On Sunday I spent the morning self-consciously traipsing around my brother's neighborhood with an apparently wierd collection of hand-held electronics and foam in the form or my APM, battery and a Hextronic 3DR knock-off 915MHz telemetry radio and Skyfun fin. We left a laptop on the top floor of the house running APM Planner with the base station radio connected directly to the USB port and using a standard "duck" monopole antenna.
The mobile unit used my experimental lightweight directly-soldered dipole and the contraption was powered by a standard Futaba NiCad receiever pack as pictured above. The aim of the new antenna design was to reduce mass and configure it so that it could be embedded entirely within the vertical stabilizer of the Skyfun.
Unfortunately, I don't have a functioning GPS at the moment, so I located our position relative to the base station by memory and adding a series of coded yaw and roll gyrations of the APM to help identify the approximate location by post-processing the tlog file and correlating with Google Earth measurements. The time-domain plot of the RSSI results looks like this:
What is immediately obvious is the elevated noise floor at the base station. I guess it could be laptop power supply related or some other component, but I think it begs some experimentation with USB cable extensions and remote antennae. I'd also like to make a dipole up to see if I can improve the efficiency.
From the post-processed position data (approximated by the ruler function in Google Earth) I have plotted an RSSI profile with radius. Note that we were not line-of-sight for a lot of the meanderings due to the geography and surrounding suburbia. There is also a huge steel girder railway bridge that we passed under during the process, which appears to have had some effect. The transmission dropout in the first graph appears to be due to terrain masking entirely.
During the log file post-processing in Excel, I concocted an error rate calculation to compare the data with RSSI. What you see here is probably of little absolute merit, but it does indicate a dramatic increase in the error rate as the RSSI value falls much below 100. The time-domain plot would seem by eyeball to indicate successful transmission with an even lower RSSI.
Since I had also wanted to see if my new-fangled antenna had made a difference, or at least no difference, I plotted RSSI against remRSSI to see if there was any bias. The answer is, if you squint a little, perhaps to a small degree. The base station appears to recieve a higher signal at range than the mobile station. Whether this amounts to improved signal transmission from the mobile station, it's hard for my RF ignorance to determine, but I will clutch at this straw for some feelings of feeble accomplishment.
So, it seems likely that the system as it stands will yield a reasonably line-of-sight data link to circa 500m. If I can improve the base station noise floor, this may be able to be significantly improved. I was actually hoping to reach the 1000m mark, but this looks like requiring more development and experimentation.
All of this is (for completeness) with the radios configured using mission planner with the frequency band from 918-928MHz, 30 channels, 20mW power. For some reason, this is the maximum power available on the selection menu when connected. There might be a firmware update to try.
Comments
Nice summary Robert! Thank you for the background info..
With the explosion of FPV related activities in Australia it has become common for people to ignore the relevant legislation. Many of those people are aware of the legislation and happy to operate outside of the rules. Their choice. Sadly, those new to the hobby are often not given the same choice and are fed advice on equipment that would have them operating outside the law, without being told that is the case.
One piece of legislation covers most of what you need, see link below. As many of the bands in use for FPV, and related activities, are allocated to amateur radio, you can get much higher power limits, the right to operate without interference and fewer limitations by obtaining an advance amateur radio license.
http://www.comlaw.gov.au/Details/F2013C00396
For each frequency range:
* Section 17 - 433.05 - 434.79 MHz for telemetry, 25mW. Pretty much all of the long range control gear operates at higher power than this allows...
* Section 19A - 5.8 GHz for video, 25mW. Some claim an OSD overlay makes FPV telemetry but this appears to be more of a failure to understand what telemetry is that a valid interpretation of the rules.
* Section 52 - 915-928 MHz, 1W. Needs to be hopping over 20 frequencies. Be careful with configuration so you don't end up transmitting in the mobile phone network ranges and being hit with a very large fine - <915 is inside the mobile range.
* Several sections apply to 2.4GHz so the characteristics of your transmitter and intended use would determine which, if any, apply.
There is nothing to allow operating in 1.2-1.3GHz which is allocated to amateur radio and other licenses. For example, transmitting video in this band legally would require an advanced amateur radio license.
Using "433" for radio control is a bit of a challenge. An amateur radio license might allow the 430-450MHz range but you still new to transmit your callsign in Australia to comply with operating requirement. There are suggestions that ACMA will ignore this requirement and would only do after people that cause interference to others. The 420-430MHz range was taken away from amateur radio last year.
Ignoring the legal issues, it is worthwhile remembering that unlicensed use of the amateur radio bands does not give any rights to prevent interference by licensed users. The power levels in use by FPV hobbyists are tiny in comparison that those used by amateur radio (up to 120W) and it is unlikely that an amateur is going to notice those low power devices so the interference is mostly one way.
Andrew - Do you know where I can go to find out what the relevant laws and limits are for 433Mhz, 900-915mHz, 1.2-1.3Ghz, 2.4Ghz and 5.8Ghz here in Australia?
With any luck I will get a chance to see how these new DIY half-wave dipoles perform over the weekend.
Quadzi, whilst you can use antenna tracking with directional elements, there are isotropic power limits for the ISM band so to remain legal you have to observe these too. My aim is to maximize the performance of a very simple and easy-to-use (and portable) system rather than get too anal about ultimate range.
I like Robert's ideas. I will be experimenting along these lines in the (hopefully) very near future and reporting back with the results
@Tridge. I wondered if that was the case, so I must have been beaming the full 100mW then.
It's not 20mW, it's 20dBm, which is 100mW
Cheers, Tridge
Try using an "air" radio on your laptop, connected with an FTDI cable. Those USB based "ground" radios are excellent noise sources... In my case, the local noise with a USB 915MHz radio was around 60 with a USB radio. Replacing it with another 915MHz air radio, connected using the FTDI cable that 3DR supplies, dropped that noise to 20-30 or about the same as the radio on my quad.
A similar improvement should mean that the dropout shown in your first graph wouldn't happen. There was a lot of discussion no how this noise issue applied to 433MHz radio, see link below, but 915MHz isn't immune.
http://diydrones.com/forum/topics/delay-in-shipping-433mhz-radios-d...
More power often just makes heat.
Seems unusual more of us are not adopting antenna tracking and better elements (even home built ones). Every HAM nutter worth his salt I speak to about my hobby says the best approach for the application is to keep power low and chase gains with the antennas (and only then looking at power for further range).
ArduStation Mega project seems not to have evolved much and few people seem interested in it. Can't understand why this is the case. Perhaps it's the reduced level of portability and convenience.
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