Helium 101: Cable Loss and EIRP.

Got ques­tions about what kind of cable you should use to con­nect your Heli­um hotspot to your anten­na? Want a Heli­um-spe­cif­ic cable loss table?

You’re not alone! Lots of folks want to know if they should use LMR 240 or 400 or 900, or how long it can be, or if they should use the cable that came with their cheap‑o eBay antenna. 

In order to answer that, I’m going to walk you through how the whole thing works. That way, instead of ask­ing me if your XX anten­na with YY feet of ZZ cable will work, you’ll know how to cal­cu­late the answer. 

First, let’s talk about the pre­cise but con­fus­ing ter­mi­nol­o­gy in the land of RF (Radio Fre­quen­cy). Well, if you want to just skip to the cable loss tables, go here.

You’ll com­mon­ly see dB (deci­bel), dBm (deci­bel mil­li­Watt), and dBi (deci­bel isotrop­ic) thrown around, as well as dBm, EIRP, ERP, FSL, and oth­ers. Sheesh, that’s a lot!

We’ll start with dB, which stands for “Deci­bel”. A deci­bel is the dif­fer­ence between two sig­nal lev­els. RF engi­neers (and the rest of us) use it to add or sub­tract the effect of cables (or oth­er “sys­tem devices”) on sig­nal strength. 

dB are log­a­rith­mic: Every time you add 3 dB of gain, you dou­ble to the sig­nal lev­el. Every time you halve pow­er, you sub­tract 3 dB. That means a 3 dBi anten­na is dou­bling your emit­ted pow­er over a zero gain anten­na! But wait…that can’t be right, can it? Anten­nas don’t “add” energy.

As I’ve cov­ered in oth­er blog posts, anten­nas focus and shape ener­gy. They don’t add ener­gy. It’s kind of like a gar­den hose sprayer with mul­ti­ple spray pat­terns. The water pres­sure going into the noz­zle does­n’t change, but as you switch from “mist” to “show­er” to “stream”, your emit­ted pat­tern changes.

That pat­tern change, and the result­ing focus and range, is mea­sured in dBi (deci­bel isotropic). 

Ok, it’s about to get a lit­tle more con­fus­ing, but I promise you can under­stand this. 

Isotrop­ic” refers to hav­ing the same ener­gy val­ue in all direc­tions. It’s the idea that an anten­na could emit a per­fect­ly shaped “globe” of ener­gy. For var­i­ous rea­sons, it’s impos­si­ble to build an isotrop­ic anten­na. Every anten­na in the real world emits ener­gy in slight­ly uneven patterns. 

Still, RF engi­neers use a per­fect 0 dBi as a ref­er­ence point. As you go up in gain (the dBi goes from 0 to 1 to 3 to…13), the pat­tern becomes less and less globe-like and more focused in a sin­gle direc­tion & plane.

That brings us to EIRP, or Effec­tive Isotrop­ic Radi­at­ed Pow­er. This is a mea­sure of the radi­at­ed pow­er com­ing out of an anten­na in the direc­tion of its largest lobe. What’s a lobe? Let me show you:

Why is that max lobe ener­gy mea­sure of EIRP impor­tant? Because that’s what reg­u­la­to­ry agen­cies (like the FCC) use to mea­sure the pow­er com­ing out of an antenna. 

In the US on the 915 MHz fre­quen­cy we use, the FCC lim­it for EIRP is 36. You get the EIRP by adding the trans­mit­ted pow­er in dBm (what gets fed into the anten­na) to the anten­na gain in dBi.

Trans­mit­ted pow­er is mea­sured in dBm, or deci­bel mil­li­watts. The max trans­mit pow­er we can use in US Heli­um Hotspots is 30 dBm, or 1 watt. For Euros, the max trans­mit pow­er for uplinks is 14 dBm. You can read more on this here, in the LoRa docs. I’m US based, so we’ll stick with the US num­bers for this article.

The trans­mit­ter used in a US Heli­um Hotspot push­es out 27 dBm. The stock anten­na shipped with the orig­i­nal Heli­um Hotspot was a 3 dBi Anten­na gain. 

