Antenna Tuners at the feedpoint or in the shack?
I had a question regarding the placement of an antenna tuner yesterday and was wondering what effect it would have placing the antenna tuner at the feed point of a GAP Titan vertical that is already somewhat tuned. He wanted to know if placing an automatic antenna tuner out at the feed point would lower the transmission line power dissipation. He didn't say what the exact SWR was at the feed point so I assumed two values of 2:1. So which will be more efficient? Instinctively I would say tuner at the antenna. But by how much?
SimSmith ApproximationsChoosing two cases involved picking two 2:1 SWR impedances that could be realized at the feed point of any antenna. This installation also involves 140 feet of Davis RF Buryflex coax from shack to antenna as well as 500W input to the coax from an Elecraft KPA-500 amplifier. (20m was assumed for this discussion). Conservative tuner element values were assumed as well as an L network. Most automatic tuners utilize this topology. And we are ignoring the non-perfect characteristics of coax and the stray reactance that is present in real tuners, etc.
I might add, each SimSmith "building block" contain information regarding it parameters. The R and X values are those values looking into the right side looking left. Also, the ^W symbol indicates the amount of power "burnt up" in that block. When you left click on that W symbol it toggles through a bunch of parameters, ^dBW, <-dBw, <-W, ^W.
^dBW is the amount of power in dBW burned up in that block.
<-dBW is the amount of power in dBW entering into that block from the right.
<-W is the amount of power in Watts entering into that block from the right.
^W is the amount of power in Watts "burned up" in that block.
The G block is the generator block, the transmitter, your rig on the right. The xMtch(a) is the setting that allows the generator to produce a constant level of power regardless of the impedance seen downstream, unaffected by reflections from an imperfect load attached to it. The (a) part is telling the source to be the value in Watts in the "a" field just below it. That way we can set the transmit power out with just one field entry.
Baseline Case: Matched Loss to 50 ohm load (85W dissipated in coax, 500W input)
Case 1A: 2:1, High Z (80+j33ohms), Tuner in Shack
Case 2A: 2:1, Low Z (27-j13 ohms), Tuner in Shack
Case 2B: 2:1, Low Z (27-j13 ohms), Tuner at Antenna
ConclusionThere is only a 15 Watt savings in power dissipation in the coax by locating the tuner out at the antenna, a 0.15dB difference. So as long as the antenna feed point SWR is somewhat low it's hardly worth the trouble.
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