How do you make sure a radio signal is within acceptable limits for Helium? Short version: Assert your antenna gain (including your cable loss) and location accurately and you don’t need to do anything else.
Wait, you want more? Dawg, why didn’t you say so? Let’s dive in!
First, let’s start with why we need an “acceptable” strength. Three words: Proof Of Coverage. We need to be able to prove that we’re actually providing coverage where we claim we are. This is important for two reasons. First, when businesses hear about the world of IoT and Helium, if we can show them a map of where our coverage has actually been proven to exist and at what strength, they can quickly make a decision regarding whether or not they want to use the Helium Network, and if they need to add a new Hotspot to provide coverage.
Second, we need to prove coverage in an accurate way in order to combat gaming, aka cheating. Understanding how this works requires a little bit of radio theory, but relax, I’ll walk ya through it.
As you read this and the example I give at the end, you’ll come to the understanding that as of now, March 2022, the RSSI limits are gobsmackingly lax. This will change. For now, just bookmark that idea as “work in progress.” Onward!
Let’s start with that signal strength, or RSSI. RSSI stands for Received Signal Strength Indicator, and, as its name indicates, is a measurement of the received signal strength. In order to know what the received signal strength *should* be, we need to know a few things. Those are broken into 3 sections.
1. Beaconer Information
- Beaconer’s transmit power. In the US, most of our hotspots transmit at 27 dBm. Your region may be different.
- Beaconer’s cable loss. (in the Helium app, this is included in the antenna gain section) This is a function of cable efficiency and length. 100′ of LMR400 at 915 MHz will lose 2.2 dB.
- Beaconer’s reported antenna gain.
Knowing the Beaconer’s transmit power comes from knowing what region of the world the Hotspot is in, and what the legal limit is for power output. For example, US915 can blast at 27 dBm while EU868 is limited to 16 dBm.
2. Distance & Region
Over any given distance a radio signal will lose a theoretical amount of strength. The actual amount can change, sometimes drastically, depending on environmental characteristics like vegetation or building obstacles, and to a lessor extent from humidity, rain, snow, sleet, and pollution. The theoretical amount is called FSPL, or Free Space Path Loss. This is the loss in signal strength in “normal” clear air.
3. Receiving (Witness) Information
- Witness’ reported antenna gain
- Witness’ reported cable loss (in the Helium app, cable loss is included in the antenna gain section)
- Witness reported RSSI (at what strength did they “hear” the signal?)
How does that look? Lemme draw ya a picture, and yes, I’ll make up a few numbers.
It starts off with the Hotspot’s Beacon output, loses power along the cable, the antenna shapes and focuses the energy out into space, where energy is lost, the receiving antenna picks it up according to its gain, energy is lost again as it goes down the cable and is finally received in the Witnessing Hotspot.
The -81 RSSI dBm that the receiving Hotspot (aka the Witness) reports is then compared against what it should have received at, given how far away it’s asserted from the Beaconing Hotspot as well as the output power for that region.
Now, there’s a problem, because there’s the theory, then there’s the real world, and then there’s our interpretation of what should “count” in the real world. Remember that term “gobsmackingly lax” I used above? Here’s where you start to understand it.
You see, as much as we’d like to think that we humans can accurately assess and calculate the world around us, we’re not always as accurate as we’d like to be. The actual path loss over 20 km may be much higher if there’s vegetation in the way (lookin’ at you, Florida) or buildings (Hi New York!). Since the radio models Helium uses don’t yet take vegetation or buildings or other obstacles into account, the actual results can be much weaker than predictions, sometimes by large margins.
How does it work? Let’s take an example. Here’s Amateur Jade Hare with a 3 dBi HNTenna Witnessing a Beacon from Amusing Eggshell Mongoose, which has an asserted antenna gain of 5.8 dBi and is 40 km away.
Ok, so that’s cool, but where did I get the info about Amusing Eggshell Mongoose? Right off Explorer.
That could be a 5.8 RAK, or it could be a 6 dBi from McGill with .2 of cable loss. By the way, you can add or subtract all this “dBi, dB, and dBm” stuff interchangeably without worrying about the differences for now. Radio geeks will bristle at that statement, by the way. I guarantee you I’ll get at least one snide comment about how you can’t possibly do that and live. Whatevs.
What about the FSPL? I headed over to EverythingRF’s calculator for that, here’s what I got:
I’ll call that 124 to make the math easy. So now we’ve got everything we need.
- Beaconing Hotspot Output (US915) = 27 dBm
- Cable Loss = .-2 dB
- Beaconing Hotspot Antenna Gain/Cable Loss = 6 dBi
- FSPL = ~124 dB
- Receiving Antenna Gain (including cable loss, in this case almost nothing because it’s 4′ of LMR400) = 3 dBi
So what SHOULD the reported RSSI be?
27dBm – .2 + 6 – 124 + 3 = -88.2 dBm RSSI
What was the reported RSSI? -108 dBm!
