Probably has to do with the direction that gain points, and whether it is referenced to a theoretical ideal dipole, or if it is referenced to an isotropic radiator.
I tend to look at most antenna claims as 99% hype, unless they include an honest 3D plot of the radiation pattern.
Antennas do not add energy to the radio signal. That is determined by whatever the transmitter feeds it. For receiving, they can not pull in more than what arrives. What they do is modify the shape of the pattern where they are most sensitive for receiving and where most of the transmitted energy is concentrated.
This comes at the expense of energy or sensitivity in a different direction.
Real world examples: A discone antenna behaves pretty close to an isotropic radiator. It also has broad bandwidth. These are often used atop air traffic control towers and other air to ground communication where the target can be anywhere between the horizon and directly overhead, as it hears and radiates almost equally in all directions. They are also used frequently for scanners because they work well across multiple bands.
I built my own 1090 mhz antenna. It is just a simple quarter wave, with downward swept counterpoise radials, partly because mechanically it is easy to make from 3/32" brass rod which solders nicely into the center terminal of an SO-239 or N connector, while the radials easily mount on the chassis flanges of the same connector.
The radiation pattern has its strongest area about 3 degrees above the horizon, and drops off with increasing angle until it has a slight null straight overhead. This can be lived with as most aircraft will be within a degree or two in elevation from the horizon, especially with very distant flights.
Higher gain antennas that are vertical whip types can include phasing coils and other tricks to effectively āstackā elements, and make the pattern look more like a flat washer than a donut or teardrop cross section up-tilted toroid. These perform incredibly well for land mobile two way radio because everything you want to communicate between ground vehicles is going to be fixed to the earthās surface and thus mostly at the horizon.
Those really donāt perform well at all with satellites, which are only on the horizon for a very brief time. Aircraft communication is going to be similar to satellite, in that they can be high overhead, but because they are about 20x closer to the earth and moving 20x slower, have a lower fraction of the time in view where they are at a high angle overhead.
Thus, a simple quarter wave antenna is probably the best trade-off for most 1090 mhz reception, mounted high enough to see over trees and nearby buildings. Consult the ARRL Handbook as to why the radials should be bent downward at about 30 to 45 degrees (skipping a large chalkboard full of mathematical equations and 90 minute classroom lecture), as this pulls the primary lobe closer to the horizon. Otherwise youāll miss a lot of aircraft out on the horizon.
That also gets the impedance closer to 50 ohms, which for receiving isnāt anywhere as critical as transmitting, but a better impedance match means more of the signal that does arrive at the antenna makes it down the coax and into the SDR, so weaker signals can be sniffed out of the airwaves.
