If I replace an old, roof-mounted TV antenna with a 2.4GHz WiFi (IEEE 802.11) antenna; can I use the existing coax? Or will I need to run all new cable?
Electronic – Are TV coaxial cables compatible with WiFi antennas
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Related Solutions
First, I would look up the datasheet of the RF chip to make sure that the output impedance is 50 Ohms (it doesn't have to be).
Then, peel the coax of your antenna in such a way that it looks like a 3 toothed fork with the inner conductor being in the middle. This is really important for best performance (otherwise your antenna may not work well due to poor balancing).
Using a sharp knife, razor blade (etc) remove the mask on the a small section of the trace leading up to the antenna so that bare copper is visible.
Cut that trance after to pad to isolate your old antenna.
Expose copper on both sides of the center trace (your ground).
Solder the center conductor of your coax to the former antenna trace pad and solder the coax outer shield grounds to the pads.
Keep these unwound wires as short as possible.
Keep in mind, that you will still lose a couple of dB in the transition and cabling.
You can measure a lot of things (impedance, velocity factor, distance to short-circuit, distance to open-circuit, ...) with a TDR (Time Domain Reflectometer) as shown and explained in multiple tutorials.
For example:
Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter
How to measure coax velocity factor VF and impedance Z
"TDR" or Time Domain Reflectometer, build and use this circuit
Determining Velocity Factor of coaxial cable
Understanding DTF or Distance To Fault, using a TDR
Determining Coax Impedance with a TDR
Best Answer
So you want to transport that 2.5 GHz (or even 5 GHz ?) Wifi signal over TV COAX cable ?
Indeed to the non-RF people you'd just think that would work. And it does BUT there will be almost no signal coming through that cable.
The Wifi signal will be attenuated so much in that COAX cable that it will defeat the whole purpose of having an antenna on the roof. The same antenna directly on the router might even get better coverage.
Why is that ?
TV COAX cables are not designed for 2.5 GHz signals, TV signals go up to 1 GHz and even at that frequency you can expect a lot of attenuation.
TV COAX cables usually have a characteristic impedance of 75 ohms, Wifi antennas routers etc. all use 50 ohms. There are no exceptions to that.
So no, in practice this will not work at all.