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The voltage at the center of a resonant half wave dipole can't be zero because the current is at a maximum in the middle. If the voltage was zero then there would be zero current right ?

How then can the type of yagi antenna where the elements aren't insulated from the boom work ?

Because apparently it's ok to mount the elements including the driven element at the center not insulated from the boom because as everyone knows the voltage in the middle is zero and so it's ok to join the elements directly to the boom.

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Without realizing it, you are asking the same question that was answered in this Ham Stack Exchange item. I considered referring to "plumbers delight" construction in my answer there, but thought it might be off-topic, so I refrained. The net is that the center of a yagi element is a virtual ground to which a conductive boom, as well as the coax feedline shield, may be connected.

Which is not to say that a conductive boom will be electrically inert. It is likely that element lengths will need adjustment when compared to their lengths for a non-conductive boom. How much adjustment depends on the details of the array: length, element and boom diameters, etc.

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The voltage at the center of a resonant half wave dipole can't be zero because the current is at a maximum in the middle. If the voltage was zero then there would be zero current right?

So you're saying zero voltage implies zero current?

schematic

simulate this circuit – Schematic created using CircuitLab

What's the voltage at the ammeter AM1?

If you're asking "which end" does it matter? What's the impedance of an ideal ammeter and what's that imply about the difference in voltage from one end to the other?

What's the current through the ammeter?

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The voltage at the feedpoint of a 1/2WL dipole is only not zero if the dipole is fed in the center. If the dipole is not fed in the center, the voltage at the center is indeed zero volts. For a 100% plumber's delight beam, the driven element is usually fed with a gamma match or a delta match and the voltage in the center is indeed zero volts.

The feedpoint of an antenna is not a point. It is actually two points. Halfway between those two points, the voltage is zero.

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