Need help to boost/repeat a 433mhz signal

I have a remote water tank level gauge.
It outputs in 433mhz to a gauge n the house.
It has a 100 metre limit range.
I need a 300 metre range.

How do I boost/repeat the radio signal?
or do i need to hack into the device output antenna cable?

You don’t. I am pretty sure this unlicensed part of the radio spectrum has an upper transmit power limit. The reasons should be obvious. Any repeat would require a frequency and/or band change and would probably intrude into the license requirement area. The ACMA website would have more info on this. in all seriousness you would maybe find it far simpler to walk down to the water tank with a long measuring stick.

I don’t know what your set up is. Is the 100 metres quoted using omni directional antennas (vertical whip types). If so and you have line of sight you may be able to consider directional antennas at each end. Possibly Yagi or Log Periodic. To extend to 300 metres you will need about 10db to 15db extra. Should be gettable with good antennas but you will then have a dedicated system for that water tank. Keep in mind cable losses. If you start running long cable runs you will probably lose more in the cable than you gain with the antennas. For this to work at all you do need line of sight.
Cheers Bob

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We may need to find workaround to this setup you’ve currently got, as Bob said, there are laws regarding transmission strength of RF in Australia without appropriate licensing, so you would need to be very careful to check regulations before attempting any modifications of your devices that could take it past the preset limit. However, depending on the microcontrollers you’re using, you may be able to switch out to another pair of modules with more range. I’d take a look at the pair I’ve linked below, which should provide point-point comms at a max range of a kilometer if you use the appropriate libraries to transmit your data. All the best with your project!