After about a week of running my Nebra Indoor Helium Hotspot, I began to look into upgrading my antenna. The stock antenna was doing pretty well and was picking up witnesses from 10s of kilometers away and its beacons could be heard by 10-12 other hotspots in my area. I was earning an average of 2.5HNT per day but on examination of the other hotspots in my area, there were beacons that I should have been able to hear given the proximity but was not.
Background
There is a lot of discussion on the Helium forums that debate the best antenna solution. But I have found two articles that detail the best antenna and hotspot placement for the majority of cases. They are a very good reference if you want to upgrade your own setup and are linked below.
To summarize, the amount of HNT you can mine is greatly influenced by your hotspot’s location, with respect to ground topography and the other hotspots that are near you. Antenna selection and placement are just secondary to this as a way of ensuring you can reach those around you. This means you need to select an antenna that fits your topography.
Since my hotspot is currently on top of a hill, I have an ideal topographic placement. I have the high ground as Obi-Wan says! Now I just need to select a configuration that takes advantage of this.
Outdoor Antenna Upgrade
Even though I had my hotspot right next to the window the double pain glass affects the signal reception. This has to do with the attenuation that the 915MHz LongFi signal goes through, from the transmitter to the receiver, which degrades it. The LoRa physical layer protocol that LongFi is built upon is incredibly resistant to attenuation, which is what makes it ideal for IoT and the Helium Network, but it still has its limits. This fact made me look into installing an outdoor antenna, but which one, and how was I going to feed it?
With the help of the above articles, I decided on a lower dBi antenna similar to the default antenna that came with my Nebra Hotspot. Since I am up on a hill I want the have as close to isotropic radiation as possible to reach the hotspots below me. A high gain antenna would compress the radiated power into more of a donut shape which would only be good for flat terrain.
I decided to go with this antenna from Data Alliance since it is supposed to have good matching for the 915MHz band that Helium operates in.
Antenna matching is important because it determines how well the antenna radiates power at the desired frequency. An antenna with poor matching causes reflections in the cable which degrades signal quality and could even damage a powerful transmitter.
Measuring the Antenna
I used a cheap VNA called a NanoVNA V2 to measure the antenna’s frequency response and compared it to the default antenna I received from Nebra.
We are mainly looking at the S11 of the antenna which measures the antenna matching at the stimulus frequency. Here we see that the new antenna matching is much flatter, this is indicative of a lower gain than that of the default, which we wanted to take advantage of the hilly topography.
Feed Line
Now, all to work out is how to feed the antenna outside. I thought I was going to need to drill holes in my rental, which I was not happy about, but I found an ingenious product also from Data Alliance called a flat coax cable. This cable is made of a very thin material that is capable of fitting through a closed window. This eliminates the need to feed a coax line through a wall!
Installation
Now that everything has been sorted out the installation was a sinch! The antenna came with its own mount and the coax fit cleanly through the window.
I took another measurement of the antenna response with the flat feed line in position. It is a bit less controlled compared to the mini-circuits cable I normally test with but that’s the price you pay for convenience!
The Results
The best way to measure if this upgrade was a success is to look at the before and after HNT mining rewards.
From the image, we can see that the weekly HNT rewards have gone up but still fluctuate with time, with a calculated 30 day average of 4.5HNT per day. That’s almost double the default antenna setup!
The other thing that I have noticed is that the cadence at which rewards come in is much more regular at approximately once per hour. Where previously I had breaks between 4 to 12 hours of non-activity.
Extras
Once you do get your hotspot up and running consider checking out Hotspots Health which combines a dashboard interface with a performance analyzer to help you get the most out of your hotspot.
