I have a remote wireless water tank level sensor. It transmits at 439 MHz with an apparent max range of 100 metres. Unfortunately I need about 200 metres.

How can I extend that range?

  • $\begingroup$ Hello Dan, and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Oct 12 '20 at 17:20

A repeater would be complicated, you'd need to get into the details of how the signal works.

Fortunately 439 MHz is in or near the amateur band, so external antennas are available. I suggest you modify the receiving end of the system to include an external antenna.

To double the range of a radio link, all things equal, you need an additional 6 dB of gain. At 439 MHz you'll easily buy plenty of gain with a yagi antenna. There are many for sale, if you search for "70 cm yagi antenna", here's one from Amazon: yagi

I suggest opening up the display unit instead of the transmitter, as increasing the transmitter effective radiated power with an external antenna will make the unit violate its type approval. Also the transmitter will be outdoors, need to be waterproof etc.

Find the antenna on the PCB, and a nearby ground point. Disconnect the antenna and solder on a coaxial cable. Ideally it would look like this photo from SE:
enter image description here
But more likely it'll end up looking like this (photo from AC0C):
enter image description here.

It's important that the ground be close to the antenna connection, no more than 10 mm away, not on the other side of the board.

Mount the yagi antenna outside, aimed at the transmitter, and rotate it to find the polarisation that works best for you.

  • 3
    $\begingroup$ Be aware an external antenna is probably not legal in the US. $\endgroup$ Oct 10 '20 at 21:12
  • $\begingroup$ Good point. Though if it's a one way link, it's OK ok the receive end. $\endgroup$
    – tomnexus
    Oct 10 '20 at 22:18

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