220V AC be interfering with RF signal (MICROCHIP MRF49XA)?

[onesixright]

Hi,

I got a question from a friend. Its way out of my knowledge, so i hope someone can shed some light ion this. This information is somewhat limited, but maybe enough to understand whats going on.

They have a electronic thermostat. It communicates (RF 433 MHz) with a heating system, via a MICROCHIP MRF49XA. Now the signal seems to be unreliable. Looking at the design we see a 220V AC wires being 'routed over'  (there is a gap of a few mm) the RF chip. 

The box is roughly 15cm x 8 cm. The power input is on the left, the power switch is on the opposite side. So the AC wires (2x) go into the switch (crossing the RF chip, and 'hanging' free inside the case), and then back to the PCB. On the PCB the 220 is switched via couple of relays to the heating elements, and there is a voltage regulator that powers the PCB). So, basically the 220V is passing the RF chip twice.

My question: could this 220V AC be a cause for interference (and with that unstable RF link)? Shouldn't you need to keep the AC lines as short as possible?

Moreover, and now it get really weird. The box has vent holes for cooling. For some reason, when you switch on the box, and wipe your hand over the box (touching the cooling holes), the signal is lost. This can be repeated with same result.  My initial thought would due that you create some static electricity somehow. But maybe thats too far fetched? Curious if this 'effect' is something common?

Thanks!

[Richard Crowley]

What is the HISTORY of the problem?  Is this a brand-new installation that has NEVER operated properly?  For HOW LONG? Was this system operating properly at some time in the past?  For how long?

It sounds to me more like 433 MHz INTERFERENCE to me than any kind of PROPAGATION problem (unless it has NEVER worked.)

[wraper]

Does the communication become unstable when switching relays? My guess is that arching contacts might interfere with the receiver. About touching box, how far devices are located? Aren't they located too far away that you interfering with a hand is enough to distract communication? Is there any shielding? Antenna is external or internal?

[Niklas]

As wraper wrote, the arching in the relays can do all kinds of magic tricks. High frequency transients on the wires can also cause problems, such as flipped memory cells. State machines, like UARTs, I2C etc, can be triggered and become out of sync.