How does antenna size correlate to gain?
Antenna gain does have a correlation to size to some degree, but gain is not a unit of measurement. Therefore, antennas that have the same gain do not have to be the same height, and a higher gain antenna can be larger than a lower gain antenna.
The first rule of thumb in understanding antenna height is knowing the outercasing is not the antenna itself. Most outdoor antennas are enclosed in heavy duty fiberglass casing- a material that does not impede signal reception, but is durable and wind resistant.
Manufacturers can use whatever size casing they want, and internally a longer antenna may have a similar gain to another manufacturer's shorter antenna. The RAK Wireless 8 dbi antenna for example is almost 3 feet tall, whereas our low profile 8 dbi antenna comes in at just over 2 feet but offers similar gain performance. This does not mean one choice is superior or inferior to another. Manufacturers make various choices when it comes to casing size, durability, and material cost.
You can also see in the chart below that a dome style omni antenna, a common design for 900 MHz and 4 spectrums can be up to 10 dbi or more, despite being less than one foot tall, and the Bobcat Miner team went with a low profile design as well for their 4 dBi antenna, which is shorter than the RAK 3 dBi.
What about multi-spectrum antennas?
You may notice some antennas are tuned to multiple spectrums, such as 600-2700 Mhz, whereas our Rokland Miner antennas are tuned to just 900-930, with an exception to our 4 dBi antenna which is 800-960. Generally speaking, the more narrowly tuned, the better performance at that spectrum, but that can also vary based on antenna design. A multi-spectrum antenna could still be a good choice.
One key takeway if looking at multi-spectrum antennas is to realize that there will be different gain numbers for each frequency. For example, a multi-spectrum antenna that may be 9 dbi for 2400 Mhz (aka 2.4 Ghz WiFi), may only be 5 dbi for the 915 MHz spectrum.
Therefore, if looking at a multi-spectrum antenna, be sure to find out via spec sheet what gain is at 915 Mhz (for US mining) or 868 Mhz (for Europe mining). A seller may be character limited when listing a multi-spectrum antenna and may put the highest gain number in the title, but this may not be the applicable gain number for your desired frequency.