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I’m going to put this in homebrew since most home brewers are into “how it works”. I’m not going to build one of these beasts.
A few weeks ago I was talking with someone about the Elk portable satellite antenna, which is a log periodic and asking how/why it works. Maybe I was asking Fred. I was wondering whether it was just a very wide bandwidth or if it had two lobes at 2m and 70cm. Well this week, ARRL’s “The Doctor is In” podcast addressed this issue in a more general way. https://www.blubrry.com/arrl_the_doctor_is_in/39781083/log-periodic-antennas/#autoplay
The general idea is that the log periodic has elements that are ALL driven (unlike a yagi where most are parasitic). The elements are of different sizes and only the size that tunes up with the frequency you are using is really radiating (or absorbing) much energy. Of course you want to design the sizes very cleverly to get other characteristics–for example so that multiple elements cause gain at various frequencies.
So essentially the answer is that the LP is in general a wide bandwidth antenna, although design choices will make lobes and/or different amounts of gain at different frequencies.
Back to the Elk: A careful look shows that there are two parallel metal booms insulated from one another with the elements to the left going off the top boom, and the elements to the right going off the bottom boom. I always thought this was odd, but now I can see that it is just a way to drive all of the elements. Fascinating! I’m not even going to ask about balanced vs unbalanced at this point 🙂
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