900MHz, thoughts and tech-stuff

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! :)

5 thoughts on “900MHz, thoughts and tech-stuff

  1. Even if the range/power on 900 is raised…..It won't matter much if the returning telemetry information on 2.4 cannot get thru………………..Another techno nerd……riverviewskater…..Just sayin

    1. Hmm. The whole purpose of 900 is to save the model if/when 2.4G band is blocked. Personally I would not care a bit about telemetry in that case, saving the model would be my priority :)

      1. Hi Terry,
        Came across your article and find it very interesting!. I'm i terested to know what your thoughts are regarding Jeti's dual band 2.4/900 band radio for mod safety. I have a very expensive model and want to avoid crashing from interference. How effective is this dual band radio? How often do you think the 2.4 band gets enough inteference to crash?
        Mike Kameya

        1. Hi Mike

          There is no definitive answer to "How often do you think the 2.4 band gets enough inteference to crash?" That depends on the site you are flying on. Some sites in congested areas are very heavy on interference, some have no issues, ever.
          One have to remember that the 900 is backup, so it should never kick in and if it does there is reason to investigate why.

  2. Nice basic analysis of propagation on these 2 bands. I would add the most significant factor: Using frequencies completely on different bands, hence having VERY different wavelengths, makes it practically impossible to have a situation where both channels (2.4 GHz and ~900 MHz) suffer simultaneously of fading caused by multipath propagation eg. via earth surface. On 2.4 GHz frequency hopping is often used, but hopping bandwith is so narrow that it actually has very little effect alone.

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