That’s almost a 20 dB difference, and it still cleared the line! What does that tell you? It tells ME that it is bloody difficult to correctly assess location based solely on RSSI and current radio modeling. I know for a fact that AJH is where it says it is; it’s my Hotspot. I don’t know about Amusing Eggshell Mongoose, but if it’s a gaming hotspot it’s doing a terrible job of earning. My guess is that AEM is where it says it is.
Where does this leave us? With a thornier problem then when we started, as we now realize that gaming through attenuation is significantly easier than you might have thought, since the leeway is so great. For now, at least you know how to check your RSSI values. If you want to see the equations Helium is using, check their github, starting here. Yeah, it ain’t easy to read.
The solution is for Helium to start ramping up the fspl_factor chain variable, which will clamp down on allowable RSSI and make it more of a real control. Before you start screaming about what Helium should do and how fast they should do it, keep in mind that they are managing an enormous and complex system, and gaming is (these are MY words, not theirs) a relatively small problem compared to keeping the blockchain running.
The great news is that if you’re not an RF geek, none of this matters. Assert your correct antenna gain including cable loss, and focus on what’s actually important, which is WHERE you put your hotspot. Need help with that? Take my Helium Basic Course or the HeliumVision Master Class; I built ’em to help you understand the things that actually matter in Helium.
Rock on!
p.s. Giant thanks to Jeremy Cooper for his help explaining this to me and fact checking my usual hasty assumptions. All mistakes are mine, all righteous accuracy is his. If you want to get an idea of the experience and skill being thrown at this problem, I strongly recommend you check out the interview we did on YouTube.
Additional Ultra Geeky Thoughts from @Jerm
Although most US hotspots can and do use a conducted power of 27 dBm when transmitting, a few things can make it different:
- Manufacturer uses a radio that can’t output 27 dBm.
- The blockchain has asked the radio to transmit at a lower power due to EIRP limits, but the radio can’t do that specific power, so it chooses the next lower level. Example: A 10 dBi antenna in the US would require a +26 dBm transmit. If the card can’t do +26 dBm it might do +25 dBm.
When either of these occur, the actual transmitted power the hotspot used is reported on the blockchain. This means that the average user just relying on Explorer may not see it, but the precise data is entered (and used) by the blockchain.
Thank you for the great analysis! I’m in the market for a new antenna, so this was incredibly helpful.
Hi Nathan
I am going to setup around 10 SenseCap in my town, have possibilty istall them around 20 meter high , all of them ( that the plan) with clear view, and all see each other. As they not yet arrived , i am studying as much us i can…Here is my question. Plan is to set them up , each one 1km each other with 3dbi anthena.
Doing a math and RSSI is around 67
SenseCap 14 dB – 1db (cable loss) + 3 (antenna g) – 85 dB ( fspl ) + 3 (antenna from other side) – 1 (cable loss) = 67 dBm RSSI
Looking at other simillar setup RSSI most of them around 120 – https://explorer.helium.com/hotspots/112eouVv2fFtMQyM3WKCGZ2EmXDebHNV3F9ES1PemN1MTG3K3PLh/activity
Am i doing something wrong , is that RSSI will cause witness_rssi_to_high ?
Greetings from Poland, amazing webpage!
Hi Pawel, I think you’ll be fine, keep me posted on how it goes! ~Nik
“By the way, you can add or subtract all this “dBi, dB, and dBm” stuff interchangeably without worrying about the differences for now.”
Subtracting pears from apples the calculations will always be wrong.
I don’t believe so in this case, but help me understand.
For example, if I have a miner putting out 27 dBm hooked to cable that has a loss of 2 dB connected to an antenna that has a gain of 5 dBi, then the “antenna gain” I enter into the Helium app to let the Network know what to expect will be 27 – 2 + 5 = 30 dBm.
What am I missing?
Nik, my understanding is that in your case, you enter 3db (5-2) in the Helium app setup, along with your antenna height. They know your power output.
Help me understand how Amateur Jade Hare can beacon and witness like this at 3dbi? I am guessing amplifier and attenuation by not reporting actual antenna dbi?
It’s a function of the location. It has a tremendously good line of sight. There’s not more than that to it; no fancy amplifier or attenuation etc. It’s just a 3 dBi HNTenna placed at about 6′ high on top of a 3,000 mtn.
Thank you for responding. I appreciate what you do.
Hi there. Very good content. In the calculations of the article I think that you are taking into account the antenna gains twice. The FSPL calculator is already using them, so no need to sum them again. Or you could just set them to 0 in the calculator. The theoretical RSSI of your example should be -88. Tell me if I am wrong. Regards,
Nice catch, that makes sense. Let me double check with my radio geek homies to make sure, then I’ll update this. Thanks Jonny!
Hi, very interesting. I’m wondering if you can help me understand an invalid witness report I got a couple days ago.
