Is the voltage at the centre of a dipole zero only at it's resonant frequency?

Question

For transmit, the voltage at the center of a dipole not cut in the center and fed with coax and a gamma match is zero in the center at the resonant frequency. This is why apparently a 'plumbers delight' yagi with the driven element connected to the boom works ok, because the voltage in the middle is zero.

And then if the voltage is zero in the middle you don't have to worry about common mode current flowing down the outside of the coax because at the point where the outer of the coax joins to the middle of the driven element the voltage is zero and no current can flow anywhere.

So what about when you move the frequency of the signal applied to the feed point away from resonance, then :

1. Do you get current on the outside of the coax now because now the voltage in the middle isn't zero anymore ?

2. And if you are outputting harmonics then will these result in corresponding current on the outside of the coax because voltage in the antenna caused by the harmonics are not at the resonant frequency and so that voltage won't be zero in the middle at the harmonic frequencies ?

3. And also does this all help to make the antenna want to have a narrow bandwidth ?

Answer 1

First let's clear up a misconception:

And then if the voltage is zero in the middle you don't have to worry about common mode current flowing down the outside of the coax

This doesn't seem quite right. Indeed, if the voltage in the middle of the dipole is zero and the boom and/or coax shield are attached there, then there won't be any common mode currents. Likewise if there aren't any common mode currents on the boom or coax, then wherever those things are attached must have zero voltage.

But one does not guarantee the other. For example, take a vertical monopole, and put a horizontal bar across the top such that it makes a "T". Now arguably, this added top bar could be considered an "uncut dipole", but does that mean there's no voltage in the middle? Given that voltage is highest at the tip of a monopole, how can both these things be true?

Now back to your original question; if the voltage at the center of a dipole is zero volts, you could say it's because (by design) there are no common-mode currents. The dipole is symmetrical, and any current is (again by design) only differential-mode, so at the center the voltage must be zero since there's no way to be both symmetrical about this point and have zero common-mode current.

Is this still true if the dipole is off resonance? Well, I suppose that depends on what's ensuring common-mode current is zero, and if it still works off resonance. If common-mode current is zero (or at least, negligible) because of a broad-band common-mode choke, then probably yes, voltage at the center of the dipole is still zero at least over the effective range of the balun.

On the other hand, if the antenna relies on something like a sleeve balun, this works only at a particular frequency, probably the same one or close to where the dipole is resonant. Away from this frequency the balun will be less effective and we'll start seeing common-mode currents, and this means the center of the dipole will no longer be at zero volts, much like the "dipole" attached to the top of a vertical.