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Helium Hotspots & RF Exposure

Are Heli­um Hotspots dan­ger­ous to be around? How much RF ener­gy do they emit? Should you or your hosts be wor­ried about the expo­sure lev­els? Let’s run through the facts, then you can make your own decision. 

We’ll start with this: In gen­er­al, Heli­um Hotspots rarely emit any RF. They’re built to receive sig­nals far bet­ter than trans­mit them. That’s because the whole point of the net­work is to “lis­ten” for real­ly faint sig­nals from sen­sors at long range.

Still, Heli­um Hotspots DO occa­sion­al­ly trans­mit out pow­er in the form of a “bea­con”. Bea­con­ing usu­al­ly hap­pens less than 3 times per day. Some­times you’ll have a ban­ner day and it’ll bea­con 5 times. That’s unusu­al, so I’ll use 3 or less when mak­ing the cal­cu­la­tions below. Just to be clear:

A bea­con is a sin­gle trans­mis­sion wit­nessed by any Hotspot.

https://docs.helium.com/blockchain/proof-of-coverage/

In this case you can ignore the “wit­nessed by a hotspot” aspect. For now we’re focus­ing sole­ly on the pow­er in a bea­con, not whether or not it was received.

You can see how often your hotspot bea­cons just by check­ing on Heli­um Explor­er. Here’s an exam­ple from one of mine on an unusu­al­ly active day:

Yep, 2 bea­cons inside of 5 hours. We are ban­gin’! By the way, for those of you con­cerned with earn­ing HNT, your “slice of the pie” for bea­con­ing is rel­a­tive­ly low, so don’t wor­ry about “not bea­con­ing enough”. Once a day is fine.

Back to RF expo­sure and pow­er! Heli­um uses LoRa as a radio pro­to­col, and what we call a “bea­con” is tech­ni­cal­ly a “chirp”. Let’s dig a lit­tle deep­er on that. From Heli­um’s blog:

LoRa uses what is known as a “chirp” pro­to­col and spread­ing fac­tors (SF) are the dura­tion of the “chirp”. Typ­i­cal­ly you’re look­ing at a range of SF7 to SF12 where the 7 is the short­est time on air. Each step up dou­bles the time on air to trans­mit the same amount of data and increas­es the range. Due to local restric­tions, we are lim­it­ed by max pay­load sizes to cer­tain spread­ing fac­tors, usu­al­ly SF8 and SF9 for most packets.

https://engineering.helium.com/2020/10/02/spreading-factor-changes-poc.html

How long does an SF8 or SF9 (or even an SF 12) chirp last? Let’s take a look at this table from Semtech (the own­er of LoRa tech­nol­o­gy). Here’s a screenshot:

There are 1,000 mil­lisec­onds (ms) in a sec­ond, so for all chirps, we’re look­ing at under a sec­ond of time that RF is being emitted. 

Cool, so now we have a time estab­lished: Less than one sec­ond per bea­con.

The next (obvious) question is: How strong are these beacons?

For those of you who’ve read the Cable Loss & EIRP post, you’ll remem­ber that the most pow­er­ful hotspots (Amer­i­can hotspots) blast out a mas­sive 27 dBm. I’m jok­ing about the mas­sive part. dBm stands for Deci­bel Mil­li­Watts, and 27 is about half a watt. You can do this cal­cu­la­tion your­self over at DigiKey’s web­site. I’ll make it easy and just paste it in here:

I can hear you say it: “Ok Nik, but what about when some­one uses a MAXIMUM GAIN! (said in my best mon­ster truck voice) anten­na? In the US, the legal max anten­na gain we can use is 9 dBi. 

27 dBm + 9 dBi = 36 dBm. So, how many watts is that? Let’s cruise back to DigiKey’s cal­cu­la­tor and see.

Holy smokes, almost 4 watts! That’s enough to fry a… Wait a sec­ond. How much pow­er is 4 watts? Is there any­thing else we might pos­si­bly use for comparison?

Yep, you guessed it. Cell phones. Cell phones in gen­er­al have 2 “lev­els” of pow­er they emit: .6 watts, and 3 watts. Typ­i­cal­ly, most peo­ple use their cell phones while hold­ing them with­in 2 feet of their face. If you have long-ass gib­bon arms, maybe you can get it 3 feet away from your face. You can reduce your expo­sure by grow­ing your arms or just fol­low­ing com­mon sense guide­lines.

When you are talk­ing on a phone is when it emits the most pow­er; it has to trans­mit your voice. At that point, you are hold­ing a device to your head for more than a sec­ond (if watch­ing peo­ple in pub­lic is any indi­ca­tion, it’s more like non-stop) that is emit­ting up to 3 watts.

Let’s go back to the Heli­um Hotspot one last time and just think about where it’s placed. While I rec­om­mend always plac­ing it out­side (NOT for RF expo­sure rea­sons, but to pro­vide the best cov­er­age), some peo­ple can’ t do that due to HOA or oth­er build­ing restric­tions. So let’s assume worst case: In your house.

