Repurposed BGAN terminal antenna


#1

If one looks hard enough one can find used BGAN terminals on eBay for around $30. I wonder if the antenna from one could be pressed into service for Outernet?


#2

I’m certain it would work; they are designed to the exact same specifications.


#3

It would seem easier just to buy the $30 Outernet patch antenna. I haven’t seen any BGAN terminals anywhere near that price lately.


#4

Strongly recommend the Outernet patch, “air gap” L Band antenna. Fantastic size and great reception. Make sure to get one from Outernet as the profits go directly to this fantastic project!


#5

Do you know of anyone else making an L-band antenna?

You said profits–ha, I wish!


#6

So what would a person search for on eBay to find these used BGAN antennas? I’m not having any luck.


#7

I think Bgan is more expensive than the Outernet patch that works well.


#8

Yeah, I suspect it might be but I’m looking for something in a nice weather proofed case.


#9

I agree on the weatherproof case being useful. But be aware you might have do a tiny it bit of filter removal or wiring adjustment as it is might have some send /receive issues / tricks etc.

Let us know how you go.


#10

Hey, the worst that can happen is that he (or others) might learn something!


#11

I’m an electrical engineer by profession. Hardware designer although only a bit of RF work.

So if I’m looking for a used BGAN antenna on eBay, and I looking for some brand or model number? I find a lot of very expensive BGAN terminals but no antennas.

ETA: Looking more carefully I do see a few older terminals for about $60. I assume these are integrated with the antenna.


#12

What antenna are you currently using to get Outernet?

If the answer is no antenna, as I do not have a Outernet receiver yet. Then buy the Outernet Patch antenna and Outernet Kit and get everything working. Its a big achievement to get the kit working properly and a lot of fun. If you use the same kit as every body else then it is easy for us all to trouble shoot your problems if you have any.

Then when everything is working look at alternative antenna designs.

If you want to use the Outernet on a boat then think a better choice might be a Furuno MARINE GPS antenna with a helical antenna internal.

I have one and intend to bypass the electronics in the antenna and go straight to the Helical and maybe put a circular ground plane reflector…

I am assuming this will work better on a moving boat.

I am assuming this will work better on a moving boat.


#13

Yeah, I’ll set up with the Outernet patch to verify everything. Long term I’d like a weatherproof patch. I can put the Outernet patch in an plastic enclosure, but it will have to be over sized. Plastic is of course mostly transparent to RF, but it has a higher dieletric constant, which will detune the antenna if it is in the near field. If you make the case bigger and keep it out of the near field you shouldn’t have a problem but you’ll end up with a bulky box. Would be much better to find an L-band antenna that was already tuned to be inside a weather proof enclosure.

On using your helical on a boat - I’m assuming that the helical has a higher gain than the patch. If so, it is going to have a higher directivity, which will make pointing more critical which is problematic on a moving platform like a boat.

Do you know the respective gains of the helical and the Outernet patch?


#14

The Outernet patch is 8dBi, per chamber tests.


#15

I’m going to guess that the gain of that helical in the picture above is maybe 11 ~ 12 dBi, based on the length.

If so, maybe the increased directivity isn’t too much of an issue. Although I do like the wide beam width of that patch in a mobile application.


#16

My thinking is that “on a moving vessel / boat” the most important point is getting a consistent signal higher than the minimum Outernet receiver threshold. Lets say minimum reception is 3 Units.

If the patch antenna is laid horizontally flat, pointing up, and works fine on flat water that’s fine . But when a boat starts to move The patch antenna will move towards the satellite but also swing away from it. If the receive pattern at this point is less than three units of reception signal we loose packets. (Lost packets may be corrected by FEC)

My understanding is a short Helix antenna with a ground plane plate / reflector attached will have a hemispherical reception pattern. As a boat moves it should receive a satisfactory signal as it changes angle to the satellite.

This is the type of Antenna Inmarsat has been selling for their L-Band Maritime Emergency Low speed data service.
The Global Maritime Distress and Safety System (GMDSS)

“Inmarsat B is a digital mobile satellite communication system providing two-way direct-dial voice, telex, fax and data communications at rates up to 9.6kbps, anywhere in the world outside of the Polar Regions.”


#17

For an unstable platform it’s always a trade off between gain and directivity. Higher gain antennas are necessarily more directional, giving you more gain when they are accurately pointed but less gain when they are pointed off axis.

That patch probably has a roughly bulb shaped beam pattern, tending towards hemispherical. Given the location of the feed point it should look roughly the same in the horizontal and vertical axis. Since someone has obviously had this antenna in a chamber to measure the gain, maybe they also generated gain plots?

The helical isn’t going to be hemispherical. It should have a more lobe shaped pattern, though given that it probably doesn’t have tremendously more gain it’s shouldn’t be too narrow. There’s going to be some optimal trade off in an application like yours between maximum gain and directivity. If you’ve got some active pointing system then you can go with a higher gain more directional antenna. To the extent your pointing system occasionally “mispoints” a somewhat broader lower gain antenna might actually be to your advantage. In unstable platform like a ship, you have to keep in mind that you generally only want just enough gain out of your antenna. i.e. you want enough SNR margin to ensure low error rate reception, but excess antenna gain beyond that will not help you and will almost certainly harm you by making your antenna more directional, thus making pointing more critical.

The proof of the pudding is in the eating. If that antenna was designed for an L-band ship mounted application it’s hard to argue that it won’t work. In that system was there any sort of gyro or signal strength based active pointing mechanism?


#18

Yes great idea. Lets ask Syed for the gain plots of the standard patch antenna.


#19

Ivo excellent reply. I am busy now but tonight my time I would like to hit you with some of my other Antenna thoughts. ie10,000 RPM hard disk DIY stabilization and Hellix conical Hemispreical antenna’s. Also why reinvent the wheel if Inmarsat already have a product or possible working design…etc.

Seasalt


#20

From the anechoic chamber.