Coil loaded 433 MHz Antenna

433 MHz Coil Loaded Antenna

In my 433 MHz projects I have been using a cheap (1 euro) pair of Tx/Rx modules. I have mostly used the transmitter and that is actually fairly  good: if used to trigger commercially available remote switches, it reaches reasonably 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, (backup PDF here) apparently once published in elektor magazine.
It is a so called coil loaded design consisting of 0.6mm  wire wrapped around a 2.5mm core. Ben’s pdf gives a good description with pictures.
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.

433MHz Coli loaded antenna
Antenna in use


31 thoughts on “Coil loaded 433 MHz Antenna”

  1. I’m doing a small project in an Altoid Tin and trying to extend the antenna outside the tin with the transmitter and receiver inside. Do I need to put the antenna external from the tin or would the antenna work as well inside the tin? Would I need to extend the 17mm length to account for the portion of the antenna inside of the tin?

    1. I havent tested it, but I presume the reception and transmission within a closed tin container is not optimal.
      I must confess that I do not know enough from Antennaś to know if you shld extend the 17 cm, but I doubt it. The simple 1/4 lambda wire antenna is not aware of the tin around it, the transmitter will just generate a proper signal in the antenna and then part of that will be shielded.
      My suggestion is to keep the onset of the antenna as close to the surface of the container and then let the antenna stick out. I also would advise to use the coil loaded antenna in this article rather than the 17 cm straight wire antenna. For me and others it really made a huge difference

      1. I would think with a tin with a flat bit of metal backing the monopole antenna, you would want to keep it perpendicular to any hole in the chamber you make thru the tin. A bit of plastic thru the coil would work.

        Otherwise you will couple a lot of the RF into the metal of the Tin and either ground it out, or at least make the antenna radiation pattern a bit odd and directional.

        I wonder the same with your problems +Android, and the short ranges you are having problems with. Did you put the transmitter and receiver on turntables to measure if you have any lobes?

      2. No i did not try the turn table. I do not know much about antenna design, so when i found and tried the coil antenna and it did what I wanted/needed, i was happy with it. My lack of antenna knowledge already becomes clear when I read yr comment as I am not even sure what you mean with most of yr terms 🙂 The chamber? is that the inner space of the coil? what is ‘the tin’?
        I have come across some other designs that I will try if I have time. This one too when I understand it 🙂
        But I am sure others might be helped by your comment. Thank you

  2. I was using one of these helical coil antennas which were made for these receivers/transmitters. They are terrible, and are very nit picky about the angle in which it was positioned. I tried this out and they worked out much better than I expected. Plus using just wires, I could make practically an unlimited amount of it with the rolls of wire I have. Thanks for this, very helpful.

    1. Yes I know the one from AliBaba/Aliexpress and though cheap, still more expensive than just a piece of wire rolled into a coil as I described.
      I am happy mine gave you good results

  3. The quarter wavelength antenna is supposed to have a reflective (ground) plane beneath it in order to work properly. Try a half wavelength dipole!

    1. Thanks, You are right and i have experimented with a 1/2 wavelength but the ‘problem’ is the size.
      I presume with the dipole the other end should be connected to earth

  4. the only difference being of its course materials, basically the same as 1/4 lambda, am I right ?, sorry my english

    1. not entirely sure what you mean, but yes the basic material same as for 1/4 lambda: stiff insulated wire

      1. I am very interested in this experiment, if this experiment requires a different calculation like in general, and how do I make it?

      2. I think I lost you. Different calculation from what? ‘How do i make it?’ well that is explained in the article you just have been reading

      3. how to calculation this antena? such as determining, L, N, C ,D gain etc. is it the same as estimates usually?

  5. the only difference being of its course materials, basically the same as 1/4 lambda, am I right ?, sorry

    1. The material you need is insulated wire, the same sort that you could use for a whip 1/4lambda antenna

  6. I’ve often seen forum posts complaining about the short range many users get with these inexpensive Tx-Rx pairs. I’ve used these sub-$1 pairs a few times and always got good results. It just so happens that I tested a pair a few days ago *without* an antenna on either side. I sent some slow code with an Arduino through two wooden walls and one brick one – all with some wiring on them. I tested them at up to 10m (measured), at which distance they still functioned quite well. Power supply was 5V on both sides. I didn’t do any tweaking on either Tx or Rx. Several years ago, I tested another pair at up to 120m in LoS with a simple 1/4-wavelength wire on each side. With careful orientation of the antennas, performance was reliable up to about 100m, and became increasingly erratic beyond that.

    1. Much depends on the transmitter/receiver pair. The ones with a crystal definitely work better to begin with. That you have good results without antenna is great, but many people see the rate ge increased with an antenna

      1. I’m not disputing that an antenna will increase the range. In fact, it’s a given that it will. I was just expressing surprise at the very short range reported by so many people on the internet. I’ve just tried out a crude version** of the coil loaded antenna and got great results. Reception was still good at my neighbour’s house (42m between Tx and Rx), through three wooden walls with lots of wiring, a tree with thick leaves and two concrete walls with wiring and steel gratings. The neighbour’s house is also twenty feet higher than mine.
        **7in. of SWG20 (AWG19) magnet wire wound on a 4mm former, 1.5cm straight at the bottom, 2.5cm at the top.

  7. I don’t understand the claims for this antenna. I get around 30m using straight 16.5cm (not 17.2 as mentioned – apparently that length is based on the velocity of light in vacuum not wire). I get similar using the small helical aerials often sold with the transmitters. I get a lower range with this coil loaded antenna. I have about 10 transmit/receive devices in use in various places and have revisited this design a few times with the same poor result.

    1. I am happy you ha e great results as you are. I only know i and others got great results with the antenna described here.

    2. Regardless, it is an interesting point you make. Is it a matter of just fine tuning the wire antenna with 7 mm? If Rf transmissions were still worth the effort it might be interesting to check it out by taking some m

      1. Good point. I searched for the mention of 16.5cm rather than 17.2cm as often stated and discovered a bit more – the velocity factor in wire seems to be changed by insulation and going by what it said I should be using an even shorter wire ! ( but when I tried 15.7cm it wasn’t better. In fact 17.2 worked just as well or should I say badly because a few weeks ago I had a transmitter/receiver pair using a 16.5cm wire on one and helical spring on the other that went perhaps 70m part of which was through a terrace of stone houses. Now I can’t repeat that so I conclude there are other factors at play. Moisture in the atmosphere perhaps as it’s damp at the moment. Or maybe the devices vary.
        (I volunteer at a charity that’s advising community buildings on reducing their carbon footprint and we need these devices to send the humidity and temperature to a hub connected to the internet. The first batch have been sent out, so before making another set I decided to revisit the radio range – I can’t help feeling insecure about the range 😐 ) . I really need a better way of measuring the signal strength than placing the transmitter down the garden and coming back to the house then going back to move it and so on. I need to go back to using the set up I had when I got 70m instead of using the community building devices I’m making which transmit only every 5 minutes. I’ll let you know when I manage to progress this further, including the coil loaded design of course.

      2. 70 m sounds pretty good for rf 433. Mind you hough that a lot of the 433Mhz transmitters/receivers that are being sold are not of great quality. Would be very interested to hear your further experiences in this matter

  8. I have to take back what I said : ( as I’ve discovered that where I stand affects the signals and further tests are now showing the coil loaded antenna beating the helical spring : ) Rain has just stopped play so can’t say more at the moment. I certainly got several metres further until I ran out of garden.

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