How to design a PCB with a helical antenna and mount it


I’d like to experiment with RF modules to build communication devices. The module I’d like to use is the HopeRF RFM69HCW, both variants for 433 and 868 MHz. They are modules to solder on SMT pads. Their pitch doesn’t allow mounting them directly on a breadboard so I’m currently designing a breakout adapter. There are only very few of them available and none have all pins in a single row. And as I’m going to have to build those PCBs for a full application later anyway, I can just start today.

I have two questions about that:

  1. There are plenty of helical antennas available everywhere. Some for vertical mounting, some at a 90° angle. How are they soldered on the PCB? Should they remain at their full length as delivered and be mounted so that only the last bit goes into the PCB hole? Or should the spring part always sit directly on the PCB and the rest be trimmed like with other THT components? Or does it not matter? There are different shapes available, some with a 90° angle look like they can only be mounted away from the board because the angled part would be too short to even go through the board if the spring lays flat on it.

    Could you show photos of how it should look like with an antenna mounted on a PCB? Most of what I find discusses designing antennas with PCB traces alone. That’s not what I intend to do, I’ve already learned that that’s too complex for me. I’ll stay with ready tuned antennas.

  2. Can I use a simple trace from the RF module’s antenna pin to where the antenna is mounted? Or will I have to consider all kinds of RF trace dimensions there? How long can such a trace be to keep it as simple as possible? (I’m not talking about tens of centimetres, just a few cm at maximum.)

Any help with this is appreciated.

I use one of these helical antennas with short 90deg bend, lay the straight part flush with my PCB so that the axis of helical part goes approximately on the PCB level. My antennas either are positioned away from the board, or I cut the slot in the PCB so the antenna fits in (no copper around, obviously).
These antennas are not perfectly tuned, so leave room for matching network and fine tune it in your application. But once tuned, they perform very reasonably.

I am not an expert on antennas, but I pick up some hints sometimes.
In the past I’ve made a breadboard adapter from a piece of 0.1" header strip. Bend the pins a bit with some pliers so they sort of fit the castellations of such an adapter print, but keep the connector long enough to add some more loose wires.

Often 8 or so wires is enough: Power and SPI or I2C bus. you don’t have to have access to all functional pins.
Note that in this image, the 2nd castellation in the lower left corner is not soldered to the header. It’s got a female socket for a 0.1" header pin on the back side instead, in which an antenna can be put directly.

This approach can be used for testing small adapter boards or for lots of SMT components. Just take a SOT-23 and a 3 pin header, then solder it to two pins and use a wire for the third.

About your antenna’s…
Those are supposed to be used unmodified. Do not cut anything off. Do not stretch, bend or compress the helical section, because this de-tunes the antenna. If you bend the straith section between the helical part and the sharp angle that is the solder connection, then that is probably OK. Use some pliers (again) to grab the antenna were you want to bend it, so do not place stress on the helical section.

They are supposed to be soldered to a PCB (duh) but it does not matter much how you solder them. Just solder them where the wire has a sharp corner.

They should not have other conductive parts nearby, especially no GND plane. You can use the PCB for mechanical strength and mount the whole thing parallel to it (with for example glue) but make sure it’s on an empty section of the PCB. Make a big fat keepout zone around the antenna.

For more inspiration:
(The first part has big ass antenna’s, but if you scroll down you get to the small PCB mounted spirals)

My experience is that these easily available antennas seem probably heat treated, and break easily if I try to bend it.

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