Simple monopole antenna testing

As outlined in the RF Best Practices guide, the Monopole Antenna is formed by a quarter wavelength vertical piece of wire, plus a “launchpad” ground around its base, and is a very good cost-to-performance ratio antenna. The quarter wavelength refers to the wavelength of the frequency at which the radio is transmitting/receiving. As an example, the for 433mhz the wavelength is 0.69m and the quarter wave is then ~172.5mm. This is a theoretical figure at which the signal would have maximum resonance in the antenna, but in real life the signal from a radio goes through various passive components (matching networks), RF switches, and other PCB traces which influence this signal, and hence the signal might resonate better at slightly different physical dimensions of the antenna.

In the video below, Marcel illustrates how you can test a simple monopole antenna by starting with a given length and then trimming the antenna length to determine which length yields the longest range. Since the radio can transmit at a maximum of 20dBm, a 30dB attenuator was added and also the transmit power was diminished in software (with this function) to reduce the total output to around -26dBm – that is many many times (-30dB = 1000 times reduction) less powerful than 20dBm (dB scale is logarithmic not linear – see this page for more on that). This allows testing the antenna at very small range without having to walk very far away to see the signal drop. The monopole is omnidirectional in the horizontal plane (ie it emits the signal in all directions at the same intensity).

Of course, this type of test could be done in the lab with a VNA (Vector Network Analyzer). But for such a cheap and simple antenna, this test shows how easy it is to do it yourself without investing an eye watering amount of cash in such exotic equipment.

You can do your own testing in a similar manner, and you can even use the Node and Gateway examples for this purpose, and simply add the setPowerLevel(0) call in setup() (and disable RFM69_ATC) to reduce signal power, or use an attenuator like Marcel did. Be sure to check out this page where Marcel has used the same technique to test several antenna types, with some interesting results – this might help you decide to upgrade from the virtually free monopole to a dipole.

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