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Yagi Udaa antenna on TV is fix or non-adjustable size but it receives almost 100MHz to 800MHz. But size of antenna should have to be exact according to antenna. How this yagi can perform best? Is it best?
If you look at the Yagi carefully, you will see that ones designed for VHF and UHF reception have 2 separate sections
a VHF section that covers ~ 50 MHz to ~ 300MHz and then the UHF section that covers ~ 400MHz to ~ 800 MHz
There's 2 basic tricks to making wide band yagi's
one is to make a log periodic yagi ... this is where the elements of the antenna start long at the back abd progressively get shorter towards the front of the antenna. ALL elements are driven ( active) They provide ~ 6dBi gain on any given frequency within the range that the antenna is designed for......
the other way is to have 2 yagi's on the same boom, one for VHF and one for UHF. like this one.....
with a dual yagi system like this, they are still NOT exact in their measurements. This can be got away with because the antenna is being used for receive only and altho there is a mismatch at frequencies other than what the elements are dimensioned for they still work well enough with acceptable losses.