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A yagi is an antenna made from a number of parasitic, parallel rods. There is usually a single driven dipole with a slightly longer (reflector) behind the dipole and a number of slightly shorter (director) elements in the front. Typical gain values are between 10 and 20 dBi depending on the details of the antenna.
One interesting aspect is that Udi (who invented the antenna [and did not speak English] that was publicized by Yagi [who did speak English]) envisioned the antenna as a wave propagating through a periodic media while antenna engineers in other parts of the world thought of phased arrays with parasitic excitation.
During the 1939-1945 war British engineers were able to trade gain for bandwidth and got about 40% bandwidth for a few dB of gain reduction.
I am surprised to hear that someone got that much bandwidth out of a Yagi. I have not tried to push one that far. I expect that the limitation is the dipole and it's bandwidth. With the usual tricks for increasing bandwidth it sounds feasible. Actually, if someone did it it is feasible. Any idea of how it was done? It might be interesting to look over the design.
If I had to go there (extend the dipole bandwidth) I would try fat elements and maybe a parasitic sleeve.
I would think that a LPD would probably be an easier path to broad frequency coverage. Many octaves of frequency coverage are not too hard to come by and the patterns are similar. The construction is also similar and feeding the LPD might be even easier than the Yagi. We could drive ourselves crazy debating that point.
I guess this is another case of where Yagi and Uda were not properly credited for their work. It happens all the time. I think their contribution was demonstrating and exploiting the idea that parasitic rods can be used to shape the radiation from a dipole. It is relatively easy to significantly increase gain. There was an interesting link to Wikopedia (vphone above pointed to it) and some interesting history on the antenna cited. I was not aware of the WWII connection. The Yagi-Uda has sure been around for quite a while. This was well before the explosion of broandband techniques discovered in the 60's.
By the way, what is the significance of the red and green bars? Mine seem to change mysteriously. I haven't figured out where the secret description is hidden.
I read the 40% information in a old book. I was not a practicing engineer as far back as 1940 so do not have personal experience.
Almost no one gets credit for what they did. Darwin plagiarized ideas from his grandfather's writings. The Gilbert mixer was around in the valve/tube era and discrete transistor models were sold before Gilbert did any publishing.
Even people quoting the past are credited with the idea. US President Kennedy quoted some ancient Greek or Roman about "Ask not what your country can do for you. Ask instead what you can do for your country." The journalists were so ignorant of the classics that they missed the original source and thought that President Kennedy was clever.
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