A 40MHz control problem recently got me thinking again about lower frequency vs 2.4GHz control. I’m still using 35MHz as are a few others locally, as well as 40MHz and 72MHz.
Local Testing
I live about 2KM from the flying field so I can listen or test for radio activity that may also affect RC flying. I generally use a Icom PCR-1000 receiver and a broad-band antenna on the roof. This allows long duration scans that indicate activity and background noise over a chosen band.
This only applies locally to Upper Hutt, which is a valley of about 3×9 KM and a population of about 40000.
Lower Frequencies – Long-Wire
Locally, the 35MHz RC band is a bit noisy. For short range RC it doesn’t matter but for FPV or large gliders, it would pay to experiment and choose a reliable channel. Noise floor is about -85dbm.
The 40MHz RC band is clear except for 40.65 to 40.71 which contains data and baby minders. I wouldn’t use channels 465 or 471. Noise floor is about -90dbm.
The 72MHz RC band is also a bit noisy. Possibly because it’s surrounded in RT channels and probably now a lot of digital traffic. Noise floor is about -85dbm.
433MHz
This is another option for (possibly) more reliable longer range, FPV and large gliders. The 433MHz ISM band is small and heavily used by not very smart short range data links like weather stations, power meters, car alarms, remote controls and toys. There is also some serious intermittent data traffic. We’ll have to see how this band develops.
Overall, long wire still works well, so being a bit biased, I wouldn’t be too quick to shift to 2.4G.
2.4G RC
This is an ISM band, also used by microwave ovens and supposedly by Ham radio, although they probably have less congested frequencies to use. It’s main use is for computer wifi and then cordless phones and short range data.
There are 8+ wifi networks at the road end of Trentham range, occupying all of the 2.4G band. Wifi occupies more of the band the busier it is. So although there is competition, it’s not all over yet.
The further out the RC plane gets the more the received RC control is drowned out by wifi etc. When first switched on it may identify a usable part of the band and then when the plane is up and away, find it’s in use by a couple of wifi networks.
A couple of hundred feet up, the plane can see a lot more interference; not detected by a range test before flight. A pre-flight range test on 2.4G is probably almost useless other than for pointing out the obvious; like the battery is not plugged in.
Some systems will just hop off to another part of the band; all good if there is a clear bit available. Some systems are not so good at hopping around the interference. Frequency hopping systems work well and at full speed when there’s little or no interference and they stay in sync. When they loose sync, it takes time to recover as they have to search the band for the correct signal. If it’s a weak signal and there’s lots of interference, it can be quite a challenge.
If 2.4G RC gets a bit dodgy you’re basically stuffed. The receivers are quite smart and work through most problems. Everything works up to the point that they cannot solve the problem.
Lower Frequencies – Reliability
The biggest danger to a 35/40/72MHz system is probably someone else turning on another transmitter. Anywhere within a few KM will be clearly picked up by the plane. These receivers are generally not super-sensitive which reduces far off interference.
Another lesser problem at 35/40/72MHz is general background noise. A combination of all other very weak interference and noise sources which adds up like a crowd chattering in the background. This “raises the noise floor” which has the effect of reducing control range. Generally, the lower the frequency, the higher the noise floor.
You can improve the robustness of 35/40/72MHz by ensuring that the receiver antenna is the correct length, straight and away from push-rods and CF. Also that the transmitter and antenna are in good condition, batteries charged and antenna fully extended. A 2.4G transmitter puts out the same power on a full or near flat battery. On 35/40/72MHz power drops noticeably as the battery goes down.
If it seems that control is fading or lost try changing the transmitter antenna orientation to the plane and holding it higher ie. high above your head and at 45-deg; but not pointed at the plane. It’s difficult to think about antenna orientation when the plane is out of control and flying away. Been there, done that.
Wifi Access Points in Upper Hutt
I’ve done two simple tests to see how much wifi there is in the area.
The first was to take a laptop and USB wifi adaptor with attached medium gain antenna to a ridge overlooking the Trentham area of Upper Hutt; about 450 feet higher than the Trentham Range (flying field) and 3KM away across the valley. Within a couple of minutes we’d logged 241 APs from within about a 3KM radius. These are only the most visible ones.
Note: the laptops internal wifi, sitting on the ground, could also detect many of these including my own outdoor AP (about 2KM away) which we could connect to and browse the web.
The second was to use the same laptop and adaptor but with a low gain antenna to drive once around the outskirts of Upper Hutt. It’s difficult to see how many APs there are, but the 2KM from home to the flying field logged 240. This test only logs active APs or those broadcasting their SSID. The round trip probably detected over 1000 APs.
Update, Oct-2013: A more recent drive around the Trentham area with a Cell-Phone logging wifi access points detected over 2000.