My apologies for this lengthy post but I think it is worth trying to explain why we are seeing such wildly different results and then discuss the options that might be available. This is a complex subject, not well understood outside the realms of RF engineers and radio amateurs. There is, however, perfectly good science behind it and I am sorry to say that I am completely unsurprised by the results that we are now experiencing. OK... here goes... deep breath!
Falcoguy's observations are spot on and many of the points have been raised in this forum before. Antenna system gain (loss) and position/shielding are the fundamental issue here and that is exactly the reason why Flarm has had to go to two antennas, preferably externally mounted.
The sums go something like this:
For every doubling of distance the free-space signal loss is about 6dB (i.e. the power drops by a factor of four. This is the inverse square law in action. So, if we start with one unit of energy at 1km, it is down to 0.25 units at 2km, 0.06 units at 4km, 0.015 units at 8km and so on. That's the reduction in signal strength due to distance. It is immutable. We can do nothing about it.
Now lets look at antennas:
The basic 1/4 (quarter)wave antenna is a useful starting point. They are commonplace - our aircraft's VHF antennas are one example. They work well because they are operating against an "infinite" ground plane - the aircraft's fuselage. Those with composite aircraft will have metal plates or aluminium foil strips to achieve a similar effect. The whip antenna that comes with the ARF looks to me like a 3/4-wave device, end fed. This theoretically gives around 3db gain over a 1/4 wave antenna but it has no effective ground plane to work against, so it is probably losing all that apparent gain, perhaps even more. Simply by fixing the ground plane problem we would get at least 3dB antenna gain - the equivalent of about a 50% increase in range. Another way of doing this is to make a dipole antenna. A dipole will always significantly outperform an end fed antenna with a poor ground plane. There are other options too. The antenna can make quite a difference to range.
But the elephant in the room is shielding:
Particularly in a metal aircraft, but composites aren't completely immune, radio waves are severely attenuated by reflective materials such as aluminium. It is very easy to achieve 20dB of attenuation with rather little metalwork, especially at 800MHz. Just look how little metalwork there is between you and the business end of your microwave. Your food gets cooked but you do not! 20dB is the equivalent to a drop in range from, say, 10km to about 1km. If significant metalwork is between your antenna and the other guy's antenna then the losses can easily amount to 40dB or more - the difference between a range of 100km down to just 1km. These are overwhelming numbers! Let's be generous and allow for just 30dB attenuation...
We can recover that 30dB in three ways:
1. Move the antenna(s) to a position that is not shielded;
2. Increase transmit power by 30dB;
3. Increase receive sensitivity by 30dB.
1. is the only practical solution. It is the reason why all installed antennas on aircraft are external.
2. is not practical. 30dB is an increase in power of 1000 times. The ARF runs, it is said, 500mW, so we would need 500W to do it this way. Quite apart from licensing issues, that sort of power is a) dangerous (think microwaves again), b) requires a lot of kit to produce it and c) would need significant battery/generator capacity to support it.
3. is not practical. The receiver is already very sensitive, close to the limits of technology, so it is difficult to make significant gains here. Notably issues with noise, both internally generated inside the receiver and external, such as solar and ignition noise, make this impractical.
Yes, a combination might be used but by far the biggest gain - and therefore where our attention should be focussed - is in the antenna system and, especially, its position, clear of shielding.
Realistically an external antenna system is the only solution and, since we need to see all around us we most likely need more than one antenna for optimal results, exactly as the folks at Flarm have discovered. This isn't PAW's fault, nor is it a failing of the ARF. It is simply the laws of physics in action.
Sorry for the long missive!
