Doug,
I think you are "doing it the hard way", but what you have proposed
along with some math will result in the correct information to give you
the impedance at the shack end of your feedline. That is if and only if
the KAT1 tuner has tuned to an SWR=1.
To figure the impedance at your antenna feedpoint, you will have to know
the type of feedline as well as its length and feed that information
into the formulas or an application such as TLW - (transmission line for
windows).
The easier way is to beg, borrow or steal an antenna analyzer and
measure the impedance at the shack end of the feedline. You will still
need to apply the feedline parameters to determine the antenna
impedance, and the feedline will act as an impedance transformer.
BTW, 24x10 is 240pF.
73,
Don W3FPR
On 9/28/2015 7:37 PM, Douglas Hagerman wrote:
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.
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