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. Nik:

    So I would be better off using my 5.8 dbi with LMR600? Hate to think of the cost of using LMR900.

    Thanks for the help. Trying to get it correct the first time so I’m not having to pay someone to run up and down a tower to change out antennas.

    Thanks for the reply.

  2. Hi Darren, I’d run the calcs to see what your end output will be, then make the call from there re. antenna gain & cable type/loss.

  3. Hi Nik,

    I recently set up a Bobcat Miner and my SNR seems quite high. I failed to witness another miner ~3.3 km away but I am able to witness one ~15 km away. Funnily enough, (not really though) as I was writing this I failed another witness to that same miner. From what I have read, the invalids would be the result of the “anti-gaming” put into effect on the Helium Network since it falls outside of the “acceptable range” for RSSI and SNR.

    I am completely new to RF and though I have been reading, I am uncertain of the next steps to take.

    My first thought was to increase the noise somehow so that it would lower my SNR to an acceptable range. Is this possible with an attenuator maybe?

    Second idea was to increase the signal strength but I already have the antenna mounted from my chimney running through LMR 400 (53 ft) to lightning arrestor to LMR 400 (10 ft) to miner and am not sure putting it higher would help considering the factor may be on the other miner’s end.

    Any input is greatly appreciated.

    Thank you.

    Invaild
    Distance~3.3 km
    DatarateSF9BW125
    RSSI-94dBm
    SNR11.5dB
    Frequency903.9 MHz

    Valid
    Distance~15 km
    DatarateSF9BW125
    RSSI-113dBm
    SNR-2.8dB
    Frequency904.7 MHz

    2nd Invalid
    Distance~3.3 km
    DatarateSF9BW125
    RSSI-96dBm
    SNR8.8dB
    Frequency904.9 MHz

  4. Nik,

    I consistently have Invalid Witnesses with a few miners around me, when I witness them or they witness me. When this happens the SNR is usually a positive value but never larger than 9db, most of the time its SNR 2-7db range but other times they are valid. How can this be corrected? (Examples below)

    VALID (my Miner):
    High Midnight Bobcat
    Distance
    ~2.8 km
    Datarate
    SF9BW125
    RSSI
    -101dBm
    SNR
    5.8dB
    Frequency
    904.9 MHz

    INVALID
    Colossal Fleece Dove
    Distance
    ~2.8 km
    Datarate
    SF9BW125
    RSSI
    -97dBm
    SNR
    9dB
    Frequency
    904.9 MHz

  5. Hi Jim, valid/invalid depends not only on your setup but the setup (and really, gain) of those miners around you. If they have a higher gain antenna (and you do as well), there’s not much that’ll help until PoCv11 comes out.

  6. In the “lobe” image its clear that even an omni directional antennae has a direction. How do to tell where that direction is? The reason I ask is I’m northeast of a large city. 99% of the miners are southwest or West of me. To the east there’s nothing. So it would make sense to align the patter to face southwest. I have an 8dbi antenna with lighting arrestor (-.5 dbi) and 50′ of rfc400 cable (-2.1 dbi) at about 35′ feet high. Line of sight is good. I get valid witnesses out to 50km. I’m thinking the lobe direction is currently facing southeast based on my pattern of witness but I don’t know how to officially tell.

  7. Hi Gary,
    The lobes in that image are for a sector (directional) antenna. Every antenna you purchase *should* have a diagram showing you the radiation pattern from a vertical and horizontal perspective. Omnidirectional antennas do have dead spots, but they’re basically not worth paying attention to for Helium deployments. An omni will “see” in all directions.

  8. Pingback: Helium Mining - Definitive Guide to Earning Optimum HNT

  9. hi Nik. I have rac v2 miner with 6dbi antenaa, but cable neth is 90 feet (LMR400) is it normal confg?
    Thank you

  10. That’s a pretty long cable, but with a 6 dBi antenna shouldn’t be an issue (depending on where you are. US no problem, Europe you’ll want to upgrade/shorten the cable.)

    Make sure you enter the loss correctly there.

  11. Thanks to this and all the rest of your article I think I have a pretty good grasp on which antenna and cable to use. Thanks!
    The one thing I’m wondering. I currently have a 10 ft. Lmr-200 cable connected to a 5.8dbi outdoor antenna. @5m height. My current further witness is 18km. If I change out for 10ft. Of lmr-400 there would be less db loss and assuming Los I should be able to get more range? I’m not quite sure what less loss is actually accomplishing

  12. I think this question is answered… It want to be sure.

    I have an hntenna 3dbi outdoor connected with 20 ft of lmr240. Looks like loss is 1.5. Should I include the loss in the Helium app to account for the loss?

  13. Does cable loss change the focus of a higher dB antenna? i.e. Would a 9dB antenna that would have a flat focused plane be changed into a wider focused plane (as in your diagram on antenna gain) if it had a 4 dB loss form a 100′ run of LMR400? Or is that not how it works?

    I am in FL with VERY flat topography (including buildings…it’s essentially wide open space above the trees), but am under the tree canopy. I have the fun but difficult task of trying to position an antenna above the canopy where it should be able to “see” for miles. By my current understanding, I feel I need something like a 7 dB antenna up on 100′ of LMR400 above the canopy, but am curious if I’ll have a flat plane shooting over everyone.

    Thanks for all the awesome info!

  14. New Bobcat arriving in a few days. Located in between Phoenix & Tucson, just a few miners nearby, but have option to also place closer to City as needed; can you please email me with information regarding consultation for my setup & strategy?

