[tedem]

Thank you very much for the replies.
The way the units are shown in the photo is not the way I have been doing testing. It was just a way to have both units in the same photo.
I will keep in mind the suggestion of antenna orientation when I try more testing. I did not take note of the orientation when I made the previous tests.
Do you think the antenna are part/all the cause of the trouble? Do you think it would be wise to order another pair of antenna? perhaps of a different design?
I have very limited test equipment so I can't really see what makes a difference except to use the receiver and go by what it says.
Is there anything else I can try?

Thanks again,
Gord

Hello Gord,
I'd echo what the others are saying about antenna separation and relative orientation. I've found the Tx modules I buy on eBay can be unreliable, so I now mount them in a socket. Confession: I've also destroyed one by powering up a board without an antena connected. I've found you need to leave the system running for several hours for the RF efficiency values to stabilise, and even then, they seem to change for no apparent reason - maybe due to interference or weather conditions (rain?).

I've had three transmitters running successfully for a couple of years. Two using antennas from eBay that look like longer versions of yours (mine are c. 16cm), and the third using a 17.3 cm long piece of wire soldered to the inner core of a length of coax cable (17.3 cm = quarter-wavelength at 433 MHz). I'm using coax cable between the Tx board and the antenna because the Tx is several metres lower than the Rx, so there is no line of sight.

So, I wouldn't worry too much about the actual RF efficiency values, but maybe experiment with simple wire antennas to find a good layout that gives you RF values in the right ball park, then leave things alone for 24 hours to see what the actual data look like.

Good luck,
Ted