Antenna advice


My current setup is a RAK2245 Pi Hat and I bought a 5.8dBi outdoor antenna I put about 8 meters up above the ground off the back of a shed.

Do I need to reduce the output power level for it to be legal? I read somewhere that when using a high gain antenna, it can be necessary to reduce output power to keep things legal. I’m not sure if 5.8dBi gain is considered high gain, I don’t know much about antennas yet. I’m in Australia and the frequency plan I’m using is AU915.

Thanks for the help. :slight_smile:

What is the maximum permitted transmit power in dBm in that frequency band in Australia ?

Often quoted as maximum Effective Radiated Power, ERP.

From what I’ve read it’s 30dBm.

Is that directly attached to the GW or is that running ~8m of feeder cable? If cable which type and what loss in db/m? What insertion losses for any associated connectors or (heaven forbid!) joints en route? It may be that the additional 2.8dbi meerly serves to offset/compensate for the connector and cable losses - need details to judge.


Hopefully I get this correct:

RAK2245 → u.fl to SMA Female (5 Centimeters) → SMA Male to N-Male (2 Meters) → Antenna

The small 5cm u.fl to SMA adapter I don’t have any data on and the 2 meter SMA to N is advertised as RG316D type cable and I found a datasheet here

That’s all the information I have. I know you obviously can’t provide exact figures if you don’t have exact part numbers and datasheets, etc. I’m just trying to see if there’s a high possibility I’m transmitting beyond legal limits despite pretty much everyone buying kits, cables and adapters off eBay and running their own gateways without knowing anyway.

1 Like

From what source? Some vendors quote numbers that are totally unrealistic. That of course doesn’t happen with reputable vendors like Mouser, Digikey and the like but when buying on Ali, Amazon and like sources you should often take the numbers with a grain (or spoonful) of salt.

I got the antenna from eBay here.

I’m only going off the information attached to the antenna and the listing, I have no datasheet so I’m only assuming 5.8dBi.

I’m far from an expert in radio communications but assuming the antenna does have a 5.8dBi gain and there’s no loss in the 2 meters of coaxial to the antenna (or if there’s a ballpark loss you could safely estimate) how would I go about finding out how far above the 30dBm limit I am?

I’m just trying to figure out if my setup is 100% legal. There’s probably no way to be sure without expensive testing equipment which I wouldn’t have a hope of affording. Or I could be 99.9% sure if the pathway from u.fl to N-Male + antenna are all branded and have real datasheets but I’m trying to do the best I possibly can with the money I have.

At the end of the day if I have to replace the antenna and get new cabling to it I can do that.

Thanks again :slight_smile:

I dont know how you have the Gateway setup, power etc.

But take an example for the UK, max ERP in the 868Mhz band is 14dBm, Gateways and nodes are configured for this power.

The 14dBm is assumed to be the limit through a 2.1dBi antenna, so using a 5.8dBi antenna increases ERP to;

14 + (5.8-2.1) = 17.7dBm

Thus you would need to reduce TX power in your setups by 3.7dBm.

There will be some cable connector losses so with that antenna, used in the UK, you would probably need to reduce TX power to 12dBm on the Gateway.

For Australia, if the max permitted power really is 30dBm (and you do need to check) and the Gateway is setup for 14dBm as standard (and you need to check this also) you would need a really high gain antenna to be getting close to 30dBm ERP.

IIRC the 2245 family boards allow upto 27dBm output - fine if used with normal 3dBi ant. 5.8dBi may push slightly depending on those losses so perhaps modify the HAL settings to 3, 3, 13, 0 (PAC/DAC/MIX/DIG) to bring down to 25dbm, then even with nominal cable, connector and insertion losses think you should be ok?

Have you reached out to your local TTN AU community or any TTN AU users for help/guidance?

To work that out, you need to be absolutly really certain;

What the ERP limit is for Australia in the 915Mhz band.

What the Gateway config has set for the transmit power.

Thanks for all the advice,

The ERP limit is 30dBm according to multiple sources. I’m thinking now I might just get a 3dBi antenna. That way with the max output of the RAK2245 being 27dBm + the antenna and taking away loss in the cable I should be fine.

I’ll login to the gateway later today and try to confirm if the output is at the maximum of 27dBm.

With a 3dBi antenna would it be worth fussing over the loss in the cable? I’m trying to get maximum distance but the terrain is very up and down.

Also I’ll check the communities out too. :slight_smile:

Thanks :slight_smile:

Don’t care abt 1 or. 2 dB plus or minus. Much more important is the position of your antenna. It should be outside on a mast - as high and as “free” as possible. The antenna should be able to “see” your nodes.


This is how I have it currently set up, a couple meters above the top of a shed. It’s as high as I can get it at the moment. I might try raise it a few more meters down the track.

What do you think?

Trees (especially when in leaf) will shield transmissions at 915 MHz quite effectively and reduce the range in that direction.

1 Like

Looks good, every meter of high will increase the coverage. You don’t need a high gain antenna, 5dBi or even less is sufficient.
If you can get a datasheet of the antenna, look at the radiation diagram. It is not senseful to radiate up into the sky, your nodes are on the ground. If you use a high gain antenna, there is a chance that it might radiate over the nodes.
btw: a forest has an attenuation of abt. 10dB/100m at 868 MHz and a RG174 coaxial cable abt 1dB/m.

1 Like

In my experience a single tree has already significant impact.

The only truth is mapping - but leaves and rain are a thing.

But get going first, don’t try to overthink just starting.

1 Like

A well placed pies of wire cut at 1/4λ is worth more than a badly placed 5.8 dBi antenna, that goes for both gateway and node.

1 Like

Do you come from Wigan? They do love their :pie::pie::pie:’s in Wigan.

Don’t forget the 0.95 multiplier for the speed down a wire rather than free space.

Hillbillies can get it done as well :grinning: :grinning: :smiley: