Two weeks ago I mentioned how good a two element yagi was on a gain for weight basis. Compared to a half wavelength dipole one could nearly triple one's effective radiated power and get better results on receive with just one extra element.
Nevertheless there are still times when you need a little more gain. When you're just on the cusp of readability an extra two or three elements can make your signal perfectly readable. That really counts if working aircraft scatter with low power or in events like VHF field days where your score increases greatly when more distant stations or grid locator squares are worked.
Bigger yagis can be difficult on HF. But on 144 MHz a 4 to 6 element beam can easily be carried places by hand. This is particularly so if you are operating from somewhere already in the clear that doesn't require you to carry a large mast as well.
4 elements will give you about 8 dBd, 5 elements will give about 9dBd while 6 elements will give close to 10 dBd. All are a noticeable increase compared to the 5dBd of 2 elements. Even the lowest of these, ie 3 dB up on what you had before, may increase your transmitting range by 50% or more. Not only that but the higher gain will let you work more closer in stations, including those running lower power and/or inferior antennas.
Interested? Some practical ideas for 144 MHz 4 to 6 element yagis are here:
* DK7ZB 4 element yagi
* IZ2UUF 100 gram 4 element yagi
* VK1AD portable 4 element yagi
* 4 element yagi dimensions (my own video - watch others linked to it)
* VE3SMA folding 5 element yagi (pdf)
* M0UKD 6 element yagi
* M1GEO 6 element yagi
PS: Want to read more about antennas and other amateur radio topics? These books could be of interest. Available in both electronic and paperback.