Practical notes on the ferrite isolator
An isolator is sort of an RF lobster trap. Unidirectional, it isolates a source and load so that any reflected energy at the load is trapped or dissipated.
The photo below shows a single-section ferrite isolator. There are three ports, labeled “1,” “2” and “3.” Port #1 is the input port, port #2 is the output port and port #3 is the load port. The input (from the transmitter or source) is applied to port #1. The output (to the antenna or load) is taken from port #2. The dummy load is connected to port #3.
The direction of power flow is from input (port #1) to output (port #2) as shown by the arrow on the maker’s plate. Any power that is reflected from the antenna or from some other load appears at port #2 and is directed to port #3, where it is dissipated in the 50Ω dummy load. If the isolator is properly tuned, the amount of reflected power appearing at the input port will be small.
Figure 1 above shows a test setup in which a broadband noise generator is used to view the response curve of an isolator in the forward direction. The frequency response in the forward direction is quite broad, as shown in the photo at the right. Figure 2 above shows the test setup for viewing the response of the isolator in the reverse direction. The photo on page 28 shows that the return loss or isolation is highest at the resonant frequency of the isolator, but it is still good at frequencies well removed from the center frequency.
Normally, the isolator is used immediately at the output of a transmitter, as shown in Figure 3 on page 28. An isolator’s primary function is to prevent signals from nearby transmitters from entering its transmitter’s final stage where they might mix with the transmitter signal or harmonics to produce strong intermodulation products. Such intermod products might cause interference to collocated receivers. A secondary function of the isolator is to provide the transmitter output with a constant 50Ω load impedance. Because the isolator dumps off any reflected power to the dummy load, the transmitter sees a near-perfect load impedance.
It is important that the dummy load of the isolator be large enough to handle any power that is reflected back into port #2 of the isolator. Let’s say that the transmitter output is 100W, and that the line loss is 1.5dB. The power appearing at the antenna is about 71W. Assuming a worst-case mismatch at the antenna, 71W is reflected back down the line. Because this reflected power again encounters the line loss, the RF power appearing at port #2 of the isolator is 50W. If the dummy load of the isolator is rated at 50W, then the reflected RF power will be safely dissipated in the dummy load, and the transmitter will never see the mismatch. However, if the dummy load connected to port #3 of the isolator is rated at only 25W, then it will soon overheat and things will go haywire.
Heat is an enemy of the isolator. So, it is important that the power rating of the dummy load be sufficient to handle a significant level of reflected power. As a rule of thumb, the power rating of the dummy load is chosen to be about 50% of the transmitter power output rating. You will see dummy loads that are attached directly to the isolator and those that are mounted away from the isolator. Dummy loads that are mounted away from the isolator tend to reduce the amount of heat transferred to the isolator in cases of high reflected power.
Isolators can be connected in cascade, as shown in Figure 4 above, to achieve a higher degree of isolation. When isolators are connected in cascade, the dummy load on the isolator that is connected to the antenna should be of a higher power rating. This is because it is the first line of defense and will receive the full reflected power (minus the small amount of insertion loss between ports #2 and #3). An alternative to connecting two isolators in cascade is to use a dual-section isolator, such as the one shown in photo at the left. As part of the intermodulation panel, a dual isolator is more efficient because it is manufactured as a dual isolator. The tuning is also more precise, with less insertion loss. Notice in the photo on page 30 that the dual isolator has a larger dummy load connected to the side of the isolator that is close to the antenna line. Also, notice that the isolator is followed by a lowpass filter between the antenna line and the isolator. This is because isolators tend to generate weak second harmonics and should always be followed by a lowpass filter to prevent these harmonics from reaching the antenna.
Several precautions should be observed when installing an isolator. Don’t exceed the power input rating of the isolator. Use a dummy load that is at least 50% of the rated transmitter power output. If possible, use a dual-section isolator rather than cascading two single-section isolators. For maximum isolation, tune the isolator to the specific frequency of a particularly troublesome collocated transmitter. Follow the manufacturer’s instructions on mounting the isolator. Some isolators might be detuned when mounted on the surface of a ferrous metal. Always follow the isolator with a lowpass filter.
By using an isolator on the output of a transmitter, many interference problems can be avoided — especially at densely populated transmitter sites.
Until next time — stay tuned!
Contributing editor Kinley, MRT’s technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, with Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. Kinley’s email address is [email protected].