That gives us a total EIRP of 27 dBm + 3 dBi = 30 dBm

If you’re US based and do the math, you’re now real­iz­ing why a 9 dBi anten­na is the lim­it for your Heli­um Hotspot deploy­ments. 27 dBm + 9 dBi = 36 dBm, or the max allow­able EIRP

But wait, wait, wait, Nik. I thought this post was about cable loss? Why are we talk­ing about anten­nas and gain? Well, when you pass ener­gy through a cable, you lose some of it. Dif­fer­ent cables lose ener­gy at dif­fer­ent rates, usu­al­ly mea­sured as dB per dis­tance. In gen­er­al, thick­er cables lose less, and thin­ner cables lose more.

Cable loss effects EIRP, because remem­ber, EIRP is a mea­sure of what is “fed into the anten­na”. So:

EIRP = Trans­mit­ter pow­er (dBm) — Cable loss (dB) + Anten­na Gain (dBi)

This is why you can have a 9 dBi anten­na with lots of valid wit­ness­es if you have cable loss that brings your EIRP down to what Heli­um con­sid­ers “nor­mal” limits.

Before we get to the cable loss table, let’s cov­er one more term that gets thrown about, which is FSPL, or Free-Space Path Loss. This is the decrease (“atten­u­a­tion” for the RF nerds) in radio sig­nal pow­er over dis­tance. FSPL is impor­tant in Heli­um because it’s one of the fac­tors the blockchain uses to deter­mine if the sig­nal strength of any bea­con is “out of bounds”.

FSPL is used to com­bat gam­ing. If you’ve got 10 hotspots in your clos­et (remem­ber Modesto?), you can “say” they’re deployed in a per­fect grid pat­tern, but using FSPL to cal­cu­late the sig­nal strength that should be report­ed is one way to make sure they’re actu­al­ly, say, 800 meters apart and not all stacked on top of each other.

You can cal­cu­late FSPL here. Here’s what that might look like for 2 hotspots 50 km apart with clear line of sight, both using 3 dBi antennas:

How would you know if that’s with­in limits?

Let’s do this! Remem­ber that your hotspot pushed out 27 dBm. Let’s imag­ine you’re not using any cables, so cable loss is 0.

We’ve already account­ed for the gain using the cal­cu­la­tor, so we just sub­tract the FSPL from the trans­mit­ted pow­er (27 dBm — 119.4 dB) to get ‑92.4 dB.

That’s a sig­nal strength with­in nor­mal lim­its. For Heli­um hotspots in the US, most com­mon sig­nal strengths are (gen­er­al­ly) between ‑90 and ‑122, though it can go high­er or low­er, down to ‑130 in some instances. Now, there are some oth­er mea­sures, both pub­lic and pri­vate, that Heli­um uses to com­bat gam­ing when assess­ing a tx/rx receipt, but these are the basics.

So, with all that as back­ground, here’s your cable loss table. Use your trans­mit­ter pow­er minus the cable loss plus your anten­na gain to get your EIRP, and make sure that num­ber is 36 dBm or below.

You can get as detailed as you want, but I’d rec­om­mend not get­ting too wrapped up about your EIRP to the thou­sandth dBm. 

Length of CableLMR 195LMR 240LMR 400LMR 600LMR 900
5 Feet0.
20 Feet2.
50 Feet5.
75 Feet7.
100 Feet10.

Final­ly, one thing to think about is this: Hav­ing more pow­er come out of your anten­na isn’t always a good thing. An effec­tive way to plan your EIRP is to go after the LOWEST num­ber you think you can get away with, say, 30 or low­er. LoRa is already pret­ty darn capa­ble, so “extend­ing the range” with anten­na gain can be point­less. I mean, I’ve seen a 3 dBi anten­na be wit­nessed 200 km away. That anten­na does a way bet­ter job of hit­ting lots of local hotspots than a high­er gain would, in large part because of the effec­tive pat­tern it has. High­er dBi does­n’t always mean “use­ful longer range” (or greater HNT earn­ings). Just some­thing to think about.

