Measuring up the tried and tested 1/4 wave ground plane antenna against a folded j-pole design. The requirement is to have an antenna that generates gain above the (0 gain) 1/4 wave.
Prototype J-Pole antenna.
With (anti-current) coax balun.
Version 2 J-Pole (weatherproof).
J-Pole with dead point mount bracket.
Fine tuning and calibration tests.
Prototype of the 1/4 wave for ADS-B.
Operational 1/4 wave 1090 MHz antenna.
The 1/4 wave is just that.. a 1/4 of a wavelength, for 1.090 GHz that is 6.8807 cm (in theory). This design requires a ground plane (virtual mirror) to operate and this is created by the four wires attached to the base. To correct for impedance the ground plane rods are bent at just over 45 deg giving an impedance of just over 50 Ohms which is exactly what the receiver requires.
J-Pole design shows a very solid short circuit between the driven element and ground. Generally enough to fool any 1st year electronic student that this will not work. Well it does and rather well too. Although this design is omni-directional (just like the 1/4 wave) with a low dispersion pattern, because of matching points (to get an impedance of 50 Ohm) it not only puts voltage on the driven element, but also current, and this we do not want. To prevent current from going to the receiver front-end, an in-line balan of four coils in the feed line is used.
Live view of this 1/4 wave dipole ADS-B can be seen at http://adsb.steeman.org
Live view of this j-pole ADS-B antenna can be seen at http://adsb2.steeman.org
Antenna gain is measured in db (decibels), but as I have no equipment to measure this at present, I use real-time reception and signal strength using two exactly the same receivers tuned to exactly the same frequency but using different antennas. If you followed the two links above and observed them at the same time it clearly shows the J-Pole antenna has "better and further" reception. This equals gain over the quarter wave dipole.
The next mission...Can a collinear antenna outperform the J-Pole antenna design, and if so by how much and where is the cut off point for effective collinear stacking.