Multiband Antenna Current Flow
Cushcraft, hy-gain, and MFJ Antennas
This is a simplified explantion of current flow in antennas. There will be no technical jargon, which tends to cloud the issure. It should be fairly helpful in finding the antenna problem. It is not meant to be strictly accurate, only a guide line. In this article, only dipoles or monopole antennas are considered. A monopole antenna is basically half of a dipole, with the other side defined by the earth ground or a counterpoise. Beam forming by reflectors and directors is not covered here, for simplicity. But, all of the info here does apply.
First, consider current flow in a single band dipole or monopole antenna. The current flows in at the feedpoint and moves out to the end of the elements in a series fashion, like water flow. I like to think of it as sap flowing in a tree. If any break is encountered, the flow stops, and reflects back, creating a bad swr situation. If the break is not entirely fatal to the current flow, then the path distance is changed, changing the resonant length, and the resonant frequency.
Second, consider a multiband dipole or monopole(vertical), using a standard three band trap antenna covering 10, 15, and 20 meters. Current flows from the feed point and out to the end of the conductor. The end of the conductor is set by the first break that applies at the operating frequency. For simplicity, you can draw each band separately, with its' conduction path. For 10 meters, the end of the conductor is located at the 10 meter trap. It can go no further, as the trap electrically chops off the current at the 10 meter trap physical location. Nothing beyond the current flow exists. It is as if it were chopped off with an axe. The current flow length defines the wavelength, which defines the frequency. You can draw the current path for each of the bands.
A special case occurs if the trap is electrically open, and defective. From a current flow viewpoint from the transmitter in this case, the current flow is the same, and stops at the same point, or very close physically. The internal trap design determines the exact stopping point of the current.
To a certain extent, the resonant frequency of the trap will affect how much of the current is stopped. If the active frequency is the same as the trap frequency, almost all of the current is stopped. The Q of the trap will determine how effectively it does its' job. If the trap is off frequency, or the operating frequency is far from the trap frequency, the amount of trapped(stopped) current will start to be reduced. Beyond a certain frequency, the trap effectively disappears, and only adds inductance or capacitance to the downstream bands. This is what you want for multiband operation with a shared current source.
At a transmitted frequency of 15 meters, the current flows through the 10 meter trap, and flows out to the next current blockage, which is now at the 15m trap.
For 20 meters, the current flow continues to the next blockage, through the 10m trap, like it is not there, and then through the 15 meter trap, until it reaches the end of conduction at the tip.
This continues through any remaining traps, such as 40, 80, and 160 meters until the current reaches the end of the line. The physical length is greatly shortened, compared to a full size unloaded dipole.
AA5MT