Rock on!

References & Resources

72 thoughts on “Helium 101: Cable Loss and <span class="caps">EIRP</span>.”

  1. Learn­ing a lot from this site — I pur­chased the HNTen­na 3 dbi out­door anten­na that you have rec­om­mend­ed else­where, but will need to run approx­i­mate­ly a 25 foot cable. It seems like this would be approx­i­mate­ly 1 dbi loss per your table (if LMR 400). If I’m under­stand­ing cor­rect­ly, this would bring the pow­er from 30 dbi (27 + 3) down to 29 dbi (27 + 3 — 1) which seems like it would still be ok. Does this sound rea­son­able, or do I need a high­er gain anten­na to off­set? I live in a sub­urb approx­i­mate­ly 15 miles away from a very large city, which is why I was hop­ing to use the low­er gain anten­na. Thanks!

  2. Nik
    Do you have any slight­ly high­er gain anten­nas you rec­om­mend than the 3dbi. If I need to run a long cable like 50ft, a 3dbi just isn’t enough to over­come the line loss. (I think) I have a 3dbi anten­na doing very well right now on a short cable of 10ft but when it was high­er on a 50ft cable it was­n’t doing well at all.

  3. Could you use a RF reflec­tive met­al, and place it next to your Omni-direc­tion­al anten­na to strength­en sig­nal in a spe­cif­ic direction?

  4. Hi Alan, sure, you’re mak­ing a home­made sec­tor anten­na when you do that. It’d work, though maybe not very effi­cient­ly depend­ing on design.

  5. John Wilchynski

    Hel­lo Nic
    I live out in the boonies Flori­da. My near­est hotspot is 32km away. I have three Bob­cat 300 units spread out in the area to form a net­work. We have many tall pine trees all over. What size tow­ers should be used? I have access to 50 foot tv masts. Was think­ing of 50 foot tow­ers with 8dbi anten­nas. Is high­er bet­ter? But then long cable runs. What do you sug­gest? Thank you.

  6. Hi John,
    The high­er the bet­ter in Flori­da; those pines are no joke! An 8 dBi at the end of a 50′ cable should be fine if it’s 50′ up. Very tough envi­ron­ment for LoRa to go long distances.

  7. I have a ques­tion about this state­ment: “That’s a sig­nal strength with­in nor­mal lim­its. For Heli­um hotspots in the US, that valid sig­nal strength is (gen­er­al­ly) between ?90 and ?122, though it can go down to ?130 in some instances.” I’m not sure what this means but if this range is some­thing to strive for, it means you actu­al­ly want more cable loss. I ran thru the cal­cu­la­tors and if my hotspot puts out 27dbm and my free space path loss is fair­ly low — for instance 3dbi anten­nas at both ends and the dis­tance is only between 1 and 3 kilo­me­ters, the loss is 95db (at 3km). 27 ‑95 is ‑68 so if you want to be between ‑90 and ‑122, would­n’t you want to intro­duce more loss with a thin­ner cable? (LMR195 at 100 feet) Even with 10 more db of loss from the cable I’m still only at ‑78 which is not with­in the range. Or maybe I don’t under­stand the com­ment about the range. Can you explain that?

  8. Hey Mike, calcs look good. I’ll pull the word “valid” from the post, as it’s not quite accu­rate. Good catch! I should have said “most com­mon”. If you’re run­ning 2 x 3 dBi anten­nas, you are very unlike­ly to run afoul of any anti-gam­ing rules. While sig­nal strengths are gen­er­al­ly between ‑90 and ‑130, I’ve seen ’em be valid at stronger lev­els, although with cur­rent rules the SNR can be too high and inval­i­date the whole thing. In fact, SNR (at least for PoCv10) is the real prob­lem, and a part of what I believe they’re pulling out in PoCv11.

