Electronics > RF, Microwave, Ham Radio

Place to connect external antenna to this weather station PCB ?

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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.

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

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

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...   :palm:

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)

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.


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