Cheapie RF Modules, Antenna Length

Question about adding an external antenna to these cheapie 315MHz or 433MHz modules.

What's been repeated everywhere I look is that adding a single wire antenna would call for 23.8cm for 315MHz and 17.3cm for 433MHz.

I'm wondering if these figures should be adjusted to account for the bit of antenna that's already on the modules, or if people actually remove the onboard ones when adding an external.

There is no onboard antenna on the modules pictured. The coils are an important part of the circuitry.

There's just a single cap between those coils and the external antenna

Typical schematics.

TX:

RX:

this is what they look like
433 Mhz antennas.JPG

The round metal can on the Tx says 433 so there is no question about that frequency but the Rx has three different frequencies silkscreened on the back. I don’t know why or how you tell which one is for that Rx or what so maybe someone else can fill in that blank.

I found this, but honestly , don’t know enough about RF to know what to do with it.
Building the circuit shown in the schematic is simple. The question is what is the application for this chip ? (as opposed to the Rx shown in the photo )

The answer seems to be at the end of the first paragraph in the datasheet:

allowing bit rates of 20kbps to 433.92 Mhz

I am really not sure how you know if you need this chip.

The transmitter shown - based on a saw freqency reference , is OK.. but the receiver is a super-regen, and will be both insensitive and subject to frequency drift. It's a bit better than most because it has a gain stage before the oscillating detector, and will so also be a bit better in resisting frequency pulling by different antennas - but still not very good.

The Micrel receiver chip linked by raschemmel would be a great deal better - and they even give you a sample pcb layout.

Note that the layout at such frequencies is critical - there's no way a breadboard approach would work

If you're unwilling to build such a device there are many proper receivers at 433MHz available on ebay etc - and matched tranmistter/receiver pairs, together with recommended antenna designs

Have you considered such devices as the nrf2401? cheap, decent range, inbuilt antenna, and lots of example code available....

regards

Allan

INTP:
Question about adding an external antenna to these cheapie 315MHz or 433MHz modules.

What's been repeated everywhere I look is that adding a single wire antenna would call for 23.8cm for 315MHz and 17.3cm for 433MHz.

I'm wondering if these figures should be adjusted to account for the bit of antenna that's already on the modules, or if people actually remove the onboard ones when adding an external.

Hello there,

Probably the best way to find out is to try different lengths. Matching is very important too but it's hard to say what works and what doesnt work with non standard antennas.

If you set up a transmitter and receiver pair you can try transmitting with different antennas to get some idea what works and what doesnt work.
The receiver should be a decent distance from the transmitter.

If you care about range, consider using RXB-12 receivers, based on the SYN470 receiver chip. They're like a buck and change on ebay, the narrow yellow ones. When I was testing their range, I got much better range with them, while the green ones like pictured above were the worst performer. All the others I could find were in the middle, but more expensive than the RXB-12. The cheap transmitters were fine.

How about the SYN480? Same rec as 470?

Yeah - SYN480 is a slightly lower cost SYN470, I think they use the same dies, just connected differently.
To save money (by fitting into 8-pin package instead of 16) they hardwire the pins that set the demodulator bandwidth and sweep mode (of course, those pins are hardwired in syn470 modules too, and internally connect the two grounds and supplies (while the 470 lets you use a separate supply for the RF section).

jremington:
There is no onboard antenna on the modules pictured. The coils are an important part of the circuitry.

The coil on the receiver literally is solder bridged to the hole for an external antenna