How to compute the required length and turns for a air cored antenna

Question

Hello,

While experimenting with various designs for the 433MHz antenna to be used with my RFM69 receiver, I stumbled across step by step instructions created by Ben Schueler in 2013 and republished a large number of times since then, such as here: https://arduinodiy.wordpress.com/2015/07/25/coil-loaded-433-mhz-antenna/

It works quite well for my application, but now I would like to dabble into the 868MHz range and I was wondering how I can calculate the appropriate dimensions/number of turns to replicate the above design, but tuned for 868MHz reception.

Would you have the link to a reference document so that I can compute it myself? Or should I just go ahead and divide every value by two as 868 is roughly 433 multiplied by two?

Thanks for any pointers.

shabaz · Accepted Answer

Hi Olivier,

I don't think there is a formula (or at least, not some single universal formula where you can plug in your desired frequency) for such an antenna. There are sets of formulas for the simple antennas (e.g. monopole, dipole, helix) and that's it. Anything else, and you're likely to either need to simulate, or try it out, or both.

The link you have specified is not an antenna calculator, it's just an inductor calculator. By the way, the formula is not exact with real life, although it's close, and in practice you'll have to try it, and tweak it. You can't get away from experimentation at such frequency ranges.

Also, as mentioned, if your board is not designed for that antenna, then there is the matching problem to solve too.

For just a hobby antenna for an off-the-shelf low-cost module, I'm not sure it's worth all that effort/cost.

kmikemoo · Answer

obones Oh! Antenna building! I'm interested. First things first, shabaz is super smart and really knows his stuff, so I'd encourage listening to his advice. As a hobby antenna builder... yes, you can try scaling your 868MHz antenna down to see if it works. If you search the Internet, you can find LoRa antennas that look something like what I believe you are trying to make. https://smile.amazon.com/G-NiceRF-SW915-TH12-Copper-Antenna-helical/dp/B07DPN47VJ/ref=sr_1_35?_encoding=UTF8&dchild=1&ke… is an example. That one is a continuously loaded spiral wound antenna. It has a low takeoff angle perpendicular to the orientation of the antenna and has an omnidirectional pattern. I second Shabaz's VNA recommendation. The Nano VNA is a great tool for the radio hobbyist. This way, you can tune your antenna to the correct frequency - given the coil spacing you end up with and the length of the radiator you choose to have. Center or end loading is supposed to be more efficient. More efficient on a compromise antenna... anyway. You can see why you're not finding a calculator. SO MANY variables. Finally, as you search the Internet, you're going to see how inexpensive 868MHz antennas are. Unless your goal is to build it yourself for the sake of building it, it's hard to justify building versus spending. You can buy a few antennas and still spend less than the Nano VNA.

shabaz · Answer

Hi Olivier,

You can just try a length of wire, roughly half, and snip bits off to experiment.

It won't be able to be an optimum value, because each antenna has an impedance. The antenna that you're replacing has an impedance, and the circuit board will have components on it to match the antenna. If you swap out with a different type of antenna then it is now unmatched to the board. Performing the matching requires a VNA and swapping out the matching components on the board with ones determined by using the VNA.

How to compute the required length and turns for a air cored antenna

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