  9. Nik, thanks for answer­ing so quick­ly. So is all the hype about need­ing real­ly low-loss cables over­stat­ed? In my case, I will need a 75 foot run. I don’t need to hit hotspots 50 km away. How­ev­er, there are a few at between 10km and 15km that I’d like to hit but I have plen­ty that are fair­ly close to me (1 to 5km). Also Vision shows that I have good line of sight for many of these includ­ing the 10km and 15km dis­tant hotspots. So if I have the 75ft run of let’s say LMR195 and have rough­ly 8db of loss and the free space path loss at 15km is 109, then 27 — 8 — 109 = ‑90 which is some­what ide­al based on your num­bers but if it hap­pens to be a lit­tle stronger that won’t be a prob­lem. For instance, right now, my lit­tle 3dbi anten­na (direct con­nect­ed indoors next to a slid­ing glass door) is solid­ly hit­ting a hotspot 8km away with an 8dbi anten­na (on a 30 meter mast). The cal­cu­lat­ed sig­nal strength is ‑71 and I’m get­ting plen­ty more rewards since he got start­ed up. So again, is all the hype about need­ing the least cable just hype? When I men­tioned a 75foot run on one of the Dis­cord chan­nels, I got a lot of peo­ple say­ing I was out of luck unless I got LMR900. Not sure I believe that. Thoughts?

  10. Prob­a­bly a lot more hype than any­thing else. “Need­ing” LMR900 is way overkill. 75′ of LMR195 is on the low side for a 3 dBi anten­na, but if your calcs are putting you well with­in accept­able lev­els you’re prob­a­bly fine. Please keep me post­ed on how it turns out!

  11. Hel­lo, i would lile to ask you about buy­ing rak’s 3 dbi anten­na with 30 ft long lmr 400 cable for my bob­cat. Will it cause too much loss or is 1.65 db loss is too much? And some resources says that bob­cat’s stock anten­na is for indoor/outdoor use but i could­nt find any offi­cial answer. Can i use stock anten­na with some insu­la­tion tape instead of rak? My last ques­tion is can i use 3 dbi anten­na for bob­cat? Because it’s stock one is 4 dbi. So will it decrease my anten­na gain or any­thing else? Thank you very much.

  12. Hel­lo, i would lile to ask you about buy­ing rak’s 3 dbi anten­na with 30 ft long lmr 400 cable for my bob­cat. Will it cause too much loss or is 1.65 db loss is too much? And some resources says that bob­cat’s stock anten­na is for indoor/outdoor use but i could­nt find any offi­cial answer. Can i use stock one instead of rak’s 3 dbi? (The rea­son that i want 3 dbi is the poc11). My last ques­tion is can i use 3 dbi anten­na for bob­cat? Because it’s stock one is 4 dbi. So will it decrease my anten­na gain or any­thing else? Thank you very much.

  13. Check the con­nec­tion loss chart on this page , 30’ of LMR400 isn’t a ton of loss. Bob­cat data sheet here.

    You can use any anten­na you want on a Bob­cat (or any hotspot), 3 dBi is fine.

  14. Pingback: Important Helium Update! How to update Antenna Dbi + location for POCV11 | ($HNT) | Chia (XCH)/Helium (HNT) Explained

  15. Hel­lo.
    Begin­ner here. I was won­der­ing if cable loss affects the sig­nal pat­tern of the antenna?

    Exam­ple: A 5.8 dpi anten­na (out­side) with a cable run­ning 30 feet to a min­er placed inside.

    Would the loss make the sig­nal behave in more a “bub­ble” pat­tern like a 3dbi. Or remain more flat like the orig­i­nal 5.8 rating?

    Maybe a bet­ter exam­ple would be with a 8 dpi antenna.

    - Thank you.

  16. Hi nik

    What is the best dbi anten­na to use on Cyprus (EU) on the city cen­ter of Nicosia
    Could you enlight me please ? 

    Thank you

  17. Thanks for all your excel­lent con­tent Nik. 

    Very much appreciated.

    From a British cousin !

  18. I’d use a low­er gain, 3–6 dBi. What­ev­er you get should be fine. Loca­tion & line of sight is far more important.

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