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 Feet0.50.40.210.10.09
20 Feet2.11.60.80.50.37
50 Feet5.23.92.11.30.9
75 Feet7.75.93.12.01.4
100 Feet10.27.83.92.61.8

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

142 thoughts on “Helium 101: Cable Loss and EIRP.”

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

  18. Nik, thanks for answering my questions above from September 8th and 9th. I ended up using 80 feet of LMR400. Got the antenna on a really nice tripod on top of my roof about 6 feet above my old TV antenna (which aIso got raised up a bit with the bigger tripod). Overall, the antenna is about 35 feet above the ground. When I had an HNTenna indoors behind a curtain, I was getting around $1 to $1.50 a day. When I moved it right next to the glass, that went up to $5 a day. When I moved the HNTenna 3dbi up to 35 feet with the 80 foot LMR400 ,that went up to around $500 to $550 a month (around $16 to $17.50 a day). A few weeks later, I switched to 6 dbi McGill tuned to see if that would do better. It does and I consider it worth it. The 6dbi really reaches out though to other hotspots that are between 10 and 30 km away. That’s really nice. The rewards are up to between $20 and $23 a day. Perhaps the length of cable was worth going up to LMR400 flex but overall, this is a very nice setup for what I was expecting. Thanks again.

  19. Right on, glad that worked out well for ya! Yeah, with those longer cable runs a higher gain antenna can make a difference.

  20. Hi Nik,

    So many of of are amending our setups. I’m in the UK currently running s 5.8dBi antenna with a 10m run of LMR-400, for arguments sake let’s say all my losses are 1dBi.

    You are asked to enter a dBi value into the Helium app, deducting the 1dBi loss from the 5.8dBi antenna and entering this value in doesn’t seem correct to me.

    My understanding is that the antenna dBi value will never change?

    Am I missing something here? As I would still enter the dBi value in as 5.8dBi because cable loss will not change this?

    Cheers

  21. Hi Nik,

    The helium app as asked for a dBi input for our antenna, I’m in the UK and run a 5.8dbi antenna with a 10m LMR-400 cable, for arguments sake let’s say the cable loss/connector loss is 1dB, can you explain to my how this makes 4.8dBi?

    To me this doesn’t make sense as my antenna is a fixed value, no amount of loss is going to change this.

    Is the value in the app ‘dBi’ incorrect for what we are now being advised to workout? (Cable and connector loss?

    Many Thanks,

  22. Well, they’re trying to calculate the output of your system, so I’d claim the loss and enter 4.8 in your case.

  23. Hi Nik,

    Thanks for the response, whilst I get that it’s a simple deduction. The cable loss doesn’t change the end dBi, that is a static number surely, as that is set by your antenna? A 5.8dBi antenna will always be a 5.8dbi antenna?

    Is dBi the correct value being used?

  24. The cable loss does change the dBi. In simple terms, dBi is a measure of the maximum power in a given direction from an antenna. With cable loss, that power will be reduced, since there’s less energy going INTO the antenna in the first place. The antenna radiation pattern won’t change, it’ll just be outputting less power in all directions. Does that make sense?

  25. Presumably there is no value in reporting cable loss if your antenna is at or below the maximum allowed before the miner reduces power to the antenna to get within the legal limit… iirc EU 4dbi and USA 9 dbi ?

  26. Not right now, but come PoCv11 you’ll need to accurately report gain or risk going over the RSSI limits.

  27. First off I want to thank you for making such a comprehensive explanation!! I’m sure it was time consuming and I appreciate all the info as well as you taking the time to answer all our questions!!

    I’m here jn Denver Colorado which by the time my unit has arrived is quite saturated! Was able to find a location with 1.0 reward scale and plan to run a 50ft cable to the roof.

    I have numerous hotspots within 2 miles (maybe 20) and probably 100 within ten miles. And 750 I believe in within 50 miles… (as I said becoming a bit saturated!!!)

    Would you recommend sticking with the 3dbi (as I feel I’m going to hit my Max witness ammt with that) or should I try snd target as many as possible with 5.8 or 8?

    Also 400 on the cable since it’s 50ft? Or is the loss going to be somewhat irrelevant since I don’t need to reach out say 50km, if that’s the case do you think I’m ok with say a 195 to a 240 or would you recommend higher?

    And while I got you (sorry!) since denver is becoming so saturated going to be placing one in the mountains as a bit of an experiment. And some more rural areas as my reward scales being knocked down due to too many units jn the “larger hex” I’ve seen some witness all the way down here to denver from roughly 50-60 miles away (if I were to guess) on that I would assume higher dbi, what would you recommend an 8? And that I want to lose as little as possible so go with a 400 for cable correct or even higher? Last part tk that question that one IDEALLY I’d like to kind of point the direction back towards denver as the other way wouldn’t get line if aight over the continental divide, recommendations there?

