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I have this board at hand:

Gate opener board

It is a gate opener, so it is installed outdoors.

The board has a 433 Mhz receiver, whose antenna goes to a 2-pole terminal block.

Normally, a simple whip is used but, having the terminal block, one can also remove the whip and connect a different antenna using an RG58 shielded cable.

A friend of mine, having seen this board, says that the detachable terminal block will give a lot of problems because of the small signal at radio frequency it has to conduct.

I tried the board and, at the moment, it performs fairly well. Maybe after some time, being installed outdoors, the contacts can wear, or oxidize, so the radio signal gets weakened by the terminal block?

I know there can not be a definitive response. I've seen that those detachable terminal blocks have generally no problems, at least with signals ranging from +5V to +24V, but I never thought about weaker signals at radio frequency.

Has someone more experience than me about this potential problem?

detached

JRE
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linuxfan says Reinstate Monica
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  • *at the moment, it performs fairly well.* Then you have your answer: there is no issue. Yes moisture etc will affect the connection **but also the rest of the circuit** so I do not see how this antenna connection would suffer "more" than the rest of the circuit. Also, moisture + contacts + DC currents will give problems due to electrolysis. With AC (and that antenna signal will be AC) this is much less of an issue. Focus on keeping your device free of moisture and there should be no issue. – Bimpelrekkie Jul 02 '21 at 07:18
  • @Bimpelrekkie thank you for your comment. About electrolysis... I can't believe that a signal coming from a small antenna has enough power to do that in a serious way! Other terminals (motor, power supplies etc.) actually could, probably. – linuxfan says Reinstate Monica Jul 02 '21 at 07:25
  • *I can't believe that a signal coming from a small antenna has enough power to do that in a serious way!* My point exaxtly, if you also have a terminal block to connect the power supply then I would be more worried about that connection. If you cannot keep the PCB 100% dry, consider putting a **conformal coating** on it. – Bimpelrekkie Jul 02 '21 at 07:27
  • @Bimpelrekkie Moisture in outdoors use will come in the form of moist air, not floating water. Oxidation will affect the whole PCB. Which isn't a big issue if the solder joints are proper, but more so in case of connectors. Gold plating/flash is a must. You can't put coating on connectors. However the main issue here is that a plain terminal block isn't suitable for RF signals in the first place. – Lundin Jul 02 '21 at 08:29
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    @Lundin *However the main issue here is that a plain terminal block isn't suitable for RF signals in the first place* I agree but I also see an RF module using pin headers to connect to that antenna, that's not optimal for RF either. The "most optimal" solution would be a to connect the antenna directly onto that receiver module. Then again, not all things have to be "optimal" for them to work. A compromise between "optimum situation" and convenience can always be made. – Bimpelrekkie Jul 02 '21 at 08:33
  • *My concern is about time: will those contacts wear out and make the board too little sensible to remote controls* They seem to work OK now so it depends on the circumstances. The terminal block could give issues in non-ideal circumstances. If you want to avoid that then remove the terminal block and solder the antenna directly to the PCB. – Bimpelrekkie Jul 02 '21 at 08:38

3 Answers3

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Even if the contact forms a thin non-conductive oxide layer, the proximity of the two contact halves will produce a capacitance of a few pF (worst case).

At 1 MHz, a 1 pF capacitor has an impedance of 159 kΩ. At 433 MHz the impedance is 367 Ω and could be fairly problematic so, if you want to know what the capacitance is likely to be; make an approximate estimate based on mating surface area and thickness of the thin oxide layer that might form. It might be several pF and, this is less likely to be a problem.

If the mating area is about 10 square mm and the oxide is 0.01 mm, then the capacitance will be 8.854 pF (based on this calculator).

So, with 8.854 pF, the impedance is 41.5 Ω and although this is somewhat of an annoyance, it won't be a showstopper.

Andy aka
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A friend of mines, having seen this board, says that the detachable terminal block will give a lot of problems because of the small signal at radio frequency it has to conduct.

Yes that's correct, you shouldn't use regular terminal block connectors for RF signals since that might screw up impedance matching and lead to signal losses (giving worse signal) or standing wave phenomenon (could cause electronics to break or misbehave). The antenna also needs to be attuned to the carrier frequency or it will cause similar issues. Oxidation will only make things worse.

