Intermodulation is the unwanted mixing of two or more signals that produce new signals (products). The fundamental or harmonic energy from strong nearby transmitters intermix to create intermodulation products. Mixing can take place in the front-end of the affected receiver, in the Power Amplifier of one of the transmitters or through "external rectification or passive intermodulation" (in some electronic device or some corroded junction between two metals acting as a diode mixer).
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Intermodulation is the unwanted mixing of two or more signals that produce new signals (products). The fundamental or harmonic energy from strong nearby transmitters intermix to create intermodulation products. Mixing can take place in the front-end of the affected receiver, in the Power Amplifier of one of the transmitters or through "external rectification or passive intermodulation" (in some electronic device or some corroded junction between two metals acting as a diode mixer).
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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A circulator is usually a three-port device; any energy coming in one port is forwarded in one direction only, towards the next port. An isolator is a circulator with one port terminated into a dummy load. With the transmitter on port 1, the antenna on port 2 and a dummy load on port 3, RF from the transmitter is routed to the antenna but any outside energy picked-up by the antenna is diverted to the dummy load and thus cannot enter the Power Amplifier where it might lead to intermodulation. Magnetized ferrite material is used in the fabrication of circulators.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Third Order Intermodulation products where the second harmonic of one signal mixes with the fundamental frequency of another are the most troublesome because of the relative frequency proximity of the involved signals: filtering them out is difficult. Given two signals, F1 and F2, the Third Order IMD product can be computed as (a) "twice F1 minus F2" or (b) "twice F2 minus F1". In this example, IMD = 146.70, F1 = 146.52 ; solving for F2 in case "a" is (F1 times 2), minus IMD ; solving for F2 in case "b" is (IMD + F1), divided by 2.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In the context of a repeater installation, a duplexer is a specialized filter which allows operating the receiver and transmitter simultaneously on the same antenna. The duplexer is built with four or more quarter-wavelength cavity resonators. The duplexer provides isolation ( 90 dB or more on 2 m ) between the receive and transmit paths at the expense of insertion loss.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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"Undesired mixing of two or more frequencies in a non-linear device which produces additional sum and difference frequencies" (ARRL RFI Book). Final amplifier stage: signals from nearby transmitters may find their way into a power amplifier creating intermodulation. Receiver front-end: strong offending signals may enter the RF amplifier or mixer and create intermodulation. Passive intermodulation (PIM), sometimes called "rusty bolt effect": passive devices subjected to strong signals cause the problem; cables, antennas, connectors, dissimilar metals, etc. due to oxidation, corrosion, metal flakes, dirty mating surfaces or poor contact.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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