Do you mean driven array as in a log periodic or seperately fed multiple dipoles? A general rule is whenever you double the amount of elements on a yagi or dipole array, the gain increases 3dB. If you start with say a 5 element yagi, you have to make it a 10 element to get the additional 3dB and if you make it a 20 element yagi you will get another 3dB for a total of 6dB gain over the 5 elements you started with. If you start with a commercial 4-element exposed dipole array like a DB Products DB-408, you could add another 4 elements in the exact same fashion to get an additional 3dB gain and so on. In these cases all the antenna elements are active at a given frequency and work together to produce a narrower radiation pattern, which results in gain. A log periodic was all elements connected to the feedline but only a few will be active at any given frequency. The amount of elements and spacing between elements is a compromise of frequency range and gain. For example the Create log periodic that covers 100MHz to 1.3GHz has about half the elements and half the boom length of a KMA Rover that covers a similar frequency range. The Rover will have more elements active at a given frequency and more gain over the smaller Create log periodic. Surprisingly (or not), Create rates their log antenna at a higher gain than KMA, go figure.
prcguy
