Add external antenna to 433Mhz remote

I have a 433Mhz remote that I'm planning to put into a metal box as part of a little project I'm working on. I think the box would completely block off the signals of the remote, so I'd like to add a little external antenna to it.

Any ideas how I could do that? Could I just cut the connection to the built-in antenna and solder a piece of copper wire to the trace? An if so, where exactly dos the antenna start and end?

Any hints would be greatly appreciated.


For good performance, 17 cm of straight wire will work as a 433 MHz antenna. Shorter antennas lead to lower operating range.

The module pictured has a built in antenna, which was designed to work in a non-conductive housing. For best performance (and possibly, for the thing to work at all) you will need to cut the connection to that antenna, as it will short-circuit the transmitter output when enclosed in a metal box.

It may work to cut at the beginning of the matching "coil" and connect the wire antenna to the via to the right of the arrow tip.

Capture

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Hello felic

Take some time and visit your local amateur radio club to get some ideas for an antenna.

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For 433Mhz a plain bit of wire 17cm long is a good place to start.

However that may not be the optimum length for maximum radiated power for that particular module but then only a big heap of testing will confirm that, so try 17cm.

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As said, 17cm of wire (a quarter wavelength at 433MHz) should work at least as well as the internal "antenna". However for optimum performance it needs to be driven against a "counterpoise", which is the internal "ground" of the transmitter. This is probably the big patch of PCB copper on the left of the first picture to which the negative battery pole spring is soldered. You need the shortest possible connection from that to the metal case, and the case will form the counterpoise.

Optimising antennas on this type of product is extraordinarily difficult and there are very few professional organisations that have the expertise and facilities to do it. But if you are fitting an external quarter-wave antenna and take care with the grounding of the box it would probably be hard to end up with something worse that the integral antenna! If the length of the wire is a problem a common approach used to be to wind it into a fairly open helix - usually covered in a plastic moulding.

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Thank you, the pcb antenna appears to be connected on both ends, doesn't it? Do I have to do this for the external antenna as well?

Could I salvage the antennas of these modules?

@jhaine Thank you, should I solder ground of the PCB to the metal box and then solder the end of the antenna to the metal box?

Goodness knows why they have grounded both ends, but it's unlikely there was much science behind it. You should NOT ground the end of the external antenna.

The antennas you linked to are the kind of thing I meant, but why spend the money on those and throw away the electronics when you could just make your own for pennies? Get some of the thick bare copper wire used for grounding in electrical installations, wind it round a screwdriver blade or similar.

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I see, okay I will make my own antenna for now.

One more question: I might end up needing a more ridget antenna. Would it also be possible to use an SMA antenna like this one?

Yes, that's probably an encapsulated helix. Make sure the sma socket is properly grounded to the box as close to where the pcb ground is connected as possible.

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Would it work to use an SMA to u.fl cable like this and solder a u.fl connector like this one to the pcb?

Because it's a loop antenna

Probably not. I doubt if the matching network for a loop antenna would work with a wire

I do not think that is correct place to cut and connect to.

Have a look at the following photograph, with both sides of the PCB next to each other.
The bottom PCB photo has been mirrored, for ease of comparison and the vias marked in yellow.
The structure of the antenna is marked in red and extends to both sides of the PGB

Look where the via that was suggested as the cutting point goes to on the other side of the PCB. It is connected through the push button to the battery +ve.

That meander line is likely to be an inductor in the supply line to the oscillator transistor (SMD marking R25).

The structure of the antenna is more complex than first impressions give you, and replacing it with an external antenna will not be easy.

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@JohnLincoln I agree. I think it is some kind of loop antenna and the feed point is in the middle

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It's easy to assume that there's some science behind antenna designs like these, but in reality there isn't because the designers have no real knowledge and often the design ends up being driven more by the product design than engineering. I've worked with two groups actually measuring antenna performance on these products using proper test facilities including anechoic chambers, and the performance is very often appalling.

In this case it hardly matters because the OP's options are very limited if he wants to put the unit in a metal case, there has to be an external antenna. A quarter-wave whip counterpoised against the metal box stands a very good chance of performing better than a loop antenna on FR4 (or worse) material in close proximity to components on the top side of the PCB. Unless of course it's multi-layer with a copper ground plane in between in which case the antenna would just be microstrip and hardly radiate at all! Without the knowledge and some equipment the best he can do is just connect it up as suggested and try it out. A saving grace is that the chip is probab;y designed for a 50 ohm load and a quarter-wave isn't far off that.

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Yes but where is the feed point?

There is the option of not putting the remote control into a metal box due to the difficulty in performing the operation with uncertain results.

What happens if I just solder my antenna onto the existing pcb antenna without cutting it? I don't need a lot of range, maybe 5m with a wooden table inbetween in the worst case.

See the zig-zag trace near the "antenna"? That is an inductance used to match the circuit to the antenna. The antenna is too long, so is capacitive and needs the inductance to make it resonant.

As @JohnLincoln pointed, that trace connects to the push button that connects V+. Why would the feed point connect to a push button?