[Elecraft] [K1] Question about antenna tuners that I should not have to ask but do.

[Douglas Hagerman]

Does anybody have a “worked example” of how to use the L and C values reported by the K1 internal antenna tuner to analyze an antenna? I feel pretty guilty asking this, because it’s sort of a “please help me do my homework” question. And I am supposed to know how to do this homework.  :-)

I have a space-limited dipole for 20 meters with drooping ends. The KAT-1 tuner will tune it. What I want to know is whether to use a 4:1 or 1:1 balun, or no balun, at the antenna.

If the tuner reports 24 “x10” pF, is that 240 pF or 2.4 pF? A straight reading suggests that it’s 2.4 pF, but I don’t see how the tuner circuit can provide so little capacitance. But C4 (82 pF) plus C5 (150 pF) gives 232 pF which could 240 pF if you include some parasitic capacitance, maybe?

Next, I need to figure out the circuit. If the tuner reports nt 2, I think that means that the coil is next to the tuner’s antenna connector, and the capacitance is in parallel with the radio connector. I guess that is designed to be 50 ohms of pure resistance, so I have a nice little circuit with one of each part. I can figure out the various reactances, etc., and combine them, but am not sure I’m doing it right. That should give me the reactance at the tuner’s antenna connector.

Then I have about 12 feet of RG-8x coax, so I can use a Smith chart to work out how the reactance at the tuners’s antenna connector is transformed to the antenna connection, but there is always the confusion about which way to go around the outside of the chart. And then take into account the 4:1 balun that’s on there. In theory, all of this should tell me the antenna's impedance at the point where the balun connects, which should suggest whether it would be better to use a 1:1 balun instead.

Does anybody have an example of this sort of calculation? I have looked in the Antenna Handbook and other sources and they all dance around it; I’m looking for something practical and cookbook-like.

Or I could spring for a 1:1 balun and try it.  :-)

Thanks!

Doug, W0UHU.