I have a old tv that only uses a coaxial cable. The tv does not have any rca or Hdmi inputs or outputs. I was wondering if I can convert a coaxial to a Hdmi input so that I can plug it in to my Android tv box or into my computer so I can view it on my old coaxial tv.
The coaxial input on your TV accepts an RF signal and it then has an analog tuner.
What your need is an RF modulator to produce this signal. You then tune the TV to a particular channel (often VHF 0 or 1, or UHF) to pick up the signal.
These are country specific.
One cheap solution is to find an old VCR with an RCA input. VCRs typically had a built in RF modulator to support this type of television.
If you want to buy an RF modulator, then again you need one suitable for your country.
Here is a model for Australia:
Models that accept HDMI input usually output a digital TV signal, so you need a digital tuner. These are not suitable.
For the cost of these modulators, you are probably better off looking for a used LCD television on ebay.
My question is can I convert a coaxial cable to a hdmi input
The input is on the television. This is what you are calling "coaxial". It is an RF input with an analog tuner.
The output is from your computer or android TV box. This is hdmi, which is a digital video signal.
I've explained what you need to achieve this in my earlier post.
I have not seen a device to downscale a digital video over HDMI to NTSC or PAL video and output as an RF signal for an analog tuner.
You are unlikely to find what you would need, and if you did it would be expensive.
You are better to buy a used LCD television with a HDMI input.