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I'm tinkering on a cheap chinaware alarm center which receives signals from its detectors on 433MHz. It is located inside a metal box which forms a faraday cage and apparently reduces the range too much. Moving the alarm center out of the box is not an option because the surroundings are very dusty, I'd like to just extend an antenna out of the box.

I found the receiver on one corner of the PCB, it's a SYN531R.

A conducting path forks off L1, passes a first solder point, goes some 4cm straight up, then 90° right and ends after another some 14cm straight on a second solder point. Apparently, the path between the two points acts as antenna.

My plan is to solder a cheap antenna directly to the PCB, however, I'm no pro, so here are some questions:

  • Is this antenna a valid choice?
  • Intuitively, I'd solder the external antenna onto the first solder point, then interrupt the rest of the printed antenna with a cutter. Once done, there's no way back anymore, so maybe there's a less intrusive solution: Could I solder the external antenna onto the first solder point and leave the printed antenna intact? Or maybe solder the eternal antenna onto the second solder point (at the end of the printed antenna)?
  • As for GND of the external antenna: Can it be soldered onto any solder point carrying GND on the PCB or does it have to be the GND pin of the SYN531R?

Here's a photo of the PCB:

enter image description here

This is the schema taken from the SYN531R datasheet:

enter image description here

Thanks a lot for your hints!

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    \$\begingroup\$ Have you considered replacing the metal box with a plastic one? Main disadvantage is lower thermal conductivity. \$\endgroup\$
    – K H
    Apr 5, 2021 at 8:56
  • \$\begingroup\$ Yeah, first thing, but that'd be only half the fun. :-) \$\endgroup\$
    – svoop
    Apr 5, 2021 at 16:33
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    \$\begingroup\$ Which frequency band are you aiming for and in what country? What modulation type and what power? This is necessary to know when picking antenna, in order to tell how wide it needs to be around carrier. You are most likely looking for an omnidirectional antenna with zero gain. \$\endgroup\$
    – Lundin
    Apr 6, 2021 at 13:08
  • \$\begingroup\$ Well, it's in the title: 433MHz, as per the schema 433.92MHz to be exact. Since it's RX only, the country shouldn't matter. \$\endgroup\$
    – svoop
    Apr 7, 2021 at 13:21

1 Answer 1

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Once done, there's no way back anymore, so maybe there's a less intrusive solution

I wouldn't hesitate to buy the evaluation board (see data sheet) and test this out first: -

enter image description here

Then you are not risking unnecessary damage to your main circuit. I read the data sheet and there are parameters unlisted such as the chip input impedance. Also, you probably cannot determine the pi-network components on your original board to infer what the printed antenna impedance is. If the printed antenna did have a clear-cut impedance value then things would be a tad simpler of course but, as it stands, it's guesswork.

Is this antenna a valid choice?

It's hard to tell because the printed antenna impedance is what the pi-network is matching the chip with and any external antenna will likely not be matched to that pi-network the same as the printed antenna. However, the one you linked is not suitable because (apparently) it has a signal amplifier built in. Choose an antenna (a monopole, quarter wave) that has a recognized data sheet and is bought from a reputable supplier for this type of product (not Amazon).

Could I solder the external antenna onto the first solder point and leave the printed antenna intact?

You could do and, due to the simplicity of such an action it's worth a go but, results may not be that clear because you are significantly affecting the pi-filter input impedance by putting two antennas in parallel.

As for GND of the external antenna: Can it be soldered onto any solder point carrying GND on the PCB or does it have to be the GND pin of the SYN531R

I'd buy the evaluation board and split the main board at the data output point.

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  • \$\begingroup\$ Thanks for your answer, Andy! Can you elaborate "split the main board at the data output point" for me? The DO being pin 5, how exactly would I do that? \$\endgroup\$
    – svoop
    Apr 5, 2021 at 18:49
  • \$\begingroup\$ There are several ways this can be done but someone skilled in pcb repair and modification is needed. You could lift pin 5 from the pad and make a connection to that pad. Else, maybe the track going to R117 could be investigated to see if that connects to pin 5. That potentially looks an easier route. \$\endgroup\$
    – Andy aka
    Apr 5, 2021 at 20:13
  • \$\begingroup\$ The layout of the actual PCB doesn't look nearly as careful as the one from the part datasheet though. It actually looks like it ends in a via. So I wonder if there is actually a PCB antenna present. "Solder blob" something by hand into that via and it won't be 50 ohm. \$\endgroup\$
    – Lundin
    Apr 6, 2021 at 13:22

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