Help finding a 915-928Mhz antenna for Davis Enviromonitor Nodes

Hey Everyone, first post, be gentle. I’m looking forward to being part of this forum.

I’m a farmer in Nth QLD with a passion for AgTech, I have fully automated our irrigation, our Job management, GPS track our workers, automate our timesheets etc etc, but that not what today is about.

I have a bunch of Davis EnviroMonitor Nodes all over the farm connected to things like soil moisture probes, pressure sensors etc, and I’m wanting to boost the radio reception a bit by adding an external antenna. These are the Nodes:

This the Antenna datasheet on the antennas supplied by Davis:

I have used the big 8dbi antenna on my shed roof in the center of the farm that is on the receiving side (that goes to what’s called and IP Gateway that is connected over ethernet to the internet) it is on a 3mtr high tripod ontop of a 12mtr high shed roof.

The nodes all over the farm create a mesh network but each node has a small internal antenna and I’m finding that they can’t quite penetrate through all the trees (its an orchard)

I know that if I were put the nodes high on a post above the trees and each node would have line of sight it would solve my problem but extending the sensor leads on every one of the 4 x sensors per 8 x nodes, plus the cost of the posts and they fact that Hawks/Whistling Kites (birds) will attack them above the trees makes this option prohibitive. Also the trees are mechanically pruned every year so they would need to be taken down every year.

So, that (admittedly) long story brings me to my question. Does anyone have a recommendation for a little 915-928Mhz antenna I could use? Speaking with Liam on the support chat the Pycom one here looks close to what I need.

For $20 I’m happy to try it out and see if it helps at all.

The final requirement is, I’d like to find one that is terminated with a small length of cable/lead with the SMA connector on the end (approx 10-15cm). This way I would not have to drill a hole in the Node casing, compromising the integrity of the IP rated case as the nodes have a hard foam sandwich at the base for the sensor leads to maintain the waterproof rating.

Anyone have any ideas?


Hi Mathew
Improving radio performance where trees are a problem is always a bit difficult to predict. Using an antenna with a bit go gain you might get some improvement but this would be very much a suck it and see operation. The only real way really is to get above the trees but you say this is way down the list of options. The whip antennas you are contemplating are omni directional. You only need to transmit one direction so a directional antenna like a 900MHz Yagi type might be worth a try. You only need to try one to see how it goes.

No matter what, there is one important thing to watch. The antenna and transmitter have to be mounted reasonably together. If your cabe length is more than a few metres at 900MHz you can lose more in the cable than you gain elsewhere. So if your antenna has to go on a pole the transmitter would have to go with it and extend your monitor connections. The cable loss is physics and there is no way around this. For instance attenuation for RG58 C/U cable is quoted between 18db to 20db per 100ft (30M) depending somewhat on manufacturer. I note Element 14 have a Yagi for cellphone frequencies multi band 900 to 1800 MHZ supplied with 10M of RG58 fitted with SMA connector. The Antenna quoted gain is 11db but you lose over 6db of that in the cable. There are other factors here that may make this loss worthwhile. Anyway this is just one thing to consider in your case.

How far from base are your transmitters. Are any over the horizon.
Cheers Bob

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Thanks Bob, I did not know about loss over the cable length, so thanks for pointing that out. I had began to think about running only the antenna up over the trees rather then the whole node up the pole but after what you have said it sounds like it wouldn’t have worked anyway.

Sticking on this topic for a minute, the 8dbi big fibreglass antenna I put on the shed roof only had about 10cm of cable, so I bought a 10 mtr extension lead from jaycar (SMA connectors both ends) so I could connect the 10cm of antenna lead down the pole into the shed and to the office where the gateway/base station is. The extension lead is probably about 3mtrs too long (it was all they had in stock) so that 3 mtrs is just rolled up and sitting on the roof. Am I losing effectiveness of that antenna by having an extension lead that is too long and 1/3rd of it rolled up? maybe if I work on getting the base station just under the roof and a shorter run of antenna cable to the station it might actually solve all my problems? I have 240v power high in the shed (for ceiling fans) and ethernet cable is cheap to run back to the office, in fact, the base station can use wifi.

Back to the antenna for the nodes, I intend for these new antenna to be attached to the node, so they will only need no more then 10cm of lead from the antenna to the plug inside the node for the antenna.

The nodes them selves (the ones struggling) are at a maximum of 600mtrs from the base station (big antenna on the shed) and none of them are over the horizon, there is though 100’s of trees in the way.

I have found an old wide band antenna for 2g/3g/4g in my ‘bits box’ so i’m going to try and put that on one of the nodes and see if it helps (The davis installation app allows me to see the RSSI, RTX and Rank) Not that I know what any of it means, but it has a nice red/yellow/green line for each indicating a weak/strong signal.

Thanks Bob

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Hi Mathew

Maybe not. Even if you lose all the gain of the antenna in the cable the better position may offset this loss and show an improvement. Only measurement would tell. Keep in mind you will lose about 6db (3/4) or so of your transmitted power in the cable for 10 Metres so I don’t think you would want to go longer than that.

YES. You are losing at least 3/4 of your received power in the cable, 6 or 7db, could be more depending on the structure and quality of the cable.

It probably will. I would do this first before anything else. You need to do this anyway as if that cable is not you beut first quality low loss cable your losses could be anything. A short cable should be OK. Try to keep it below a couple of metres.

I mention horizon because of the frequency involved. 900 to 1000MHz is the range used for Troposcatter communications. Telstra use such systems for remote area telephone and associated comms. If you were trying to sneak over the horizon you may have got some funny results due to some unwanted secondary scatter signals going in and out of phase with the primary signal.

If one of the operating points is about 900MHz it can’t do any harm and point you in the right direction. I would still do something about that long base antenna cable though.

RSSI = Received Signal Strength Indicator.
RTX = I have no idea
Rank = Probably what your TV calls signal quality. Based on the BER (Bit Error Rate) and probably just as if not more important as actual signal strength.
Let us know how you get on.
Cheers Bob
PS. Be a bit careful with what you connect to the transmitter nodes. Make sure any antenna suits the frequency in use as a badly matched antenna could possibly damage the transmitter. If in doubt go for the ones sold by the manufacturer. May be a bit more expensive but could pay off in the long run. They may sell directional ones if that proves to be required.

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