Place to connect external antenna to this weather station PCB ?

[haydent]

Firstly thanks for dropping by, Please ignore the yellow trace, that was my first attempt but i took it off. Ive made what i hope is a better coil antenna but would like to check with someone more in the know the best place to connect it ? The PCB is in the outside section of the weather station, at the top of the PCB you can see the 433MHz transmitter.

[cdev]

What is under the red blob (of wax?) - the circular thing near the end of the L-shaped slot?

[haydent]

i just took the wax off and a small phillips head screw, i imagine for tuning ?

[cdev]

Maybe its a trimmer cap - for its 'tank circuit'.. ?

Try taking an oscilloscope or RF detector (germanium diode + cap + earphone? ) and probe around, maybe you'll find it. If not, maybe just inductively couple your antenna to that loop. Use that loop next to a 1/2 wavelenth long wire (dipole, or driven element in a coathanger Yagi and ground it in the exact middle. Use the loop like a 'gamma match'.

But you may detune it doing that if it doesn't have a crystal...   

Do you have a DVB-T dongle/rtl-sdr? Almost short the end of a length of coax with a very small loop and use that as a probe. Put some ferrites at both ends and on the USB so the coax shield and USb wont pick up common mode stuff. You can also use it for receiving the weather station signals (you likely already know that)

[voltz]

The PCB track is a coil and the red blob of wax hides a trimmer cap. These two are in series forming an acceptor tuned circuit. So when tuned, current is maximum. It forms the TX aerial and should be tuned for maximum radiation with a wave meter / receiver / analyzer.
Its crystal controlled, 433 Mhz, so frequency shift not be too much of an issue (hopefully)..

If you wanted to connect an external antennae to it, its worth remembering the circuit is not designed for it. Also it may not even be legal if the radiated energy exceeds the original design. That said, i've seen yagi beams connected to wifi routers to extend range for up to a mile or so, so i wouldnt worry too much about it... This is quite minor in comparison. But worth pointing out. It could be illegal depending what you connect it to. 20 element beam?? lol.

The actual feed point is the other side of the cap where you had your yellow wire. Your original point with the yellow wire is in the middle of the acceptor circuit. You could i suppose remove the cap to disconnect the existing tuned circuit and take a thin coax from the feed point to an external socket, like a BNC. Shield side to the ground plane. Or just solder a wire to the feed point.

[haydent]

firstly if i disconnected the red blob cap, wouldnt the trace that leads around the edge of the cutout become part of the antenna up to the point i connected onto with the yellew wire ? and wouldnt that make the pad near the silkscreen "WHP0063B1" the feed point ? and if i connect an antenna there, would i cut that trace that leads out from there and around the cutout ?


thanks for your help this sounds good. my problem is that overnight it drops out and doesnt reconnect to the base till during the day. now this is despite the batteries being good, and the solar being disconnected. If i sit the transmitter inside next to the reciever it stays connected for days on end no problems.

 its always had woefully pathetic range, but seems to have become worse at night or during rain. have tried to test if it temperature related by putting it in the fridge for a few hours to chill it then pull it out and start it up and it still works straight when right next to it. (i must do this test again from outside though)  but it doesnt even get that cold at night here in summer (17degC) and it used to work fine at 1degC in winter night.

maybe its condensation/humidity, but it never looks noticeably wet when opened up

Im wondering if some other component could have degraded ? we live in country, so no neighbours and no interference.

Any way im thinking of trying and antenna to try and get a bit more out of it to hopefully keep it connected.

[voltz]

"firstly if i disconnected the red blob cap, wouldnt the trace that leads around the edge of the cutout become part of the antenna up to the point i connected onto with the yellew wire ?"

No, because that track at point next to "WHP0063B1" is the earth side. So removing the cap makes the track redundant - not resonant.

[haydent]

ah ok, so my antenna connects onto the red blob cap where ive disconnect the trace  ?

[voltz]

Like this

[cdev]

Maybe you can find an article like this one, except about YOUR weather station...

http://goughlui.com/2013/12/20/rtl-sdr-433-92mhz-askook-decoding-of-various-devices-with-rtl_433/

[haydent]

thanks, will give that a go

[haydent]

so it didnt help  im using these antenna design here: https://arduinodiy.wordpress.com/2015/07/25/coil-loaded-433-mhz-antenna/ which seems good, i just added it to another 433 system i got (tank level gauge receiver) and its performing much better now, no drop outs (fingers crossed)

whats with all these shite 433 devices on the market with over spec'd transmission distance, they all have the same crummy $1 ebay style modules in them...

anyway have to work out why its cutting out overnight... could a 433 crystal degrade ?

[voltz]

UHF being UHF, the distance is pretty much stated for line of sight only. Put objects in the way of the path and its drastically reduced. So it may be advertsied as "100m" or so but in real life situations its probably more like 10.
I just wonder if your drop out problem is actually signal strength related or something else. It only happens during the night?

some ideas:
Is is solar powered - if so, could simply be power loss.
Frequency drift during cold nights.
Some other transmitter locally working at night wiping your RX out.

[haydent]

thanks for your suggestions, its about 10m away with only a glass window between it... it hard to know if its the tx or rx modules or the circuitry leading up to them, will have have a sticky beak with my scope, i wish rf analysers where cheaper, ive also ordered some better 433 modules super heterodyne, and will try them instead