The goal of detuning is not to avoid resonance, but rather to minimize re-radiation and/or current in the interfering structure.
When large enough, structures can re-radiate and cause severe pattern distortion even when they are NOT resonant. Consider, for example, how effective a large non-resonant screen or solid disk is at reflecting signals even when it is nowhere near resonance.
We can minimize re-radiation by making an area or areas of the structure "electrically vanish". We often call this "de-tuning", even though it is more correctly electrical trapping or sectionalizing of a structure.
Most structures or towers, when detuned, have a section adjusted to represent a parallel tuned circuit.
Section A and B carry out-of-phase currents. Picture the current flowing upwards in A. It must then flow downwards in B. Since it is a closed loop, these out-of-phase currents are equal and flow in opposite directions at resonance. The result is radiation from sections A and B cancel each other. When section A and B are exactly resonant, sections D and C are isolated by a high impedance. The high impedance is caused by or related to the high current though the capacitor and the inductance of section A. When current is maximum, voltage drop is maximum.
This results in the electrical structure on the right, with section A and B removed!!
In effect, we have created a trap much like the trap in a dipole. As in the trap dipole, current is maximum in the trap at the trap's resonant frequency.
NOTE: The condition of proper tuning occurs with MAXIMUM current in section B, NOT minimum current!! To electrically sectionalize the tower and isolate C and D (and minimize radiation from A) section B must be tuned for MAXIMUM current!
As either section C or D approach resonance by themselves, the tuning condition will change. This would occur when D is grounded and near 1/4wl or an odd multiple of 1/4wl long, or when C (with whatever is mounted on it) is self-resonant with section A removed!!
Under this condition, you would either need to sectionalize and detune C or D with additional detuning, or move the location of sections A and B to a new point that (when isolated) prevents resonance in C and/or D.
A few general rules apply. Pay attention to these guidelines to insure best results:
Note: I added the section about cables on Feb 17,2003. I'm surprised cables are often not grounded at the top and bottom of tall towers, and that unshielded control cables are not passed through the inside of towers. Cables should always be treated that way for lightning protection if for no other reason!
The amount of capacitance and the voltage rating of the capacitor is not easy to predict. The size depends on unwanted power levels that excite the detuned structure, the electrical characteristics of the detuned structure, and the Q of the detuning section. Capacitance values will be fairly high with short sections on lower bands like 160, almost certainly in the range of a few thousand pF for ~20 foot long sections. The exact value would depend heavily on dimensions of the A to B loop.
Voltages across the capacitor are generally not high, although they can be at times. The "loop Q" of A and B affects voltage, as does the amount of excitation and load presented by the impedances of C and D.
MFJ sells a clamp-on calibrated current meter that will not perturb the system. It is a cheap version of a current meter I designed. This is a calibrated meter with internal amplifier that measures current from a few mA to 3 amperes, not the uncalibrated RF-sniffer commonly sold. Some RF-sniffers, including those by MFJ, actually change the impedance and resonant frequency of the system because the pick-up transformers are not properly designed and terminated current transformers. Avoid loop-stick type current meters, since they measure ANY external field and can provide misleading results. Use a current meter that is directly inserted in line B, or clamps around line B with the closed core of a terminated current transformer. Use a meter that does NOT perturb the system when removed!
Lacking a current meter, it is possible to tune this system with a grid dip meter, by forming a small one or two turn coupling loop. As an alternative, the loop can can be broken at any point near the capacitor and a MFJ-259 or similar antenna analyzer connected in series. Proper adjustment is at the point were minimum impedance occurs. If that impedance is not low, you probably are not effectively detuning the structure.
Multiple Stacked Antennas or Tall Structures
When multiple stacked antennas are used, especially on a fairly tall tower, it may be necessary to sectionalize multiple points. Individual sections between antennas can be resonant, or appear electrically long.
If the tower or structure or any part of the structure or tower becomes resonant when section A is tuned to present a high impedance, then we need to move section A or tune it to some condition other than maximum current (resonance). Adjustments under this condition can only be made two ways: