433 MHz Coil Loaded Antenna


Introduction: 433 MHz Coil Loaded Antenna

About: I am a physician by trade. After a career in the pharmeceutical world I decided to take it a bit slower and do things I like. Other than my hobbies that involves grassroots medicine in S.E.&P Asia. I have bu...

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 fairly OK with just a simple 1/4 lambda antenna, but is open for 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.

For any serious project that involved receiving data it seemed I needed the much better (and more expensive) RXB8 receiver. But as said, also the reach of the transmitter could use a bit of improvement.

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 (back up) 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 diameter core (Ben advises to use 1.5mm²black installation wire for this. I just used a screwdriver)

The results with this antenna are very good. 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.

I am not the only one with this experience. Many people confirm to me that it dramatically increased the range of the cheap Tx/Rx pair, read the comments!

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In your text you describe using a 0.6 mm wire. Is this for a specific reason? Would a thicker wire get a different (worse) result? Or would I need to adjust the core/number of turns?


Ah yes, I also have that pdf and saw some time ago that it was taken down. Very valuable piece of work

I mentioned that as it was in the original (but difficult to find) article.
nevertheless, I used justthe wire available to me and had great results

Ben, Thanks. That design changed my 433 world. When i was just using the 1/4 wave whip antenna, I couldnt do anything serious with those cheap transmitters and then I found your design: BIG difference and I mean BIG!!!!.

I trust I gave you enough credit :-).

I am slowly moving away from 433 in favour of the Wemos... but I just seamlessly shifted my existing 433 infrastructure into Openhab with a simple MQTT-433 Gateway, again with your antenna. Even at 3 Volt there is considerable reach with the el cheapo transmitters, yet I use it at 5Volt, but may try 9Volt.
Thanks again for your design

6 replies

Hi diy_bloke,

I'm currently buildung exactly the same thing here, a 433MHz-MQTT Gateway. What microcontroller did you use? I have an ESP8266-12F in a "Witty Cloud" board. Just am fiddeling with the voltages, the Witty Cloud exposes 5V but the GPIOs work on 3V3...

I'm gonna build this antenna for sure, when I was playing with the receiver before the reception was abysmal, but what do you expect without any antenna :D

using a Wemos board (D1 mini). I am not sure how the witty cloud board exposes 5V to the pins. It has a 3v3 adapter and one can use the voltage from that.
Though the 433MHz Transmitter works better on 5V vcc, it is no problem (as I experienced) to have the signal come from a 5V pin.

yes, antenna makes a big difference. Before I made this one, only using a 17 cm wire, 433 Mhz wasnt a serious option

I ended up using a voltage divider for the receivers data pin because the ESP on the witty cloud runs with 3v3 and the GPIOs expect that too. The receiver itself is rated at 5V anyway...

I already received some codes (even without antenna, will build that now), but unfortunately not the temp/humidity sensors I have :D
Have to try some other library, I'm using the ESPiLight one right now.
Do you know if I can run several libraries from the same GPIO interrupt?

indeed for the Receiver it is a different matter. I understand though the receiver does work on 3V3 too and unlike the transmitter that wouldnt affect its sensitivity. Having said that, those cheap receivers are usually crap whereas the transmitter si fairly OK
ESPILight is the pilight library I think. I dont have any experience with that.
Whether you can run 2 libraries from the same pin depends much on the libraries itself. I know I have run RCSwitch and RemoteSwitch from one pin

Oh ok, but doesn't matter, it is running and receiving something... I'm gonna let the lib send the RAW data to me for some debugging, perhaps I can add that protocol in use...

Looks like running two interrupt routines from one interrupt is not possible, the newer attachment overrides the older one and I would have to change the libraries to "merge" them...
I'll simply connect the receiver to several pins (hopefully they are 'interrupt-able') and then I have several possibilities for libs. Although the ESPiLight is already quite good.

You are sending, right? So you are triggering a pin, for the receiver I have to use interrupts and sometimes the libraries are doing that by themselves.

Btw.: the maker of ESPiLight has already done exactly what I'm doing here, but I'm still gonna do it by myself, you know the drill ;)


yes you are fully correct. I am sending. Though I have done projects that involved receiving, my main use is sending. The receivers are commercial wall switches.
Indeed, with interrupts and one pin it is a whole different ballgame.
Yes had seen the pilight MQTT433gateway. You also may want to check github for a guy called "1technophile" he has an IR/433/MQTT/RFM69 hub

These are awesome. I made them a while ago (and I think I left a
comment about it) and they work quite nicely. One question I have,
though. Which library do you use for your modules (if any)? I use the
Radio Head for Arduino and it doesn't seem to support the ability to
send different commands based on sensor input. The reason I ask is, I'm
trying to use them for robot communication, which really only works if
the robot can send/receive more than one set of signals.

don't seem to be many other reliable libraries... So I was wondering how
you do it since it sounds like you've been using these things for a

3 replies

I have used 3 different libraries, depending on what I wanted.

Manchester library - for between arduinoor attiny communication

RCSwitch for remote control

yet i prefer the

RemoteSwitch for that purpose and I used a completely updated fork of that one.


If you need it for a robot i presume the Manchester or virtualwire library would be the best

Thanks! I'll look into those... And hopefully get my robot working!

Anyone know what stores in the Midwest would carry semi-rigid copper wire of this size for this use? Radioshack is gone from my area.

3 replies

Sorry, cant help you there

No problem! Any idea what temper bare copper wire would work best?

I used 18 gauge copper modelling wire. Found it in the hardware isle at Lowe's, it was low down under some random stuff at the end of the isle. They also had steel wire, from 24 gauge to 18 gauge. That would've worked too.