on 400 Mhz and above the impedance is far from optimal.

Smith chart shows that the impedance is then 48.9 ohm +j6.8 which is pretty good. in that frequency antenna works optimaly. I did reflection measurement (S11) with my VNA and as you can see in results minimum power (-24 dB) is reflected back from antenna at about 313 Mhz ie. I set the span frequency on VNA from 150 Mhz to 500 Mhz. I have not tested this antenna in practice wtih a transmitter but I suppose that transmitter's output impedance is 50 ohm as normal. Based on these results it looks to me that this antenna construction works better on 315 MHz ISM band due to fact that antenna impedance is closest to 50 ohm at 313 MHz. I build the antenna and did some impedance measurements with my R&D VNA and got some interesting results. First of all I must say that I am not an rf engineer but I do know how to perform some basic antenna measurements. Many people confirm to me that it dramatically increased the range of the cheap Tx/Rx pair, read the comments! I am not the only one with this experience. The distance (with the cheap receiver as well as the transmitter) 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. The results with this antenna are very good. Then goes into 16 turns over a 2.5 mm diameter core (Ben advises to use 1.5mm²black installation wire for this. The picture gives a clear description: a length of 25 cm wire should be enough. It is a so called coil loaded design consisting of 0.6mm wire wrapped around a 2.5mm core. A reference to Ben's pdf ( back up) would suffice to build it, but so is my picture and I can add my experience with it as well. 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. But as said, also the reach of the transmitter could use a bit of improvement.

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.įor any serious project that involved receiving data it seemed I needed the much better (and more expensive) RXB8 receiver. I have mostly used the transmitter and that is actually fairly OK with just a simple 1/4 lambda antenna, but is open for improvement In my 433 MHz projects I have been using a cheap (0.70 cnts) pair of Tx/Rx modules.

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