I have seen both patch and helical antenna when shopping for video receiver antennae. All I know is that they are both directional antennae. Do these types of antennae have any inherent performance differences? (I´m talking specifically about 6-turn helical antennae compared to patch antennae)
tl;dr: In terms of radio performance, they're almost identical if you compare similar-gain models, so choose based on dimensions, durability, maintainability and other practical considerations. If you're buying, patch is usually better simply in terms of size and toughness. However, consider DIYing a helical; it's cheaper, a nice project to do, and makes a very good antenna. Only buy a premade helical if you wanna do very long range flights and don't want to bother with making one.
If you only consider the strictly RF differences between these antenna classes, they are rather subtle. These indeed are both directional antennae, and the directivity for both classes can be varied in a rather wide range depending on the particular variant of the antenna. A helical antenna would get longer as its directivity increases, while a patch would get wider. One thing that may or may not be significant is that helical antennae tend to have a very symmetrical radiation pattern, i.e. a very round beam, while the radiation pattern of patch antennae tends to flare out slightly more to certain sides. This can be significant if you're using highly-directional versions and need to point your antenna to keep your aircraft inside its cone; for a helical antenna the gain reduction would be the same no matter in which direction the antenna is misaligned, it would only depend on how much it's misaligned. For a patch antenna the direction contributes as well, though maybe subtly.
Otherwise, there's not much to say. While in theory a helical antenna is slightly more efficient, in practice each antenna's efficiency would depend more on the design and fabrication of the particular antenna than on its class. Speaking of fabrication, I must also point out here that since patch antennae are PCBs, and helical antennae are likely at least in part assembled by hand, I would expect the characteristics of patches to be a bit more consistent antenna to antenna within the same model/manufacturer. You probably wouldn't notice either way.
The more significant characteristics are dimensions, endurance and ease of manufacture. In the first two, patch antennae win by a long margin; they are both more compact and harder to damage in transportation or rough handling. For ease of manufacture... it depends. If you're looking at producing thousands of them, the patch also wins unequivocally, as it's just a PCB that can be automated and made very consistently and cheaply. If you need just one or two, however... The helical is far ahead. It's a very simple design that doesn't depend on things like copper thickness or the grade of material of the circuit board, its directivity can be easily set as needed by simply changing the number of turns and without recalculating anything, and it is very easy and cheap to manufacture at home using a minimum of parts and tools; the rarest of them is probably the coax connector and maybe the wire former (if you don't have plastic tubing of the right diameter or a 3D printer); the rest of the materials are probably already lying somewhere around your house.
So if I were to recommend, then:
- If you're flying near yourself, don't want to worry about transporting/damaging it or want a premade antenna, go for a patch.
- If you're willing to make one and/or you want to do very long range flights, AND you're okay with it being a bit bulky and less durable, helical is better.
It is my understanding that in general a helical antenna will perform better than a patch antenna. I should really find a source for that though. However a patch antenna is flat or nearly flat and thus easier to pack and transport. The bulkiness of the helical is probably the main drawback.
RcModelReviews has some great instructions on building your own helical antenna. Another nice thing about the helical is that the number of turns determines how directional they are, so it is easy to see exactly what you have, know that they are polarized and all that. With a patch, it is not so easy to tell exactly what they are.