For Yagi antennas where the driven element is electrically connected to the boom like the parasitic elements, how can this function without diminished performance outside of the design frequency? In other words, how can it achieve any kind of bandwidth? Frequency is the centre of the 10 metre band. Antenna feed is via the usual gamma match.
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3 Answers
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The elements of a Yagi are dipoles. If you look at a dipole at any instant, the voltage at one tip will be some value, and the voltage at the other tip has the same magnitude, but opposite polarity.
So let's say at some instant, the left side is at +50V and the right side is at -50V. Consider these relative to the soil at the base of the mast, if you like. What about other points on the dipole? The voltage distribution is sinusoidal, so it must be 0V in the middle.
The boom and mast must also be at 0V (ie, the same voltage as the ground rod at the base) if we intend for these things to not radiate. So, the voltage at the ends of the antenna can oscillate symmetrically but the center remains always at 0V, not unlike a seesaw (except the function is sinusoidal, not linear). Since there is no difference in electric potential between the center of the elements and the boom, no current flows in this connection, so it doesn't matter if the impedance is zero (connected) or infinite (insulated).
answered Nov 1, 2020 at 14:26
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1$\begingroup$ @Andrew The feedpoint is a physical object with two terminals separated by a non-zero physical distance. The electric field potential at the point between them is zero volts if there's no common-mode current. $\endgroup$ Nov 4, 2020 at 15:40
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1$\begingroup$ @Andrew Consider that (for these purposes) the electric field is always differentiable, so if one terminal of the feedpoint is at a positive potential, and the other at a negative potential, there must be a zero electric potential, somewhere, right? If that's not in the middle of the antenna then where do you think it is? $\endgroup$ Nov 4, 2020 at 15:44
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1$\begingroup$ @Andrew and if you don't think the middle of the antenna is at zero potential, how do you think plumber's delight antennas work? How is it that a folded dipole can be feed by coax but without common-mode current? How does a folded balun work? There are quite a lot of antennas which demonstrably work in practice which rely on that point in the middle being at zero potential... $\endgroup$ Nov 4, 2020 at 15:46
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Bandwidth of a yagi is largely controlled by the diameter of the elements. Generally, the larger the diameter, the larger the bandwidth. However, the yagi is also helped by very slightly tapered elements.
The impedance matching network may also affect the SWR bandwidth.
The boom conductivity does not affect bandwidth, as well explained by Phil.
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1$\begingroup$ As you have correctly stated, boom conductivity does not affect bandwidth, but note that your comment "as well explained by Phil" is incorrect and this is not explained well in Phil's answer, in fact he doesn't explain it at all. $\endgroup$– AndrewNov 5, 2020 at 0:44
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The boom is like two end-fed right-angle fan-dipole elements for some completely different frequency added to the driven element. Elements of a fan dipole for a very off frequency typically do not take much of the Rf current away from the on-frequency resonant element of a fan-dipole.
My guess is that some RF current will travel down the boom (and back), but in opposite directions from the feed point at the same time. Thus the EM field generated by the front and back of the boom will cancel out, as well as being orthogonal to the beam direction, and thus have little to no effect on the pattern or bandwidth of the Yagi elements.
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$\begingroup$ The boom is at RF ground potential. Its sole function is to support the driven element, director, and reflectors. $\endgroup$– Mike Waters ♦Nov 2, 2020 at 3:54