emonTx just out of range of RFM12Pi


A couple of weeks ago I have bought an emonTx (V3 - 433Mhz) and RFM12Pi (433 Mhz).

The problem I am currently facing is that the RFM12PI is not receiving the data transmitted by the emonTx.

If I move my raspberry pi with RFM12Pi and also the emonTx a bit around then the data sometimes get transmitted.  So this indicates that they are just slightly out of range.

The distance between the 2 devices is about 10 meters (not much in my eyes) but the signal needs to cross 3 indoor walls (assuming the signal has to go straightforward).  One of the walls has a metal plate which I guess might also hinder the signal.  If the signal doesn't need to travel straightforward then it only needs to pass a wooden door (:-).

So what can I do to improve the reception ? Most likely a small improvement might be sufficient for good reception.

I know that moving and changing the direction of the antenna might help but a bit more specific advice would be great.  

I have posted this in the hardware forum but any software solutions are of course also fine.

many thanks in advance

Robert Wall's picture

Re: emonTx just out of range of RFM12Pi

Radio signals at that frequency can be reflected by large conducting surfaces, but a clear line of sight is best.

I'm assuming you have the single wire antenna that's 165 mm long on each. Take a look at the amateur radio websites and see what the radiation pattern of a 1/4 wave whip antenna is. That should give you some idea of the direction in which to point them.

What is limiting the positions of the two units? How far can you move one or both to improve the positioning? (I've extended CT cables by about 10 m, there should be no problem with extending the ac input by a similar amount.)

As far as I know, the transmitter power in the emonTx is set at maximum, so there's no opportunity of increasing the range there. But you could slow the data rate, which might be enough. There's a thread which you should look at http://openenergymonitor.org/emon/node/6178 and searching for "bit rate" or "data rate" should show up some more.

jvda's picture

Re: emonTx just out of range of RFM12Pi

Hi Robert,

Thanks for the fast response.

I can move it a little bit, but there will never be a clear line of sight: there will always be at least 2 walls to pass. Allthough these obstacles, I have noticed that moving it a bit might also seems to help - but I have no clue which moves really help - I am just moving it randomly around.  Some guidance tips regarding how best to move especially in combination with directing the antennas would also help.

Regarding the antennas: it is not exactly what you have written above:

  1. The RFM12PI has a wire of about 165 mm (I think it is a bit shorter about 160mm),
  2. but the emonTx has small SMA antenna (its total length is 59mm) it looks like the left antenna on : http://tekfun.manufacturer.globalsources.com/si/6008823829919/pdtl/Two-way-radio/1068517955/Stubby-Rubber-Monopole-Antenna.htm

Would it help if I replace the SMA antenna with another antenna ?  Allthough I have ordered emonTx (including antenna) very recently, it got delivered with a different antenna than the one shown on the website: http://shop.openenergymonitor.com/433mhz-sma-antenna-included-with-emontx-v3-4/  (the length of this antenna is 106 mm = double length of my antenna).  So would using a different SMA antenna (e.g. the one mentioned in the link) for the emonTx help ?



Robert Wall's picture

Re: emonTx just out of range of RFM12Pi

Email the shop and check that you do have a 433 MHz antenna. I'm not a radio expert, it could be right - it depends on what's underneath the cover. Without knowing full details of each, it's impossible to say whether changing it would help. If you can move it enough to remove one wall, that too will help. Without knowing your building, there's little more I can say about positioning. Have you looked at radiation patterns?

Do you have a programmer, and can you reduce the data rate? Of course, you'll need to do that in both the emonTx and in the RPi (hopefully, Paul - pb66 - will be able to tell you how in the Pi)

pb66's picture

Re: emonTx just out of range of RFM12Pi

"(hopefully, Paul - pb66 - will be able to tell you how in the Pi)"

The serial baud is easily set in the Pi using a "com-baud = 12345" line for the RFM2Pi settings in emonhub.conf, that baud has to match the baud set in the RF12demo sketch (RFM2Pi firmware).

However I think the setting you require is the Data Rate which can be also defined by the RF12demo sketch using rf12 control codes. either way, changing the Data Rate or the serial baud will involve editing the RF12demo sketch. You can maybe get info needed to do this from Jeelabs rf12 wiki

Once you have edited a copy of the sketch that then needs to be loaded to the RFM2Pi, See "Re: Flashing RFM12Pi with Raspberry"  I'm happy to help "install" any changes, but probably not the best guy to decide what changes to make. (a couple of threads on this)



Regards the RFM2Pi antenna, if it's is roughly right, possibly a little short, is it feasible to perhaps insert a steel pin (common "dress making" type) into the cut end of the wire (very carefully without stabbing yourself) to give a small range of adjustment from say 160mm to 170mm to "tune" by trial and error whilst sliding the pin in and out? I know copper to steel probably isn't the best connection but even if this is done to establish the best length for a potential replacement wire. If you do try it warming the wire end will make inserting the pin easier.


