Sunday, March 3, 2019

So much gain for so little metal - the 2 element yagi

If you're after the most dB gain for the least metal, the two element yagi cannot be beaten. A half wavelength dipole and a parasitic element spaced 0.1 to 0.2 wavelength in front or behind it is all it is. That will give you about 5dB forward gain over a dipole - equivalent to tripling your transmit power.  Plus, due to the sharp nulls off the sides, reception will be better as noise is reduced.

Sure, adding a third element will increase gain further; 7 to 8 dBd is usually quoted. And you will get much better rejection off the back. But three elements requires a boom is twice as long - something that might not go down well with sensitive neighbours around. And your tower will need to be heavier duty to support the increased weight. When you add all this up a two element beam can be a good proposition, with the performance difference versus three elements falling greatly if its lightness permits it to be erected higher. 

The gain per metal economies of a two element beam are particularly apparent when compared to other antennas. Consider for example the quarter wavelength ground plane that 27 MHz CBers often used at home.  That had one vertical element and three drooping radials. A total of one wavelength of element stock. All for 0 dB gain over a dipole.  It's a similar deal with other antennas such as folded dipoles and J-poles. Whereas about the same amount of element material made into a two element yagi will give so much more gain.

Excited? So you should be! Follow these links for some two element beam ideas applicable on bands from low HF through to high VHF.

* WB2VUO 2 element direct feed yagi
* 2 element parasitic yagis by DK7ZB
* M0MCX bidirectional 40m wire yagi
* VE7CA's portable two element triband yagi (pdf)
* More VE7CA portable yagis (pdf)
* 2 element wire beam for 24 and 28 MHz by KL7JR
* IW5EDI 2 element yagi for 28 MHz
* K3MT rabbits ears 2 element beam for 144 MHz (pdf)
* Folding beam for 144 MHz portable (video)

PS: Want to read even more about beam antennas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.

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