the basement with a piece of soaker hose that is the wrong dimensions. Kind of a  bad analogy because they also have coaxial cable that is more like a soaker hose (RF distribution used in mines), so maybe call it a aquaduct, idk. Because there are two ways to screw this up, using the wrong impedance, and also the wrong type, you did basically both.

Did you check your phones reception in a place where it used to bearly work?

this is what you did



wheras a good coaxial cable is this
(their not perfect!)


Well, if you put a proper twin lead in there (engineered), it would also not leak much (I suspect it would still be worse then a coax), but thats not what you got there.

I did check my phone in places where it barely worked. My phone signal is actually better now than it was before it was broken. Maybe I just got lucky.

[coppercone2]

or they adjusted a transmitter. The telecom company goes out to measure signal strengths and stuff once in a while. I saw them do it. Or they put up more equipment to service more people, or adjusted equipment as result of a conflict, carrier change, system mod, fixed a mispositioned antenna (i.e. rectified wind damage), replaced a dodgy amp, fixed a supply problem, etc. Those things are not static. Unless you have a graph over time with a more stable receiver in the same place, those results are suspect. And its also susceptible to atmospheric conditions. It just takes one mod or company going out of business or something for them to start repositioning antennas height on a tower.

And it could have to do with transient phenomena like environmental changes involving physical obstructions near the transmitters, like roof work, scaffolding, or what have you.

Plausible reasons why it improved ASSUMING that the RF signal is constant (likely not).

1) From the very point of manufacture, the cell phone had some seriously dodgy solder joint or something, which greatly degraded the proper hardware, and then when you put a wire, it improved because there was a real connection path due to a solder touch up. Or like a damaged connector with a deformed pin or receptacle, damaged coaxial, bad crimp, etc.  Those micro connectors like SMB or smaller are VERY easy to damage by kids working in the factory. or the factory wanted coax and they sold it gold medal flour 

2) And you also mention a new frame. It does not sound like you tested the unmodified hardware in the new case before you broke it. A case change is something I would think to possibly improve SNR if the original is shit, but replacing a RF line with dodge city is not something I would think would help anything. Even if they are totally plastic, you still might have different spacing between the hardware parts, which effects the RF "ground plane" most likely.  I don't know what a frame is, I assume its the phone enclosure (not the extra case you put the phone in). Could even have to do with just reseating some connectors that were not making proper contact due to improper assembly that made the phone work as it should, dislodging some contamination, so forth.

 

Just so long it does not turn into a stupid tik tok about improving your phone by putting speaker wire in it you know? mr bean could make a skit about this (advanced version of the TV antenna bit).

[bozza]

Just to make it clear. I don't recommend this method of fixing the coaxial cable.

This was the most practical fix for me at the time. It worked perfectly for me so I don't see the need to solder in a coaxial cable.
Your own mileage may vary.

What I do however recommend is to experiment and try different solutions. You will see that many times a practical solution is possible without compromising any fundamentals!