Sunday, July 20, 2014

Thoughts on GoTenna

[Update Nov 30th 2016: The ARRL has asked Bay-Net to attempt a modification of the GoTenna for use in the amateur radio bands.  Also I should note that since I wrote this article, GoTenna has backed off on their performance claims to levels which are more in line with reality.  I'm looking forward to seeing the inside of these units and doing some real world testing.]

[Original Article posted July 20th 2014] GoTenna seems to be doing a good job of generating "buzz" for this proposed product - a cursory search of Google+ and Facebook turned up a HUGE number of posts.  Many people have forwarded this to me via email or social tagging.  The proles are calling it "Text messaging for CB radio".  facepalm

GoTenna's radio works in the MURS band - three channels at 151 MHz and two channels at 154 MHz.  Given what I know about their product, MURS rules will require them to operate on the two 154 MHz channels.  They say the product emits 2 watts, which with a 3 dB antenna would be 4 watts effective radiated power (ERP).  At that level they're going to have to undergo safety exposure testing, which is expensive - I guess this is why they're crowd-funding the project.

To be fair - GoTenna has an engineering advantage in that they're not dealing with large data streams - they send GPS coordinates, text messages (< 200 characters), and it's not real-time.  They claim 20 - 30 mile range in a "typical" urban environment - I'm struggling with the idea that something which emits 4 watts ERP can give you 20 - 30 mile urban range.

Coding gain from a forward error correction engine could help, but I can't imagine an effective coding gain of more than 10 dB.  (The device is battery-powered, and claims a 30 hours continuous on time - processors which could give them > 10 dB coding gain are going to wipe out a battery fairly fast.)  Let's be generous and say their coding gain 10 dB - this gives effectively 40 watts.  I suppose depending on your definition of "urban" this might be enough to cover 20 - 30 miles.  A more reasonable coding gain will be in the 6 dB range, which means effectively 16 watts - and I don't see 16 watts covering urban areas very well.

Here's a post on from Raphael Abrams, the GoTenna RF designer:

GoTenna claims that the device can be used in-flight on commercial airliners.  This is a Bad Idea™, and I don't see the FAA signing off on this any time soon.

Lots of speculation on GoTenna right now in the radio community, but not many answers to the head-scratcher questions.

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