Bill Thomson's picture

Re: emonTx just out of range of RFM12Pi

Hello Jan,

The addition of a counterpoise to your SMA antenna will help considerably.

See http://openenergymonitor.org/emon/node/6238 for more info. (picture 3 posts down from the top)



jvda's picture

Re: emonTx just out of range of RFM12Pi


Thanks for all the advice. 

So far I figured out on the internet that the antenna of the emonTx must be oriented vertical for optimal reception (as in my case the receiver (RFM12Pi) is located on the same floor).  Not sure if the antenna of my receiver should also be oriented vertical - but I did.

I experimented with moving around a bit and managed to get better reception but the problem is that for this I need to position the devices at a place where I totally do not want them to be located. I tested also with moving them to possible solutions (which I can sell to my family members as well) and none of them gave proper reception.

I am totally not convinced that the antenna (=small SMA antenna with total length 59mm) of my emonTx is doing a good job.   Note that the signal only needs to cross 10 meter on the same floor through a few walls.  Note that I found similar antenna on the internet with a gain of only 0 Dbi while the good ones have a gain of 2.5 Dbi - in other words the signal of a good antenna is about 75% stronger at reception than the antenna I found on the internet (which might also be the case for my antenna).  I am contacting support@openenergymonitor.zendesk.com to check this out.  If this doesn't lead to proper solution then I am thinking of buying the following "1/4 Wave Scanner Antenna - Trimmed For Best Frequency Performance - 5yr Warranty"


Regarding the other suggestions:

  1. using a counterpoise: available space where I can put my emonTx is a bit too limited for adding a counterpoise to antenna.  Note also that the antenna is directly attached to emonTx without any cable in between.  So adding a counterpoise would no longer make it possible to mount the emonTx agains a wall.  
    Nevertheless did someone try this on emonTx and can he share the improvement it has brought ?
  2. lowering the data rate / bit rate: this indeed seems promising to improve the signal to noise ratio but it requires a software modification of both emonTx and RFM12PI.  Based on what I read it didn't look that easy to do (I also do not have the serial interface to upload software to emonTx) nor something that has been tested thoroughly by other people.
  3. Extending the RFM12PI antenna a bit (through sticking a metal pin into it): I don't think this will help a lot as its size is really close to 1/4 of the wave length and I am also afraid that i am going to disconnect this antenna if I manipulate it too many times.  Maybe I will it a try later.

Many thanks anyway for all the feedback.


Bill Thomson's picture

Re: emonTx just out of range of RFM12Pi

Hi Jan,

The antenna with 2.5dBi gain will help, as the antenna that's rated at 0dBi is actually operating at a loss compared to the antenna rated at 2.5dBi. Antennas are a "double-edged sword," a loss or gain in one direction e.g. transmitting, happens in the other direction, i.e. receiving. So your 0dBi antenna is not only causing a loss in your transmitted signal level, it's causing a loss in the received signal level too.

You were correct regarding the physical orientation of both antennas. If one is oriented vertically, and the other horizontally, that will cause a significant reduction in received signal strength.

Regarding the counterpoise. The photo in the post I referenced shows the antenna/counterpoise combo with a short cable attached. The idea is to locate the antenna a short distance, a meter or two, perhaps even three, from the EmonTX via the cable. But, if you don't have the room, you're more or less locked out of that route.


Robert Wall's picture

Re: emonTx just out of range of RFM12Pi

Given the counterpoise ("ground plane" in English), the antennas should be aligned parallel to each other and at right angles to the line joining them, then you might see a small improvement if you tilt the tips towards the other one by about 10 - 20° (This is assuming the radiated pattern is not quite symmetrical and points slightly "up" in the direction of the tip of the antenna, which is likely).

glyn.hudson's picture

Re: emonTx just out of range of RFM12Pi

Your emonTx was supplied with the following antenna: http://www.rfsolutions.co.uk/acatalog/info_ANT_STUB_433SM.html#SID=129


• 1/4 Wave Monopole Antenna 
• Omni-Directional 
• VSWR <2.0
• SMA Male Connector
• 50ohm Impedance 
• 0dBi Gain 

Jérôme's picture

Re: emonTx just out of range of RFM12Pi

Regarding the RF bitrate modification, I'm afraid it is not so easy on the RFM69 because the feature is not implemented in the driver. But I may be totally wrong. See the post Robert linked to for further info.

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