[Elecraft] KXAT1 with dipole

[Karl Larsen]

Dan Presley wrote:
>>    Guys, my money is on the connector on the feedline. It must have a 
>> short developed since the last time you used it. Or the short might 
>> be at the antenna feed point.
>>
>>    The feed line length is just not a factor even when there is RF on 
>> the shield, and there is some.
>>
>>
> Sorry Karl-Ron had it exactly right.Feedline length is always a 
> potential factor; I've run into it before and just forgot.
    I'm sorry Dan, I have no idea what was wrong because I'm not there. 
But to say you changed the feed line and then it tuned up fine is not to 
me correct. I am an Electrical  Engineer and I studied feed lines and 
antenna design. I do things with a Smith Chart.

    If your dipole has an impedance of 72 ohms, which it really does at 
it's resonance frequency, then that impedance is what your feed line is 
seeing and it's characteristic impedance (RG-217) is 50 ohms. I plot 
both on my Smith Chart and look for a feed line length that will give a 
really bad low or high impedance at the transmitter end. I only need to 
look at the first half wavelength since it just repeats itself.

    I find the lowest impedance occurs at 240 degrees and is 28+j30. The 
highest is 120 -j12 at near a half wave. Now either of these should be 
possible for your tuner to tune. Your right the impedance changes 
between these values and if your very careful you can get close to 50 
+j0. But why?

    Now in my house I have a 88 foot dipole fed in the center with 450 
ohm windowed feed line of random electrical length. The SWR can get to 
12:1 on some bands and the loss goes up to near 5%. I get out just fine 
and am Net Control for several nets on 80 and 40 meters.

    I am sorry but my experience and training make it hard to understand 
your results.

73 Karl K5DI

PS: If you want the Smith Chart I can mail it to you.


> 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.