We ran some field tests today using a whip antenna (standard) and a ducky
antenna (well-known brand) mounted on equivalent Futaba transmitters using
the same RF module. Measurements were made using a calibrated HP spectrum
analyzer as the calibrated receiver. The receiving antenna was a 50 ohm resistor
with no directivity or gain (a "monopole"). The distance between
the transmitter and the receiving antenna was about 60 feet.
We found a 4~6 dB difference in received signal strength between the factory-standard whip and the ducky. This was (to be) expected, and the amount of margin built into today's receivers tolerates that, as long as we remember that we are eating up safety margin.
The ducky is much less direction-sensitive than the long whip. The best angle range for the ducky in the vertical field is: up 30° up to 30° down from horizontal, ok to 60° down, start seeing signal loss at 60° up but not much; measurable loss at 90° (straight up). The best angle for the ducky in the horizontal field is: 30° left to 30° right from pointing at the rx antenna; still ok at 60° left to 60° right with small loss, then, again, a measurable loss at 90° to the receiving antenna. We therefore recommend flying with antenna pointed at the plane plus/minus 30°.
DIRECTIVITY (Whip Antenna):
The standard whip antenna helps create the strongest overall signal strength at the receiver antenna, but we found the directivity to be much more critical than that of the ducky. The best angle in the vertical field: 60° to 90° (vertical). The best angle in the horizontal field: 60° to 90° degrees (max radiation out of the side of the antenna). The worst angle was when the whip antenna was pointed "approximately" at the receiving antenna and we measured a sharp dip in received signal strength of more than 25 dB. At this short a distance, this would mean a reduction in received signal strength of more than 99.7 percent!
Pointing a whip antenna at the plane is BAD, and pointing a rubber duck antenna at the plane is GOOD. These measurements proved nothing new. We have known for a long time that the antenna patterns are as described above.
The above measurements are approximate, made with calibrated and sophisticated test equipment but not on a calibrated test range. Overall test data is believed to be accurate and representative.