Hi Bill -
The information you provide is interesting.
I have essentially 2 concerns with 2.4 GHz radios. Note that my primary concern is not about spread spectrum - it is about the carrier frequency. Let me explain my concerns:
1) 2.4 GHz attenuates much more in free space than 75 MHz (about 10 dB more Free space path loss)
2) 2.4 GHz is much more directional (sensitive to shadowing) than 75 MHz (hence the Line-of-Site concerns).
The best I can figure, the free space path loss difference between 75MHz and 2.4 GHz ia about 10 dB (at distances 100-500 m). That means that (given that all other losses and gains are equal) that the modulation and coding technique would need to provide a 10 dB gain to make up for this difference in path loss. OK, that is possible. As you probably know, a large part of the gain realized by spread spectrum systems can be quantified by dividing the data rate into the spreading rate, and converting to dB. Gain = 10*log(Spreading rate/Data Rate). So I think that would be my question - what is the data rate (I don't know what data rate is required to support 3 channels of throttle, rudder and mixture information), and what is the spreading rate? If the gain is not more than 10 dB (to make up for the path loss difference between 75 MHz and 2.4 GHz), then I would say I'll be sticking to my digital 75 MHz PSK radio. If there is more than 10 dB of spreading gain, then...cool!
Beyond that, I agree that we have to talk about cable losses and antenna gains. Certainly higher frequencies do not require antenna lengths as long as lower frequencies to achieve the same gain. However (I think you touched on this), higher frequencies do suffer from higher losses in the cable. How do FASST receivers account for this? It is not nearly as big a question as the one about spreading bandwidth and data rate, but cable loss does have an effect.
When you say "multiple receivers", do you mean that these receivers have RAKE receivers? How many fingers?
The satellite part totally got me confused, but I am guessing that that part doesn't really have much to do with boats. Please correct me if I am wrong. I would love to know the satellite application that is being used! Especially if is is not GPS!
The eight antenna part confused me as well. Can you offer a further explanation of how the 8 antennas are used / connected to the receiver(s)? Is there 1 per RAKE finger?
The shadowing/Line-of-Site fading part will never go away at 2.4 GHz. 2.4 GHz will always shadow more than 75 MHz. But once again, it comes down to the link budget. If there is enough headroom in the maximum allowable path loss, then it is possible that shadowing will not be an issue.
Additionally, the part about "essentially immune to interference"...I have done a LOT of interference testing of spread spectrum systems, and have found that outside interference definitely can affect spread spectrum systems, although I agree with you that SS sytems do have some interference rejection ability. the only problem is that this interference cuts into the link budget. So, while in good path loss conditions, the link budget hit might not matter, but the maximum range will definitely be affected by interference. The more interference (higher power levels), the bigger the link budget hit and the smaller the range of reliable operation. Correct?
I look forward to increasing my understanding of how the spread spectrum radios work! I appreciate you assistance!
Thanks
Jay
What you see on the Futaba FASST receivers is not all antenna. The antenna itself is ONLY the small bare wire looking portion at the end. The rest is coaxial cable that is designed to allow you to get the antenna out and away. The Spektrum RXs have very short antennas on them and they are ALL antenna.
The problem with using a longer run of coax cable is that at 2.4 GHz it is very lossy. Signal attenuation is a physics problem that cannot be easily dealt with.
And I know this may sound odd, but with SS you are far less likely to ever need failsafe since the signal integrity is so much better. SS is far less likely to have a problem with RF generated inside the model and is essentially immune to outside interference. The biggest problem for SS is keeping the TX and RX antennas in line of sight contact. JR/Spektrum addresses this issue with multiple antennas and satellite receivers (one new airplane model has THREE satellites plus the main one giving you EIGHT antennas!!). Futaba does it by making it easy to get the antenna up and out. Longer would be nicer, but as I said, it has a limit based on the signal loss caused by longer runs of coax.
