Introduction: 433 MHz Coil Loaded Antenna
In my 433 MHz projects I have been using a cheap (0.70 cnts) pair of Tx/Rx
modules. I have mostly used the transmitter and that is actually pretty good: if used to trigger comercially available remote switches, it reaches very far with just a simple 1/4 lambda antenna.
The receiver however is a bit crappy: without antenna the reach is maybe no further than a meter, but even with a 1/4 lambda antenna it is marginally more, even with free Line of Sight.
For any serious project that involved receiving data it seemed I needed the much better (and more expensive) RXB8 receiver.
However, when mining the internet for a coil antenna (trying to improve on the lengthy 17.2 cm stick antenna) I came across a design of Ben Schueler, apparently once published in elektor magazine. A reference to Ben's pdf would suffice to build it, but so is my picture and I can add my experience with it as well.
It is a so called coil loaded design consisting of 0.6mm wire wrapped around a 2.5mm core.
The picture gives a clear description: a length of 25 cm wire should be enough. At the base it is 17 mm long. Then goes into 16 turns over a 2.5 mm core (Ben advises to use black installation wire for this. I just used a screwdriver)
The results with this antenna are very good. The distance (with the cheap receiver) that can be covered easily goes to 25 m with line of sight, but also in-house the distance will be increased reaching other rooms with concrete walls in between, were earlier 3 meters with line of sight would be pushing the limits already.
I am not the only one with this experience. I have at least 2 people confirm to me that it dramatically increased the range of the cheap Tx/Rx pair