(May 2 2006)
A yagi antenna is a high Q resonator. There is a lot of energy stored in the electromagnetic field surrounding the elements and the bandwidth is relatively narrow. This is the price one has to pay to get the very high gain figures associated with yagi antennas.
Saying that the Q of a yagi is high is the same thing as saying that the impedance of the elements is low or as saying that there are very high currents flowing on the elements in relation to the magnitude of the forward far field.
Among other things this implies very strong electromagnetic fields in the near zone compared to the far zone. The yagi antenna can be described as a group of radiators that are arranged in such a way that the electromagnetic field cancels more or less in ALL directions. Cancellation is not good in the forward direction so some energy can leak out there, but cancellation is nearly perfect in all other directions.
Because of the high currents and the associated strong near fields a yagi antenna is very sensitive to conductors or materials with permeability above that of vacuum placed anywhere where the magnetic field is strong (at the element centers). The high Q also causes correspondingly large voltages at the element ends and a yagi antenna is also very sensitive to conductors or materials with a dielectric constant above that of vacuum placed near the element ends.
It is well known since 50 years or so that a yagi antenna that was designed on a wooden boom will not behave the same if the elements are moved to a metal boom. It was found at a very early stage that one can compensate for the influence of the boom tube by making all elements a little longer. In some old book (abt 1955, we do no longer have it) it was suggested that the elements should be lengthened by 60% of the boom tube diameter for an element in good electric contact with the boom tube.
With time more detailed information has become available and graphs showing the boom correction for elements in contact with the boom as well as elements through insulated holes can be found in many books describing antennas.
The boom tube changes the magnetic field near the element center. An element in electric contact with the boom behaves more or less as an element with varying diameter. The center part, with a length similar to the boom tube diameter has a diameter comparable to the boom tube diameter. The thicker diameter reduces the inductance so the length has to be increased to add a capacitance that will restore the same overall characteristics.
Equivalent elements are elements that have the same phase between an incoming wave and a reradiated wave. This means that equivalent elements (in a yagi structure) have the same ratio between the real part and the imaginary part of the impedance. Compensation is not 100% because the longer element that goes through a boom tube and that has the same phase will have a slightly different ratio between the amplitudes of incoming and reradiated waves. The amplitude errors are small and do not cause any problems on the medium size yagi antennas that we plan to put in production.
The boom tube affects the reactive part of the element impedance by allowing less magnetic field near the element center. The reduced magnetic energy means a lower inductance. The coupling to the far fiels is also changed, part of the magnetic field is screened by the boom tube causing a lower resistive part. When the length is increased to compensate the lowered inductance, the coupling to free space is increased which causes an increased resistive part.