Telemetry Radio Range testing with home-made dipole antenna

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.

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Comment by Gary Mortimer on April 28, 2014 at 11:14pm

I think you would see a dramatic improvement if you put a dipole on the laptop as well and raised it above the laptop at very least. If the laptop was outdoors you might very have got your 1km especially if you were in the air. Why not put the APM on a kite and drive the laptop around?

Comment by Andrew Rabbitt on April 28, 2014 at 11:26pm

Some plans are afoot to test it from a motorway footbridge overpass where we can maintain line-of-sight for a reasonable distance.  I really wanted to confirm whether my antenna concoction had done irreparable damage to the module before I laminated it into the fin.

Comment by Quadzimodo on April 29, 2014 at 3:34am

Thanks Andrew.  I just started on a set of Dipoles after reading this, and this.

Comment by Ted Van Slyck on April 29, 2014 at 6:29am

With my stock xBees 900 mhz I find I can receive out to about 2 km, but I cannot transmit commands to the a/c beyond a few hundred meters. I find antenna polarization makes a big difference as well as how high I have it off the ground. And oddly enough when I use DroidPlanner (through Android phone) the range goes down even more. I'd be interested to see how well it transmits back with a patch antenna and the dipole on the plane.

My bet is that a directional antenna on the ground is what we need to get a couple of km transmit ability. 

Comment by Quadzimodo on April 29, 2014 at 7:06am

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.

Comment by Gary Mortimer on April 29, 2014 at 7:13am

More power often just makes heat.

Comment by ikrase on April 29, 2014 at 8:57am
Im going to go with complicated unreliability?
Comment by Robert Palmer on April 29, 2014 at 2:26pm

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.

Comment by Andrew Tridgell on April 29, 2014 at 3:10pm

It's not 20mW, it's 20dBm, which is 100mW

Cheers, Tridge

Comment by Andrew Rabbitt on April 29, 2014 at 10:22pm

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.


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