If it’s in your house, it’s unlike­ly that you hold your Heli­um Hotspot to your head. It’s prob­a­bly on a table or by a win­dow. Let’s say it’s rea­son­able to be at least 3 feet away from the anten­na at all times. We’ll do a quick cal­cu­la­tion check on that dis­tance with the MAXIMUM POWER anten­na to see what you’re being exposed to, using HintLink’s RF Expo­sure Cal­cu­la­tor. By the way, a 9 dBi omni anten­na (the max­i­mum gain you can legal­ly use) is about 4 feet long. Most peo­ple don’t like the way a 4′ long fiber­glass pole looks inside the house. 

Since I’ll assume that you’re not work­ing in a Con­trolled Envi­ron­ment (think RF labs, or near super high RF emit­ters like a cell tow­er site), we’ll cal­cu­late the expo­sure based on the max lim­its you’d encounter in your ordi­nary life. In that case, (ah, fuck it, I’ll make this huge because this is my final answer.)

A Helium Hotspot emits .07% of the Maximum Permissible Exposure to RF devices. It does that for less than a second, less than 4 times per day.

I do know that some peo­ple are super sen­si­tive to RF. If that’s you, it may not be the best idea to par­tic­i­pate in the build­ing the world’s largest wire­less net­work. For the rest of ya, go deep­er if you want, but after look­ing into it, this seems like enough for me to say that the RF being emit­ted from a Heli­um Hotspot is not some­thing I’m going to wor­ry about.

If you need more help with under­stand­ing Heli­um, whether you have anten­na ques­tions, want help with opti­miza­tion, or just want a cus­tomized walk through of the Heli­um ecosys­tem and how you can fit in, con­sid­er hir­ing me.

Rock on!

8 thoughts on “Helium Hotspots <span class="amp">&</span> <span class="caps">RF</span> Exposure”

  1. John Hickmott

    Hi Nik,
    This con­firms my spec­u­la­tion on this mat­ter, espe­cial­ly as regards bea­con­ing, pow­er and lis­ten­ing time. I do won­der about p2p and OTA comms, though, most­ly as relat­ed to all these updates that come from Heli­um and the ven­dors. It sounds like gobs of data going over air rather than the inter­net. A com­ment on that and maybe a link would be appre­ci­at­ed. Thank You!

    John

  2. Thanks Gris­tle, you have been grant­ed PhD lev­el cert in Quan­tum Splaining.

    Tox­ic RF effects from HS is FUD fod­der and seem­ing­ly not a ratio­nal consideration. 

    Sooooooo many oth­er things to wor­ry about. Death by land shark or Dis­cord over­dose lead the list, well behind lethal anten­na instal­la­tion mishaps.

  3. Nik,
    Thank you for shar­ing your knowl­edge of Heli­um. I have found your expla­na­tions very help­ful. How­ev­er, I don’t find the state­ment that min­ers only trans­mit when they are send­ing a bea­con to be true. Per­son­al­ly, I think the RF expo­sure is insignif­i­cant, but I just want to set the record straight.

    Remem­ber, that the point of Heli­um is to cre­ate a world wide LoraWAN net­work so that sen­sors and “things” can con­nect wire­less­ly to the inter­net from any­where. You are cor­rect in stat­ing that min­ers are opti­mized for lis­ten­ing to sen­sors, how­ev­er they can still trans­mit to devices. (I am not the most up to date on Heli­um hard­ware, but I think each min­er can lis­ten to eight sen­sors at a time, but can only talk ot trans­mit to one sen­sor at a time)

    I will pro­vide an exam­ple: There is a GPS vehi­cle track­er avail­able that can also immo­bi­lize the car if it is stolen. It uses the Heli­um net­work to com­mu­ni­cate, as it is sig­nif­i­cant­ly cheap­er than pay­ing for a cell­phone plan for the device. Nor­mal­ly the track­er is send­ing GPS loca­tion data to a min­er, and the min­er relays that data to the inter­net, so the own­er can look at it on a web­site or app. This rep­re­sents a lis­ten or receive case. How­ev­er, if the car is stolen, the own­er can immo­bi­lize the using the inter­net. The min­er then talks or trans­mits RF to the vehich­le track­er sig­nal­ing it to stop the car.

    In con­clu­sion, min­ers do emit RF ener­gy. How often, depends on what peo­ple are will­ing to pay for.

  4. Jacob, total­ly makes sense, thanks for clar­i­fy­ing. Do you have any info regard­ing duration/power and if that will dif­fer from a beacon?

  5. Thank you very much! Very informative.
    So is the max 9dbi because above it is harm­ful? For exam­ple a 15 dbi anten­na and the hotspot will pro­duce 15 W. It still does­n’t look like dan­ger­ous if comes only 5 times a day and each time less than a sec­ond. Right?

  6. 9 dBi is the max in the US due to FCC regs. Due to the way LoRa prop­a­gates, there’s no real sense in using a 15 dBi anten­na for Heli­um Hotspots

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