This board wasn't designed by a professional RF designer. If so, there would be a 50 ohm antenna connector such as for example a SMA one.

You could ask the manufacturer to provide their third party test protocols EN 300 220 compliance to see if it's a legal radio or just "certified by Ali Baba".

Lundin
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  • Yes, the receiver has been tested by SGS, and found compliant, on january 28 2021 (I have the test report on my desk). The target market of these boards is very "consumer" (it is difficult to realize how much). This particular board, at the moment, performs reasonably well compared to the competitors. My concern is about time: will those contacts wear out and make the board too little sensible to remote controls (which, BTW, costs about $2)? – linuxfan says Reinstate Monica Jul 02 '21 at 08:34
  • @linuxfansaysReinstateMonica Regarding how sensitive against noise it is, it entirely depends on the radio design. Mostly what modulation it uses and how broad it is. Oxidation will not make it more sensitive to noise but could lead to reduced range. – Lundin Jul 02 '21 at 08:41
  • @linuxfansaysReinstateMonica Ironically, a surface mount, gold plated SMA connector is about as expensive than a terminal block + wire part, if not cheaper. These green terminal blocks cannot be reflow soldered, so they have to be soldered in a separate process, which is where all the cost goes. Very few companies solder through hole components for free... – Lundin Jul 02 '21 at 08:46
  • things are not simple. SMA connectors are not good for the people who installs the gate: they want relays, terminal blocks and they never connect the protective earth... – linuxfan says Reinstate Monica Jul 02 '21 at 09:15
  • @linuxfansaysReinstateMonica I'm pretty sure that the average chimpanzee could be trained to connect a SMA antenna to a SMA connector... If you want something more rugged, then go with TNC/BNC/N etc but it's the same principle. – Lundin Jul 02 '21 at 09:36
  • What about the length of the wire? If it is too long? – linuxfan says Reinstate Monica Jul 02 '21 at 09:50
  • @linuxfansaysReinstateMonica What wire? – Lundin Jul 02 '21 at 09:57
  • the wire from the board to the antenna... – linuxfan says Reinstate Monica Jul 02 '21 at 09:58
  • @linuxfansaysReinstateMonica Are you saying that the white thing in your pic is a wire and not the actual antenna? – Lundin Jul 02 '21 at 10:03
  • No, that's the poorman antenna but as gates are metallic and the gear+board are near the ground, sometimes an external antenna is preferred. – linuxfan says Reinstate Monica Jul 02 '21 at 10:06
  • @linuxfansaysReinstateMonica Then you provide pre-made coaxial cable assemblies of various lengths... but that's a different issue than picking the proper connector. Though if the antenna is placed at 90 degrees angle from the gate, it won't be an issue. If you mount it "in parallel" with the gate, then yes that will be a big problem. – Lundin Jul 02 '21 at 10:08
  • I don't provide cable assemblies. People who install buy antennas wherever they want, put them wherever it seems fit, cut the RG58 cable, and connect it to the terminal block. This particular market works like this. – linuxfan says Reinstate Monica Jul 02 '21 at 10:11
  • @linuxfansaysReinstateMonica That doesn't make sense. Where do you connect the RG58 shield if handed some terminal block? If you don't connect it, then why buy expensive RG58 when you could as well just use some random copper wire? – Lundin Jul 02 '21 at 10:15
  • The terminal block has two poles for that: shield and core. I don't want to say you are wrong, things could be better, but they are not. Some people wants remote controls with 12 bit code and dip switches, and don't want to buy a board with encoder (like the one in this post) just because it *has* an encoder. – linuxfan says Reinstate Monica Jul 02 '21 at 10:21
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The only real issue I can see is weatherproofing - and absolutely nothing to do with RF. It is a small signal connection, and if it is weatherproof enough for the proposed environment then it is fine.

As far as the impedance matching comments go - look at the antenna! It's clearly a low power radio link where the antenna, typically installed by a non-RF person, is a length of hookup wire that usually works, and if there is a problem them read the manual and try a different length / orientation. This reminds me of the receiver on my garage door opener which is mounted near the ceiling of my garage. This works at 300MHz, the receiving antenna is a length of hookup wire hanging from the unit, and it is connected to a screwed connector block. It has worked fine of over 20 years. I've never felt the need to convert the connector to SMA.

Tesla23
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