As many of you know, optimizing RF systems for really long range can be tough.¬† There are many factors that impact the range of an RF system including output power, reciever sensitivity, and antenna type and placement.¬† Many people under-estimate the value of the last one, but it has the biggest impact.¬† Two pointers to consider when maximizing range:
Fresnel Zone - the concept here is that between two antennas for a given RF wavelenght is a football shaped 'zone' that must be free of obstructions to maximize range.¬† At 900 Mhz, this zone is as fat as 95 FEET in the middle if you are tracking at a range of 10 miles.¬† At 1 mile, it is 32 feet.¬† Put another way, if you have a transmitter and a base station being held in hand on flat plane, you will interrupt the fresnel zone with a transmission of only a few hundred feet.¬† This degrades signal dramatically.¬† At 2,000ft it is unlikely the data will make it.
HOWEVER, if you put the antenna up 95ft off the ground on both ends, 10 miles will be possible with no interference.¬† OR, if you are tracking something in the sky, your fresnel may be clear because you are pointing UP.
Radiation Pattern - Your simple dipole antenna has a 'radiation pattern' that is not really omnidirectional as is often discussed.¬† It is 'pretty much' omni directional, but only perpendicular to the antenna.¬† In other words transmitters 90 degrees to the side will have maximum range, and transmitters the antenna is 'pointing at' will have the worst range by far.¬† This is the OPPOSITE of directional antennas that are designed to be pointed at the transmitter. (Courtesy of Maxstream/Digi's web site)
What this illustration shows us is that for our OMNI antenna, your range at the tip of the antenna will be terrible - even worse with high gain omnis. Range may be perhaps less than 2,000 feet or so even with a clear fresnel zone, but if you rotate that antenna to the side, you will be back in business.¬† Typically for tracking terrestrial items, omni antennas are mounted up/down to maximize their range.¬† Further, notice the higher the gain of the omni, the FLATTER the dougnut'.¬† Actually a 2db gain antenna might perform better at harsh angles compared to higher gain.
If you are tracking flying objects, you should consider mounting your antenna HORIZONTALLY.¬† This will provide a longer range UP, but not as good along the horizon.
We know this is true from tracking balloons.¬† At 20 miles downrange and 50k feet up, we get signal on a 2db antenna with only a 100mw transmitter just fine as long as it is not pointed at the balloon.
As with everything RF - test to see what works best in your particular situation!