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 buil…

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!

13 People Made This Project!

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79 Discussions

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ari.torpstrom
ari.torpstrom

1 year ago

Thank you for your article. First of all I must say that I am not an rf engineer but I do know how to perform some basic antenna measurements. I build the antenna and did some impedance measurements with my R&D VNA and got some interesting results. 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 have not tested this antenna in practice wtih a transmitter but I suppose that transmitter's output impedance is 50 ohm as normal. I set the span frequency on VNA from 150 Mhz to 500 Mhz. 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. in that frequency antenna works optimaly. Smith chart shows that the impedance is then 48.9 ohm +j6.8 which is pretty good. on 400 Mhz and above the impedance is far from optimal.

I am not saying that this antenna does not work on 433 MHz band but from my point of view the best frequency band for this antenna is 315 MHz supposing that the transmitter's output impedance is 50 ohm. There seems to be no theory available for the calculations any more but perhaps somebody can explain the deviations...

Refl.jpgSmith.jpgAntenna_20190517_131117.jpg
0
nzhorty
nzhorty

Reply 8 months ago

Excellent post and great reply from Ari, thanks for both.
I'm trying to reconcile theory that I have found on the internet and the experiences of people here, so that I can make my own design. I would also like to share where I found the theory so others can follow too, as it seems quite tricky to find something that can string together all that's required to design your own.
Also, I'd appreciate if anyone could correct me where I'm wrong here as I know very little.

Ari, I think the reason why yours is different is you've got a different coil density from the OP. Looking at your photo, I make your average diameter to be around 5mm, (looks like OD is ~6, insulated wire is ~1). It looks like you have 17 turns not 16, and your overall coil length is quite a bit longer, at ~30mm. It also looks like yours is not copper (not sure how much difference that might make but some sites factor material)

I plugged those values into two sites:
https://m0ukd.com/calculators/loaded-quarter-wave-antenna-inductance-calculator/
has a nice calculator which is based on loaded 1/4 wave antennas, yielding an inductance required
http://www.66pacific.com/calculators/coil-inductance-calculator.aspx
has an inductance calculator for a basic coil

For your basic aerial dimensions I got a required inductance of 0.240uH for 315MHz (0.10 for 433MHz). When I put your coil in, it came out around 0.220, so pretty close, and goes a way towards explaining why your results showed resonance at 315.

Just to update on this, I then tested a few more examples, the original above, MJ Ward's assertions around a coil only option, some stock items on alibaba that have dimensions, and as above couldn't quite get a match with the two sites above (not sure why - hopefully someone more knowledgeable might explain).
I decided to make my two aerials on a 'coil only' approach, and used this site (which is a coil only):
https://hamwaves.com/inductance/en/index.html
I installed one design in a 433MHz site, and another design in a 315MHz site, and both were really good improvements on the original.

0
KenC7
KenC7

Reply 11 months ago

OK, how would you make it better?

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Diamond Troller
Diamond Troller

Question 8 months ago

Hello diy_bloke.
Where could I get that pdf?

0
mjward1
mjward1

11 months ago on Introduction

By tightly wrapping the coil, you are basically trying to create a tuned circuit with a higher Q factor than you need. ari.torpstrom has calculated this to be a tuned aerial for around 315MHz, which I have no doubt is true. As soon as you wind an aerial with a tight coil you introduce an inductor with close cross capacitive windings, in other words a high Q factor filter.

0
KenC7
KenC7

Reply 11 months ago

So, how to fix it?

0
mjward1
mjward1

Reply 11 months ago

Just use a single piece of wire, 17.3 cm long and make a single loop if required for physical constraints I calculated the aerial with a single turn on a 1cm former and its resonant frequency was 430MHz

0
KenC7
KenC7

Reply 11 months ago

So you think this antenna design is worse than a straight quarter-wave antenna? Is there any way to do a coiled design like this, but change the lengths, coil diameter, or number of turns, and improve this design? For example the picture is a wearable I did, and the length of this antenna design is very helpful. It fits pretty well, with only a small part of the antenna wire protruding from the case. (I added the coil at right this morning to make it a dipole.) The way I wear this, a 17cm wire would be stabbing me in the throat. But the current design picks up signals from just under 200' away with no problems. Doesn't seem to pickup signals from farther than that, however. I added the dipole bit to try to improve that. A better tuned coiled antenna would be terrific, but a straight wire, not so much.

motion sensor pendant.jpg
1
mjward1
mjward1

Reply 11 months ago

17.3cm is the correct length of wire for a 1/4 wavelength straight antennae. Once you start to wrap the wire around a former, then the resonance of the aerial will drop in frequency, it wont stop working however as an aerial just the distance will be reduced. For the most efficient use you will want the aerial to receive around its resonance point. I just did a calculation for best nearest case. 10mm former with 10 turns with a total coil length of 30mm with 1mm wire (wire diameter won't make that much difference at these sizes. 0.6mm is just as good) Total physical wire length would be 316mm. Resonant frequency should be around 426 MHz

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KenC7
KenC7

Reply 11 months ago

So it's basically all coil then? I think I can make that fit - thanks! I'll try that out.

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diy_bloke
diy_bloke

Reply 11 months ago

yes it is. goodluck

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0000007
0000007

Reply 9 months ago

@ diy_bloke - > " yes it is " == mjward1`s design better than " coil loaded antenna "design ?

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diy_bloke
diy_bloke

Reply 9 months ago

he asks "So it's basically a coli then?".My reply: "yes it is"
I did not try his design

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SaifullahK5
SaifullahK5

Reply 10 months ago

KenC7 .... Did you tried that 10 turn coil antenna design suggested by mjward1 ? If YES then was it a better success over the antenna design(coil loaded) as given in this instructable ? please share your experience ...

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KenC7
KenC7

Reply 10 months ago

Sorry, I haven't had a chance to try it. It was the middle of hunting season, and it wasn't a good time to make changes. I'll try to dig that up and see, but I don't have a good way to measure signal strength with that setup. Probably better if someone who can get a signal strength reading tries it out. I have other devices that do give an rssi, but that'll take even longer to rig up.

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rbantin
rbantin

Question 1 year ago

I have a door chime that is 433Mhz. The receiver is about 5 feet out of range. I took it apart and it has a spring looking antenna with about 20 coils in it. If I were to drill a hole in the case and stretch out the coil a little would that help? I was also thinking about putting a rod7 inches long in the coil through a drilled hole...

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diy_bloke
diy_bloke

Reply 1 year ago

That is hard to say as antennas are funny things, but it is certainly worth a try

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diy_bloke
diy_bloke

2 years ago

Look great, thanks for the upload