ORIGINAL: franklin_m
The manufacturers / distributors have yet to tell anyone what the design limits are....and they've got incentive to continue to keep that quiet. For all the ad dollars spent touting the robust nature of 2.4, to come out now would create havoc in their marketing / consumer relations. Don't kid yourselves, there's huge dollars involved, therefore huge incentive to try and blame something else rather than the most obvious - a crowded spectrum at the site and radios that are not so robust as advertised.
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Regardless of what the marketeers say, there are limits to these systems. I just wish they'd share them so we can make informed risk decisions on when to fly and when not to fly.
IMHO this is a good post. And thanks for your service, BTW.
I think it was in late 2003, Bob Wilcox and I built a 2.4 GHz spread spectrum 16 channel radio and flew it successfully. We took it to the AMA to lobby them to approve the use of spread spectrum radios (at the time they only allowed the specific narrow band frequencies)
We used single TX and 4 satellite receivers in the plane, in a digital architecture that (I GUESS) is similar to the new futaba S Bus (just a guess). We had continuous feedback to the TX, in real time, of the RSSI (received signal strength) of each receiver. You could see the RSSI fading as the plane maneuvered and as you moved the transmitter antenna, and not a little bit, the signals were going up and down many dB. You could see when the .40 sized glo motor got in front of one of the antennas. Imagine what a turbine tailpipe does!
After this we built a TX with 2 transmitters. And we were about to start working on 900 MHZ for better signal propagation. We quit about that time as we felt there was not enough Intellectual Property (patents) that could be protected, in other words if we got this thing working, everyone and his brother would be making a radio and the competition would be fierce. We were too small to compete.
Like 2.4 GHz, 900 MHz is one of the Industrial, Scientific, and Medical (ISM) bands. 900 MHz has a bigger aperture (size of the radiating element, the antenna if you will) so metal “things” look smaller to it. And (in part) because it has less bandwidth (OK for us), many of the users of 900 MHz have moved to 2.4 GHz, and now even to 5.8 GHz, so the band is less crowded.
As for design margin (or design limits as you call it), if you look at 72 MHZ operation, with ideal antenna orientation, you might be able to get 1.5 miles of range (just for sake of discussion). So let’s say we typically fly our planes no more than a ¼ mile away from us. That would mean we are only using 1/6 of the potential range, so we have a lot of extra design margin.
I have no analytical way to back this up at my fingertips, but I think in most cases, we are flying at about 75% of a 2.4 GHz radios max range. So we only have 25% margin for poor installation, high noise floor, etc. So the spread spectrum designs have to be more robust.
In my mind, an ideal system would have multiple TX antennas (or actual radios) similar to Weatronics, it would have satellite RXs like spektrum, and it would have FHSS (or a hybrid FHSS/DSSS) like Weatronics, Futaba and maybe others by now.
I have a feeling that Futaba S Bus might eventually get 2 out of 3 of these (depending on how the RXs can be configured) , or maybe a Weatronic if you ran multiple RXs through DPSI “like” power boxes.
And of course it would be on 900 MHz but nobody seems to be looking at that. Or it would be a dual frequency system like ACT makes, one side is 72MHz (or 35 or whatever you use in your country) and the second TX is on 2.4 GHz.