    Sorry for the 10 part question! Been a 6 month long wait so I would love to get it right the first time!! Willing to spend the money to do it correctly the first time around but obviously don’t see the need to spend the extra money for say a 400 cord when I can get away with the 195 here in denver. But the main goal is to maximize the gains as with us all I’m sure 😉

    Again thank you so much for taking the time this will be the first out of ten I deploy so ANY information is GREATLY appreciate!! BY FAR the most informative post I’ve seen!! Can’t thank you enough! Keep up the great work it’s greatly appreciated!! Some of this is a bit over my head so thanks for holding my hand lol

    Thanks again! Best of luck.

  28. Hi Kevin, stick with low gain antennas and short runs of antenna cable. A 3 dBi HNTenna is fine, and 20′ or less of LMR400 for that is also fine. As you start to go beyond that you probably want a 5.8 antenna, but it really won’t make much of a difference compared to *where* you put the the antenna & the lines of sight it has. 3 dBi antennas have been routinely hitting 30 km with clear Line of Sight over the hills/mesas in San Diego, and 200+km over water.

  29. Hi Nik, thank you so much for all the useful info you share that helps us to be be better Helium miners.

    I just have 1 question: I see a lot of reference to the cable loss and not as much to the connectors. I’m just wondering, when you add up the cable loss to the connector to the miner, the one to the lightning arrestor and the arrestor to the antenna, shouldn’t you be factoring those in as well or are they so minute as to not make an appreciable difference?

    Thank you

  30. Hi JV, cable connectors do add to the loss and you can include them. Any good connector should come with how much loss it induces, and you can add that into your calcs.

  31. What if it doesn’t include that information, Nik? Is there a general rule that applies or a place with info we can consult?

    Thank you

  32. NIK, thank you for the excellent article. I must admit a lot of the technical terminology went above my head.
    My hotspot is a RAK miner with a 60 foot LMR400 cable that runs all the way to the top of my house. It is attached to a 10 dbi antenna. With all that, I am not even witnessing. BTW, my miner is ‘Acrobatic Wooden Beetle’. Feel free to look at it. Do I have the wrong antenna. How much loss do I have on this cable? Should a find a way to run an electrical line and ethernet cable to the roof and attach my miner up there inside a weather proof box? Sorry for all the questions and feel free to make fun of my ignorance. I just want to get this right. Thanks again.

  33. Check lines of sight to local hotspots. Are you not witnessing *at all* or just not as much as you’d like? The antenna is too hot, for sure, though the cable length will offset that a little. I’d run much shorter antenna cable and a much lower dBi antenna.

  34. Hey NIK. I can confirm that I am not witnessing at on this 7th day into the HNT mining world. My antenna is 3 feet long and is sitting above any obstacles at the top of my chimney. So It should be true omnidirectional and have a 360 degree line of sight.

    What dBi would you recommend for a suburban house where 90% of homes are 1 story sub division homes?

    Again, I cannot thank you ENOUGH for your insight and expertise. TY TY TY!

  35. Hello NIK.

    I can confirm that 7 days into HNT mining, I am not witnessing at all. My 10 dBi is sitting at the top of my chimney with nothing blocking it. It should be a true omnidirectional 360 degree line of sight install.

    For a suburban house with nothing but 1 story subdivision homes, what dBi should I be using?
    What would be the max length cable I could get away with?

    Again, I cannot thank you ENOUGH for your expertise and guidance.

  36. 5.8 dBi will be fine for what you’re doing; 10 dBi is way overkill. Florida is a tough environment for RF; flat, with LOTS of trees.

  37. Hey Nik, first off great info and love your YouTube videos.

    I’m using a 5.8 antenna with a 25 foot lmr 400 cable. Cable loss calculator shows a 1.1 dbi loss. Should I be updating my antenna info to 4.7?

  38. Nik, what do you think about ground planes. I have read somewhere they can helo focus signal. Thanks.

  39. Yep, they can help shape the signal to go where you want it. The Bobcat stock antenna benefits greatly from a ground plane.

  40. Nik, I read about deducting the cable loss and entering the value in the miner. What happens if we do the opposite? Say 8dbi antenna with 20 ft of 400 cable, instead of deducting 0.8dbi, can we add 0.8dbi in the miner (8.8dbi) so the antenna will get 8dbi of power and not 7.2dbi? Thanks

  41. Hi Shakir, your miner will output more power which will translate at the far end into a signal that looks suspiciously strong, and may invalidate your witnesses.

  42. Nik,

    I’m looking at putting an antenna on top of a tower that is about 100’ tall. Then another 20’ to enclosure. So around a total of 120’ of cable. I plan on using a 8 dBi antenna. Can I use LMR 400? By my math I would loose 6 dBi. Or should I go with the LMR 600? I don’t want to put miner and router on the tower because I want to be able to get to those items without paying a company to climb the tower every time I might have to reset, etc.

    Thanks!

  43. I’d use a lower loss cable, which will be a pain in the ass to work with. LMR600 or 900. And probably a lower gain antenna.

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