Connecting An ATU
Typically the ATU is connected between the antenna and the radio transmitter or receiver. More specifically, the ATU can be connected between the feedline and the antenna to minimize loss, or between the feedline and the radio for convenience. However, SWR in the feedline must be considered in this second configuration.
It is a common misconception that high standing wave ratio (SWR) in and of itself causes loss. This is not the case. An antenna with a high SWR (e.g. 4:1) when properly configured with an ATU could have only a few percent additional loss compared to an intrinsically matched antenna. The ATU essentially redirects the reflected energy back along the feedline and antenna path. The additional losses come from the inherent losses within the feedline and antenna itself. SWR causes feed line losses to be multiplied. Low loss feedline would have minimal loss when tuned with an ATU whereas a "lossy" feedline/antenna combination of the same SWR could have significant loss.
Without the ATU, SWR from a mismatched antenna could cause reflections of power back into the transmitter, which will cause heating in the transmitter and significant power loss.
While modern solid state power stages do not like an SWR above 1.5, in reality an antenna VSWR of 2 means 11 percent of power is reflected and 89 watts sent forward through the antenna. Tube amplifiers using a Collins filter could easily remedy this.
Read more about this topic: Antenna Tuner
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