I'm attaching a schematic of an RF transmitter made by Synoxo. Chip is the SYN115. The schematic is found in the datasheet.

I do not understand why L3 which is the PCB antenna (pointed with the red arrow) is connected on both ends. As far as I know, antennas are connected on one end on a PCB and the other end is not connected anywhere. Also why there is no value to L3?

Also what is the black arrow (pointed with my blue line?)


  • 5
    \$\begingroup\$ Site policy to to credit the work of others when posted on this site. Please edit your post to give credit to the owner of the schematic you've posted. \$\endgroup\$ – Transistor Apr 2 '20 at 12:17
  • \$\begingroup\$ how to edit it? its a schematic made by Synoxo \$\endgroup\$ – ChrisT Apr 2 '20 at 13:53
  • \$\begingroup\$ @ChrisT Did you draw the schematic yourself? If so, then you should know what the symbol means. If not, provide a link or citation to the original source...a web page or a document. = \$\endgroup\$ – Elliot Alderson Apr 2 '20 at 14:01
  • \$\begingroup\$ I already said its made by Synoxo! \$\endgroup\$ – ChrisT Apr 2 '20 at 14:17
  • 1
    \$\begingroup\$ Hit the edit link below your question ... \$\endgroup\$ – Transistor Apr 2 '20 at 15:34

why there is no value to L3?

L3 (or at least the loop part) is printed on the PCB (as indicated on the schematic) hence isn't a lumped component you need to provide. The antenna is shown (somewhat vaguely) on the PCB diagram in section 15 of the datasheet you took the schematic from.

It would have been helpful if you had provided a direct link to the datasheet you are using with your original question rather than adding a bit more information for each comment/answer received.


The black arrow just means that the antenna/coil has an adjustable tap, that allows it's performance to be adjusted. It shorts out part of the antenna, thus changing it's electrical length.

EDIT 1 - Added IEEE symbol for adjustable inductor

From IEEE Std 315, here's the symbol for an adjustable inductor:

enter image description here

  • \$\begingroup\$ No, that would be a shorted turn - very lossy. I suspect the connection from GND to tap IS the antenna ( a loop) : tapped inductor L3 is an autotransformer, providing impedance matching for best efficiency between the TX output and the loop antenna. \$\endgroup\$ – user_1818839 Apr 2 '20 at 17:25

You don't choose a value for L3. It is a specifically shaped trace on your PCB.

The circuit you gave is a "typical application," but isn't a full design.

Later in the datasheet, the manufacturer shows the circuit and layout for a demo board they produce using the SYN115.

Here's the circuit:

enter image description here

Here's the layout:

enter image description here

L5 is the antenna in the schematic, and L5 is that thick trace along top half of the layout on the right (the top layer.)

The parts list also says that L5 is the antenna and that is part of the PCB:

enter image description here

It is formed and dimensioned to be resonant at the transmitter operating frequency of the transmitter.

There are no details given for the dimensions of L5.

You have a few options:

  1. Contact the manufacturer and ask for more details.
  2. Place a jack for an antenna connector and use a normal antenna.
  3. "Eyeball" the dimensions of the example antenna and hope for the best.

If you work for a company that will be making a whole bunch of these things, then you should go with option 1. Options 2 and 3 are really only "usable" if you are doing a one off for a hobby project.

Another option would be to study antenna design, and design your own loop antenna and matching network.

If you'd like to install a jack and connect a regular antenna to the chip, then have a look at Figure 2 from the datasheet. That's a test circuit followed by a PCB layout. It includes a circuit for connecting a 50 ohm antenna jack.

enter image description here

As a final option, you can just ignore matching and antenna design and just do what all the manufacturers do that make and sell the cheap SYN115 modules on AliExpress: Include an eyelet where the antenna should be (the antenna jack in Figure 2) and just hang a random piece of wire on it as an antenna.

Most of the projects you'll find on the internet that use these modules don't even bother putting an antenna wire on the module. They just assume (as you seem to) that the modules have an antenna on board.

  • \$\begingroup\$ Regarding L5, you said to place a jack for an antenna Connector. But place it where? On the gnd? Or on the right of L4? Because L5 seems to have 2 "jacks" \$\endgroup\$ – ChrisT Apr 2 '20 at 19:36
  • \$\begingroup\$ Also that schematic seems too big. I want to replicate syn115 transmitter(google it) pcb to embed it on my board. The first schematic seems closer to that transmittet \$\endgroup\$ – ChrisT Apr 2 '20 at 19:43
  • \$\begingroup\$ Do you mean any one of the dozens of projects google finds, or do you mean the module. Google returns different results for everybody, and I have no idea what it turns up for you. Why don't you just link to it instead of telling me to google for something to help you? \$\endgroup\$ – JRE Apr 2 '20 at 19:48
  • \$\begingroup\$ All the modules I found have an eyelet where you should attach an antenna. It's pretty much just hang a wire in it and hope it works. They aren't properly matched to the transmitter. It's all just "meh, it'll work some how." \$\endgroup\$ – JRE Apr 2 '20 at 19:53
  • \$\begingroup\$ At the bottom is the tranmitter ae01.alicdn.com/kf/HTB1BcpAau38SeJjSZFPq6A_vFXab.jpg \$\endgroup\$ – ChrisT Apr 2 '20 at 20:38

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