OpenEnergyMonitor Community

Improving RF signal strength emonTH emonTX

Probably one for @Robert.Wall but does the gauge of the EmonTX/TH antenna affect its signal strength?
(I know it’s length needs to be fairly precise).
I’ve moved my base station, and trying to improve their signal strength.



There is a minor effect with thicker marginally better (recall that with skin effect, the currents are flowing only in the outer “skin” on the conductor). A more signficant improvement can be made by improving the ground plane that the quarter-wavelength antenna is attempting to reflect in.
A quick experiment is to simply place a chunk of plain FR4 board under the vertical antenna - it doesn’t need an actual ground connection to work. If it is available, monitor the RSSI level for a quantative assessment.

Bill is the practising radio expert. My ½p worth: I believe the antenna bandwidth for a yagi is a function of the element thickness, though whether that holds for a whip, I know not.

But I second the suggestion of a ground plane. It’s effective on both the receiver and transmitter.

0000 (4/0) AWG will give you an easy 3 db gain! PS - Where is April Fools Day when you most need it?!?

Not sure how much gain you need but as Robert stated a yagi would give you good gain. A ⅝ wave might help with a good ground plane.

I didn’t, actually. It would, but at the expense of directivity. If all the transmitters are in the same direction, then a Yagi is good (as it would be for the transmitter - assuming only one receiver). But if the transmitters are scattered in different directions, then it’s a bad idea.

Thanks all, the original antenna was a 82mm length of CAT5 wire, and get the following db results;

and as you can see, lots of missing datapoints and a mean average of 81.8db

But… replacing the CAT 5 antenna with a 82mm length of 2.5mm solid copper (13A mains cable), I now get the following results, with both transmitter & receiver in exactly same positions;

…and a mean average of 75.4 db

So after telling my wife the opposite for years, size does matter!



A single strand, pulled out of the bundle?

I’d be worried about pulling the pad off the PCB should that get caught in clothing or a duster, if it’s not otherwise supported.

You could still try adding a ground plane - it should give you a few more dBs of signal. It doesn’t have to be a solid sheet, a conducting mesh or even 4 more pieces of 2.5 mm^2 radiating outwards from the base of the whip antenna would help.

Yes. For the past 2 years it’s provided about 60db as the base station was about 4mtrs away, but the weakness has been exposed by moving the base to another room.

I did think about that, but its supported from lateral movement by the case (protrudes through a hole!), and it’s wall mounted at about 2mtrs high in my garage, so should be ok.

I’ll try adding a ground plane to improve ss further, and good point about adding one to the base station too.



The main beam radiates above the ground plane, at 45° but less as the ground plane improves, so you want the transmitters and receiver to ideally be in the same plane, with the antenna ground planes parallel to that, or even better, inclined slightly down in the direction of the other end.

The radiation pattern is omnidirectional, i.e. a somewhat flattened toroid centred on the whip.
The signal strength is plotted looking broadside at the whip.

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Adding a ground plane

Measured signal strength tonight over 1 hour without a ground plane and got:

Mean Min Max Diff Dev npoints
-74.2 -82.0 -68.0 14.0 2.2 362

But adding a ground plane to the transmitter, in the form of placing a small sheet of foil beneath the casing, I get a further 3db gain:

Mean Min Max Diff Dev npoints
-71.1 -80.0 -66.0 14.0 2.0 362

Quite happy at that!


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Further improvements…

I made 2 ground planes earlier from 165mm diameter pieces of cardboard, and stuck aluminium foil to the uppermost surface (see image below).

I sat the emonTX on one (instead of the sheet of foil I tried yesterday), and also sat the Raspberry Pi base station on the other (no earth connection) and immediately saw yet further signal strength improvements, despite both units remaining in exactly same locations.
Measuring over a 2hr period, I now have;

Mean Min Max Diff Dev npoints
-66.1 -72.0 -54.0 18.0 3.0 720

Which seems a big difference from the -81.8db mean average, which I had 2 days earlier.

Any particular way of attaching it to the PCB?

No, just place the Pi on top of the disk so the antenna is centered in the circle - there are no electrical connections to the foil.

NOTE; I made it 165mm diameter because I understand that it’s radius should equal the antenna length. Which in my case it was 82mm (868MHz).
I assume that it would need to be 330mm diameter for a 433MHz system which has a 165mm antenna.

I wonder if sitting it on a metal shelf would achieve the same outcome?
I think I’ll give that a try when I get home tonight!

It should do.

@Bill.Thomson contacted me earlier today, and sent me the below video that he took several weeks ago regarding the ground plane effect.

To set the scene, Bill’s emonTx is located about 75 feet from where the emonBase in the video clip is, and there are three walls between the two. Watch what happens to the RFM12 LED when he removes & replaces the aluminium foil from the emonTX…


I was thinking more of the connection to the PCB for the antenna. Straight down or a slight bend on the bottom of the wire and if so, does the bent part get included in the 82mm length?

I also wonder about putting a hole in the ground plane and putting the antenna through the hole and sitting it on the case.

The wire is completely straight & vertical. Distance from PCB to tip of antenna = 82mm (for 868MHz)

No, the complete antenna must be above the ground plane.


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Ah, and for 433MHz?

Ahh, you mean for the emonTX.

My emonTX is in a bespoke case, which enables me to have the antenna vertical, however with a emonTX in a openenergymonitor case, it’s problematic to use a ground plane, unless you drill a small hole in the case, pass the antenna through, and lay the emonTX flat.

I think that I gave details for the 433MHz antenna in a post above.