This started from a simple range test. It became a lesson in signal tech for me so I thought I’d share the thoughts and perhaps some of you have some better knowledge drop me a line. Range-test write-up will be on separate post in near future.
As usual with matter that may cause discussion, a small disclaimer: I’m not a radio signal specialist, as always source criticism should be used.
Technic used in Jeti
We know a few things. 2.4GHz uses frequency hopping and listen before talk. And in EU the ERP (Effective radiated power) limit is 100mW (= 20dBm). Also we can find out that in EU the 900MHz Jeti uses is actually 868MHz and allowed ERP is 25mW (= 14dBm), if listen before talk and adaptive frequency adaptation is NOT used the allowed ERP is only 10mW (= 10dBm)
If it only was that simple. It’s not.
When we are talking about radio signals with different frequencies we need to think about different factors that lower the range, that means of course different things obstructing the radio signal.
One thing we tend to forget is very simple, humidity of air. And in our case this does make a difference. The 2.4GHz is more affected by high humidity in the air than 900MHz band. In fact, 2.4GHz band is ten times more affected.
Also free space loss is different, with 2.4GHz and distance over 2 km the free space loss is 106dB, same values for 900MHz is only 97.5dB. (Keep in mind decibel is logarithmic scale.)
What does this compute to? Well, confusing to say at least. And there’s more. There’s also the latency to think about. I need to build a test-bench for some latency-testing before I’ll dive in to that subject. so I’ll leave that for now.
What is the 900MHz for?
According Jeti it’s a backup in cases 2.4GHz band get’s too busy on some locations. And this it does, according different scenarios I tested it works really good. But why leave the subject there?
There’s more. It came to my attention that RSat900 might not be in its final form, for example the antennas may be changed. This is as of today not confirmed nor can it be confirmed until Jetimodel comes from their well reserved vacation next week.
EU vs US
And then there is the good old EU vs US. As I wrote earlier in the first 900MHz article the frequency is different, EU has 863 – 870MHz and in US 902 – 928MHz.
Looking datasheet for the dual band antenna included with RSat900 is also interesting. (Can be found from manufacturer Molex site here.) What pops out is the sensitivity-diagrams, they are not that close each others. Of course this does not mean a lot due the fact that I’m currently unable to confirm the power used in 900MHz link in US.
What I do know and have tested is that 900MHz does NOT give any significant added range compared to 2.4GHz band if the latter is not crowded and works. Also we have to remember that Jeti goes to 900MHz backup only when 2.4GHz is out.
Where to from here?
The topic “2.4 plus 900” is not over. And all we can do at the moment is wait. We need to wait for range test’s done by users in US after they get their equipment, wait for information on power-levels in US, wait for news on the possible change in software or hardware.
But in the mean time, grab a model, get out on the field and have fun! :)