by Tim Gorter, AV/IT/Wi-Fi Virtual AVIT ETO (teletechnics.com)
More and more vendors are trying to move into the cellular space, wanting to provide you the ultimate 4G connection. VSAT providers try to give you the complete connectivity package, cellular modem manufacturers are slipping in the SIM card option, and vice versa 4G SIM card suppliers add the perfect antenna option to their monthly subscription.
All of them a touting the connectivity range that their solution will provide, how many miles offshore you will still receive a signal!
When you consider who to go with, what hardware to install and which SIM card to use, I wanted to lay out a few basics that you should always keep in mind. From there the salesperson, or the marketing material should only be the final turning point for you to choose who you believe in most.
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I wont hide the fact, that I can also assist you in a SIM card subscription, and happy to provide you with the best, that won’t change the fact that the SIM card I supply very likely is similar to that of the other supplier and that is why you should look at the whole picture.
I will go from top to bottom. The antenna is a crucial part of the design. It comes in various types and shapes, with the dB performance number being the most crucial. Be clear that a passive antenna will always output the same power, no matter what enhancements are made to it. The improvement in performance comes in shaping the RF emission of the antenna. i.e. the “beam” is shaped to change the reach of the antenna, this shaping improves the performance in one direction at the expense of decreasing it in the other direction. That is why a dish antenna can reach out into outer space, whilst an omni-direction antenna (normally a vertical stick antenna) would not be able to.
But you can get very nice 7-8dBi omni-directional antennas, that improve their performance by flattening their beam, i.e. decreasing the spread in the vertical plane, whilst increasing it in the horizontal plane. This means it reaches further.
How much further? Well RF needs line of sight. Any obstruction will decrease the performance, and if it is a metal obstruction… even more so. So that means height normally, the higher the better (same goes for your vhf antenna!). Now a bit of mathematics, let’s say your mast is 40m high, and the cellular mast on shore is also 40m high, that gives it a 28nm distance. (a search on the internet for a RF line of sight calculator will give you a place to enter these numbers if you want to check.) So anything above that number has obstruction (the curvature of the earth!).
And note, we are calculating the distance between 2 antennas… not the shoreline and your vessel!
Next important factor is the cable run between your antenna and the modem. The longer it is, the more loss. The more connectors there are, the more losses. The more cables run parallel to each other, especially electrical, the more influence on losses. Get the picture? Simply said the closer the modem to the antenna the better the output performance, as every decimal of a dB is a gain! 3dB is half the antenna power. A connector in itself can be 0.5dB (or more) loss.
This is why you see more and more modem manufacturers trying to sell you a “dome” antenna where the modem is physically built in the same housing. The problem is the SIM card normally needs to be slotted in by the modem… which in this case is up the mast! But there are solutions for this.
Next, the SIM card! This little plastic piece with strange metal engravings come in various types. Or better said, are bound to various performance factors. And this very much has to do with the network provider and the agreements they have, and how the card registration is programmed into their system.
A business grade card outperforms a consumer grade card. It has higher priority; this is why it costs more money! A local card has priority over a roaming card, that is the privilege of the local operator! And an unsteered card will search for a better signal, whilst an steered card will be bound by best agreement rules (i.e. a network operator will have better agreement with one overseas operator than with other, and thus steer you to their favoured business partner), this will mean you may get a weak signal, whilst others around you have a stronger signal.
And the last factor that strongly influences the performance is how many modems are sharing the same network. This has nothing to do with signal strength or SIM card performance, but rather bandwidth from the cellular provider at the antenna mast that everyone is trying to connect to!
I have not spoken much about the modem itself here as that is a completely different article, there are a few dominant manufacturers out there. What you need to make sure that you have one that is multiple antenna (MIMO) and of at least cat-12 specifications, for the region that you operate in.
So, when they sell you that 80km range, do ask what their tests are based on? And can you have exactly the same environment on your vessel where you intend to be sailing?!¿
This article was written by Tim Gorter, Virtual AVIT ETO, www.teletechnics.com. I provide support and training to crew assigned to look after their AV & IT system onboard. Making sure you understand how it works, and that you get the best out of it. Call for an AV & IT health check, more on teletechnics.com
