RF 433mhz inside plastic enclosure doesn't work well

Hi everyone.

I made my first PCB. The PCB mainly consists of a attiny and an rf433 schematic embedded on it(all in 1 PCB), that I found online. As a big surprise the rf433 transmitter works very well. I am on the second floor and the receiver which is on the first floor receives the signal without any issues. Big surprise for a noob like me.

However problems start when i put my PCB inside a small plastic enclosure. The receiver receives the signal only when its close (few meters away). If i change floor then it does not work. If i put the PCB inside the plastic box(plastic box consists of two parts, the back and the cover) and i do not cover with the plastic cover(and the antenna is on top, not covered)then it works from far.

So is something wrong with my plastic box or its normal?

thanks

Plastic won't affect RF, its some else like a loose antenna connection perhaps?

433MHz antenna should have good range as its large - longer wavelengths benefit from this effect, but eventually there isn't room for a 1/4wave antenna - 433 is a good compromise for portable equipment.

In free space you'd probably expect a km or so range from a low power 433 TX.

A thought - with everything in the box is there a decent ground counterpart to the antenna?

If not you'll be best using a dipole antenna, most are 1/4 wave (which assumes groundplane or similar).

Does putting the lid on cause anything inside the box to move?

Do you get the same poor range by just placing the lid on the box instead of screwing it down?

What else is in the plastic "mix" that your box is made of? Is it an anti-static box?

Actually, the change in density of the material causes the RF to be reduced. You have RF in the board traces being propagated to free air, and you have shown that works. But in the box, the RF must be coupled to the plastic material, with loss, and then back from plastic to air, with more loss. If your plastic is colored at all, then there is a mixing of the base plastic material and the particles of color, which may even contain metal atoms. Each time the RF is coupled from plastic molecule to color molecule and back, there is loss.

Radar sometimes uses 450 MHz and it requires a special fiberglass enclosure that limits the loss as the RF passes thru the enclosure.

The best design would use an external antenna.

Paul

Some plastics also can retain moisture and some black plastics contain some carbon, which could interfere. Made that experience with drone antennas.. enclosed in black 3d printed casing, the range was reduced. Used yellow, all good.

Plastic may have a small amount of RF absorption, but nothing significant enough to be detectable without
careful measurements, perhaps 1dB at most. This study of several radome materials for instance shows
that sort of loss for 2.5mm thick radomes at UHF: https://digital.library.unt.edu/ark:/67531/metadc5801/m2/1/high_res_d/thesis.pdf

Carbon in black materials is a valid concern however.

Another thing, any change you make that changes the location of any physical part of an RF device, will usually radically change the transmission characteristics. This can trick you - for example imagine I have a unit, it works fine here on the bench, now I put it in a box, and not so good. But I moved it a few centimeters in the process. That is almost 1/4 wavelength at 433MHz. Even putting a metal object near the antenna can make a huge difference, and it can randomly be a good or bad difference. If I had somehow positioned a typical plastic box around it, without any movement to the device, there would probably be no change.

By that same argument any deep nulls will short lived as you move and breathe!

Significant reduction in indoor range (where everything is multipath and reflected!) means significant increase in loss somewhere, perhaps 20 to 40dB's worth. A broken RF connection is a common way to get this, as you'll easily get -40dB leakage across a small gap at UHF.

When I was working with 433MHz RF, a few times I wound up with the parts and myself located such that it would or wouldn’t work depending on where I stood…

That was back when i was using those godawful green receivers, which are total trash. With the RXB-12 based on the Synoxo receiver IC (SYN470 - SYN480 is comparable, but physically smaller), you’ll probably be fine, as it’s receiving range is so much longer as to make up for that. I’d probably just put pins to connect the receiver with - though the SYN470 is surprisingly easy to implement on a PCB as far as receivers go - I did it a few years back when I wasn’t very good at PCB design yet, I think a few of my inductors and capacitors in the RF section were off, and my range was still 80% of what the RXB-12 was. But the RXB-12 is a buck and change each, so very hard to justify building my own - it’s not worth the time to place the parts on the PCB, even if they were free.

An aside on range: All the transmitter designs you see around are fine. There’s like a 20% difference IIRC between the best (STX882) and the worst of the common ebay ones (again, a couple of new designs have come out since - IIRC the most common green ones were near the top of the pack within that… though it’s easier to build one based on the SYN115, which was closer to the bottom of the pack) - though a $8 US-made transmitter I tested had very poor range, oddly enough.

The receivers, on the other hand, varied between a range of under 50 feet to over 1000 feet under comparable conditions. The RXB-12 was the winner, beating out all the other ones available on ebay at the time (though a few more designs have come on the market since. I haven’t tested those yet). The other SYN-480/SYN470 modules were about the same.

As long as your antenna in each case is 17 cm long and extends from the PCB in a straight line, it should not matter that it is in a plastic box.

If you have the antenna anything but straight and a similar distance from metallic objects you are likely to be preventing it operating properly. :cold_sweat:

No matter how good your receiver is, it will receive the same trash as the cheapest receiver if the digital logic on your board is radiating interference. So things like dozens of jumper wires between the board and say, an LCD display or other module, can create a real receiver de-sense problem. If you fold your antenna into close proximity to the rest of the wiring (such as inside a box) that phenomena will worsen the performance.

Also, standing waves certainly do generate peaks and nulls. I was waiting in a pandemic caused line up last week, trying to lock my car with the remote. Other person in line sees it, says, "hold it up to your chin". I gave him a look like, "pffft..." but tried it and it worked instantly. RF is magic.

Hi,

I made my first PCB. The PCB mainly consists of a attiny and an rf433 schematic embedded on it(all in 1 PCB), that I found online.

Can you post a copy of your circuit and a picture of your PCB? Have you got an aerial on the NRF units?

Thanks.. Tom... :)