I’m up little over 100 meters, and most of the other sites I can see are about 20km away on the other side of San Francisco Bay. I put up an 8 dBi antenna a few days ago, with 15 feet of LMR-400 on a Bobcat 300, rated loss through the cable of 0.7dB, so I’m reporting 7.3dBi. For the most part it’s working well, but there are a couple of invalid witness reports that don’t make sense to me. Example: the other station is a Cal-Chip Connected Devices something-or-other, reporting its location as 33km away, reported as 30m up (probably a mistake for “above ground”, but close to correct for ASL height), Explorer shows ~33km distance (measured 33.39km), with a 5.8dBi antenna. I get an FSPL of 108.94434015dB from the everythingrf.com calculator. Assuming 27dBm and zero cable loss for the transmitter, their EIRP would be 32.8dBm, and 32.8 – 108.94434015 is -76.14434015. Explorer tells me the transaction had an RSSI of -80dBm, which would make sense if we add some cable loss on the transmit side, and an SNR of 7.8dB. The SNR does seem high, but–maybe we just got lucky, and the noise happened to be low?
edit [Just realized I neglected to add my antenna gain into the final RSSI calculation, which brings it up to -68.84]
Anyway, I don’t understand why this was flagged as invalid. Can you explain? Also, I’m very confused as to the current state of PoCv10, 11, etc., and frustrated at how difficult it seems to be to get definitive, official information. Comments on that?
Thank you!
Apologies for being so chatty. As Jonny noted above, the “FSPL” calculator is not really calculating FSPL, it’s doing path loss, including antenna gains. True theoretical FSPL then would be 122.0521234dB. With that correction, my predicted RSSI comes out to -81.95212337dBm, so very close to the actual value. If the transmitter’s antenna gain is actually 8dBi, and my cable loss is just a bit higher, it’s almost an exact match. In any case, the numbers are reasonable, and both stations are within legal limits for the US.
How much are your services?
Hi Toby, you can find all services & prices via the tabs on the home page.
Hi Nik!
I just got a kidney transplant, your interesting writings will heal me! When I go home I will set up the miners again based on your descriptions! Greetings from Europe, a small country, this is Hungary! Vilmos Opra / no Oprah/
Great informative site – thanks for sharing all this knowledge! A quick question if I may. If I have a 6dBi antenna with a 2dB loss cable is that the same has having a 4dB antenna with zero loss cable (hypothetically obv!)
It is as far as signal strength. It’s not as far as signal pattern. Does that make sense?
Thanks for your reply. Yes it does make sense, that was what I was getting at. I was wondering if mounted on my roof andmy 6.2 dBi antenna with a 30ft LMR400 cable would behave like a 5dBi antenna in all respects! So, my antenna is rated as 6.2dBi and I entered 6.2dBi in the app when I asserted my hotspot as I assumed that was the correct thing to do. Assuming a cable loss of 1.2 dBm should I edit my hotspot to show a 5dBi antenna as that is roughly what it is I guess…?
Yep, exactly right. You’re aiming to let the network know what to expect as far as signal strength.
Cool, thanks again for taking the trouble to answer. I am in the UK and I only setup my first hotspot 10 days ago, I bought a 6.2 dBi antenna a week ago and put it on my roof. My stats are 39 witnessed in three days and 103 beacons 7 day average according to Helium Explorer. I am wondering though if this is likely to be improved by a lower gain antenna as I have fairly good lines of sight for about 15/20km in a 270 degree radius and line of sight for about 7km for the other 90 degrees. The biggest concentration of other hotspots though are in the 270 degree arc about 10km away and a lot of people seem to just plug them in and put them on a windowsill by the looks of the stats of some random ones I have clicked on, so would a lower gain antenna be more likely to reach these? When I say lower gain I mean one of the 3dBi ones. All the diagrams and explanations seem to suggest that a lower gain antenna is better in a block of flats, which is fine, but other hotspots are likely to be only meters away in that scenario. How does a lower gain antenna fair when the lines of sight are good to the next town but it is 15km away..?
Hi James, the only accurate results will be found from testing, but at a guess I wouldn’t expect a huge improvement from changing your antenna; they just don’t make that much of a difference.
Hi to everyone.
I hope you guys have a wonderful day and hopefully someone can help me a bit.
I have 3 antennas. 8 dbi, 6 dbi and 4 dbi. Hights of the antenna is around 20 m high (Lodnon/UK) I have a 5 m cable LMR400 and also 10M cable LMR400. Miner is bobcat 300. What I am wondering is this. With 8 dbi, I get so many invalid witness because they’re too close. With the 6 dbi not so many and with 4 almos none. So my question is why does this 8 dbi have so many invalid witness? Should it not reach those miners that are a bit outside of london? I thought if I use the 4 dbi, it will not leave the central london where I live so I got the 8 dbi to get out of london and witness those that are over 10 km away. My other question is what if I connect two antennas to the same miner. Will that work? Has anyone tried it? The 4 dbi and the 8 dbi. The 6 dbi I have does fine on its own.
Thanks
Hi Tony, there are a few answers that are complex regarding how RF works, but one useful very simple one: Choose either the 4 or the 6 dBi, sell the rest, and work on other more productive aspects of Helium. Optimizing antennas is a fun game for geeks, it’s not really important for the rest of us. :). Let me know if that helps.