Changing feedline length makes two important differences. One in the
internal path length (which makes an impedance transformation) and one in
the external path (if there is not an adequately effective balun at the
feedpoint). The impedance of this outside path can vary over a considerable
range, and depending on the impedance of the antenna this extra wire
connected to the antenna can have a wide range of effects. Model your
antenna as a simple vee with a source in the middle, and then attach a wire
equal in length to the feedline and see how the current and impedance are
disrupted. The extra wire (outside of the coax) is end loaded by the rig
and whatever else is at that end of things, so it is hard to model
precisely. The end loading will make the extra conductor much longer than
actual length, and it may be strongly coupled to earth.
-- Alan wb6zqz
At 06:42 PM 8/23/2006, Don Wilhelm wrote:
I believe two points need to be made about this:
First, if the SWR on the feedline was 1:1, changing the length of the
feedline would have no effect. The conclusion is that the original SWR had
to have been something other than 1:1.
Secondly, the inductance and capacitance range of the KXAT1 is limited (3
inductors and 3 capacitors), the result of that is the impedance matching
range decreases as the frequency is reduced. On 80 meters, the matching
range is quite narrow, a bit better on 40, but on 30 and 20, the range
becomes broader so that one can match most anything on 20 meters.
73,
Don W3FPR
> -----Original Message-----
> Sorry Karl-Ron had it exactly right.Feedline length is always a
> potential factor; I've run into it before and just forgot.Both with
> coax and ladder line sometimes you have to add or subtract a bit for
> impedance mismatch. You can look it up in the ARRL handbook In fact,
> you can even do a fair matching job with ladder line bypassing a
> tuner with the correct lengths.I added two feet of feedline and it
> loaded right up sweet as pie. I also should have realized it wasn't a
> shorted feedline as it still took some power and signal strengths
> were not hugely different as when there's nothing connected.
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