When excessive reflected power is detected, self-protection circuits in modern transmitters automatically reduce power to safe levels, and hence reduce the power of the signal leaving the antenna even more than the loss from some of the power being reflected away from the antenna.Īutomatic power reduction by safety circuits typically causes most of the power loss ( see below).īecause of this, feedline matching is a standard part of almost all radio transmitting systems. In high-power transmitters it may overheat the transmitter's output amplifier. Without a matching system, in addition to reducing the power radiated by the antenna, the reflected (or "backlash") current can cause signal distortion and overheat transformer cores. Use with transmitters Īntenna tuners are used almost universally with solid-state transmitters. If the impedance seen by the transmitter departs from this design value due to improper tuning of the combined feedline and antenna, overheating of the transmitter's final stage, distortion, or loss of output power may occur. Transmitters are designed to feed power into a resistive load of a specific value: 50 Ω (Ohms), by modern convention. In transmitting systems with an antenna distant from the transmitter and connected to it by a transmission line ( feedline), in addition to a matching unit where the feedline connects to the transmitter, there may be a second matching network (ATU) near the antenna or incorporated into the design of the antenna, to bridge the transmission line impedance over to the antenna's feedpoint impedance.Īntenna tuners are particularly important for use with transmitters. The system transmatch may be a circuit incorporated into the transmitter itself, a separate piece of equipment connected to the feedline anywhere between the transmitter and the antenna, or a combination of several of these. To avoid possible damage resulting from applying power into a mismatched load, and prevent self-protection circuits in the amplifier from cutting back the power output, matching networks are a standard part of almost all radio transmitting systems. In addition to reducing the power radiated by the antenna, an impedance mismatch can distort the signal, and in high power transmitters may overheat either the amplifier or the cores of transformers along the line. However the impedance of any antenna normally varies, depending on the frequency and other factors, and consequently changes the signal impedance that appears at the other end of the feedline, where the line connects to the transmitter. Transmitters are typically designed to feed power into a reactance-free, resistive load of a specific value: Essentially all radio transmitters built after the 1950s are designed for 50 Ω (Ohm) output. Īntenna tuners are particularly important for use with transmitters. English language technical jargon makes no distinction between the terms. Various alternate names are used for this device: antenna matching unit, impedance matching unit, matchbox, matching network, transmatch, antenna match, antenna tuning unit ( ATU), antenna coupler, feedline coupler. Its purpose is to optimize power transfer by matching the impedance of the radio to the impedance of the end of the feedline connecting the antenna to the transmitter. To get the rest of the way there I had make the rest myself.Front view of a modified ‘ π’ type antenna tuner, with interior partially exposed.Īn antenna tuner is an electronic device inserted into the feedline between a radio transmitter and its antenna. I had almost enough balanced feedline given to me for free to make it to the tuner. I also found that in order to not have arcing on 80m with my linear amplifer I had to update the capacitors for higher voltage. I mitigated the issue by unwrapping the wire on the wooden coil form and replacing it with plastic zip ties. time after applying power, culminating in the arc shown here. This at first manifested as an increase SWR vs. I later had issues with arcing, presumably due to a high impedance at the end of the transmission line, especially on 20m. The link coupling is adjusted using a loading capacitor.Ī few images of the coil winding process. The balanced feedline connects to a parallel-tuned circuit, which is link coupled to coax going to the transmitter. The hard part was winding the 4" coil out of 8 gauge wire, which took 3 days! But that is a different story. It is very simple and I was able to assemble the circuit in afternoon. (Transmatch/Antenna Tuner)Īfter some searching on the web I came across the K1JJ design, posted to the AM forums. I recently put up a doublet antenna, and I quickly realized that I would have the best efficiency with a proper balanced antenna coupler. Homebrew Balanced Antenna Coupler Go Home
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