  15. HI buddy, how would you go about reducing the transmit gain? If you found u are running higher then you should and setup in the helium app to a 15dbi antenna and still was too high what can you put in the line to reduce the transmit gain?

  16. Hi,
    I’ve read through the article and embarrassed to say still stumped… I’ve got a 5.8dbi antenna on a 10m long LMR400 cable with a lightning arrestor between the aerial and cable. Is the figure I put into the helium app simply 5.8 less the loss in the cable which I think = 4.4dbi

    Any help would be really appreciated thank you!

    All the best

    Az

  17. Yep, bang on. [Antenna gain] – [cable loss] – [insertion loss from lightning arrestor] = Asserted gain in app.

  18. I live in the suburbs on a hill and have an antenna on top of my roof about 40~ feet above ground. I am running 40ft of LMR400, which comes down to about 1.57db of loss. I used to run a 5.8db antenna when I had near 0 loss. Would it be more beneficial to swap to a 8dbi antenna due to the loss introduced with the new cable?

  19. Probably not, I’d stick with the 5.8. Focus on keeping the antenna up high and you’ll be getting the most benefit.

  20. hi Nik, how are you – great post!

    I am trying to figure out my best setup. I have helium miner (bobcat 300) with the stock antenna on my roof. my house is prob 180m above sea level +10 meters for the antenna. most of the antennas i want to hear /see are below me or very far away (closest is 4km) . I actually think that I have to gain from a high gain antenna (8dbi is prob the sweet spot + 20 feet of cable + a lighting protector.)

    https://explorer.helium.com/hotspots/115s2Kxkwc5QMnBS3qLW2Bz7CBmwGRt7nFB9YRqQAiQ7C8pKUBP/activity

    does a high gain 8 or 10 sound too much? or a 6.5 better?

  21. my fav part of this article is how GK says ‘i’m showing you how to do the math yourself!’ and all the comments are like ‘what about my setup, it’s like this:’ Thanks for the info, way to always make it digestible

  22. @nik ty very much – appreciate all your “guardian angel like approach” – i went with 8 to try and reach the remote islands 🙂

    @tanner you are right 🙂 but as to myself and in my defense there are components implied in the distance and height which make the decision a little more “experience” based vs math based. I liked this calculator also https://www.pasternack.com/t-calculator-antenna-downtilt.aspx – it helps understand how the height and distance play into it as well as the vertical lobe degrees

    Alex

  23. I messed up and ordered a LMR600 cable with the wrong connector. Will using a female to female converter result in a big lose? Is it worth replacing the cable or would it not make much difference?

  24. Depends on the connector quality; anything you use should list its “insertion loss”. Probably not a huge deal, though if you’re running LMR600 you probably have a reason to avoid loss. 😉

  25. Hey NIK, is it worth runing 25 metter cable in order to climb antenna to roof of my 45metter building and completly open the view in full 360 circle? Or should I rather have around 150 degree open view and stay much lower but run short cable. Also which cable would you suggest for such a long setup? Is LDF 7/8 okay or should I stick to LMR – 600 or something else? Thank you for the answer in advance.

  26. I’d say getting it high offsets any cable loss issues. Calculate cable loss for your length & frequency and decide on your cable choice from there.

  27. I already did, but I am not sure how much loss is a red flag? Is 2 db loss acceptable for 8-9 dbi antenna? I am from Europe btw.
    I saw that everyone just talking about LMR-400 as a standard and LMR-600 for longer set ups. So not sure if LDF cables are okay for hnt mining usage because no one is mentioning them even with the fact they got lower db lost.

  28. Hey Nik – Thanks for this awesome post – When you change the gain in Helium, this will only affect the Tx, not the Rx, correct? Say you have a 15dB antenna, if you set it to 15 in Helium, the Tx will be brought down to 36 (USA) while the Rx will remain at 42. Am I understanding this right?

  29. Haha thanks! I’m not actually using a 15dBi – I was just using it as an exaggerated example. I run a 10dBi antenna and I’m planning to install a 2-way amplifier with a SAW filter on it and I want to make sure I’m in the legal range for my Tx without losing any Rx. Thanks again for all of your work in teaching us everything you know!

    Actually while I have you on the line – do you know if the Bobcat puts out 25dBi or 27dBi? I know the max for the US is 27 but most of what I can find in forums says Bobcat puts out 25.

  30. HI Nik,

    First what an amazing article, I’ve read hundreds and by far this is the most comprehensive.

    I’ve just setup my Bobcat Miner, live in Scotland in U.K. However, I messed up with initial setup, I got conned on some cable.

    So my setup is bobcat 300, McGill 6DBI tuned antenna, I’m 63M above sea level and my antenna is 13m high (top of house). However, I have 15 metres of RG58 cable (I know right). I calculated the loss it’s over 6dbi so I’m not sure why I am seeing witnesses 24km away. I’m generating around .4HNT per day but only 5 witnesses/2 becons a day….. In 3 days time I am replacing the cable to 10 Metres LMR 600 (£100 just for the cable), once I setup I’m going to update Helium app with a .4dbi loss is that correct? So new antenna will be 5.6DBI. I’m more keen to see what impact it will have with the new cable? What’s your thoughts? Hopefully I’ll have more witnesses.

    I’ll update you on the results hopefully the upgrade of the new cable will pay for itself and show the results.

  31. Right on Ben. You may have more witnesses if the extra signal gets you through a few more walls/trees. Depends on how many Hotspots are around you. .4 HNT/day right now is about 4x global average. 🙂

  32. Hi nik, How to calculate db , dbi, Eire for miner using 860 to 870:mhz…pls advice which antenna should I use.

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