[600MRG] Scopematch question

[J Mcvey]

Frank had a very good scenario for how to calibrate and use the scope match using X-Y and liiajous patterns to determine phase.
https://en.wikipedia.org/wiki/Lissajous_curve

  
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Lissajous curve
 which describe complex harmonic motion. This family of curves was investigated by Nathaniel Bowditch in 1815, an...  |   |

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However, the coax mismatch at the antenna has it's own "transformational"  consequences , so the length of the coax will effect what impedance you see at the shack end.The only way to get the  approximate  antenna Z in the shack would require a lamda/2 length of coax .However, that is not practical to do when the that length would have to be 315 METERS X VF.. So if your coax has a VF of .66 , you would need about 208 METERS of coax for the actual antenna Z to be seen at the shack!
Shorter lengths will have varying results . In fact, it may be possible to use the cable length to transform the impedance somewhat. None the less, you will need to know the impedance at the feed point before any of these calculations can be done. 
Knowing the staring Z, the Smith chart could be used to find what  length of coax might work as a matching device.Other than that, do the best match at the antenna to minimize the coax reflection issues.

In other words, you have an antenna SYSTEM, which includes the antenna, loads, and feed line. All three are interactive and will affect the impedance presented at the shack.
Jim AC2EU






 

    On Tuesday, December 12, 2017 12:49 AM, N1BUG <paul at n1bug.com> wrote:
 

 Pat,

I too miss a lot of things because I am interested in, and 
distracted by, a lot of different things! And sometimes I fail to 
see or understand what is right in front of me, which seems to have 
been what led me to ask my question about this.

You are correct. I would not use this method for calculating EIRP 
regardless whether there are matching or coupling circuits at the 
antenna.

I like to understand the technology I use to the fullest extent 
possible. Learning and understanding is part of the fun and the 
challenge for me. I was curious to see, in the absence of any 
matching or coupling devices at the antenna, if one could measure 
with a scopematch, calculate R + jX, transform this to the other end 
of the coax (in my case using a Smith chart) and get a reasonable 
approximation of how much inductance to add/remove for resonance 
and/or what the R component to be matched is. To me it is an 
interesting, if not the most practical exercise. Obviously it's a 
lot more work than looking at the antenna with an analyzer! I'm 
doing both and comparing results to see if I've got this process 
into my head correctly.

73,
Paul N1BUG / WI2XTC



On 12/11/2017 03:03 AM, patrick hamel wrote:
> I may be missing something. But then I miss a lot of things
> because I am interested in, and distracted by, a lot of different
> things. As I remember; The measurements at the transmitter can
> show what is at the other end of the transmission line with a
> Smith chart, but, unless you feed the coax directly to the
> radiator with no other matching inductances or capacitors between
> feedline and radiator you won't be able to determine the R, X,
> and I where it leaves the tuning box which is what the FCC says
> you must do to show you are EIRP legal. Rr and X may be
> calculated, Rtot and I are measured, but it must be at the
> radiator. . The only way I found to measure my radiator was with
> the Autek vector analyst, I know there are newer and possibly
> better analysts out there, but the steps are the same. For a base
> loading coil first measure the radiator and find the frequency
> where X=0, add coil until X=0 at the frequency you want to tune
> for, measure Rtot, then ground that point and add a link coil. If
> you have turns left below the grounded point you can use them to
> match the feedline to the coil. 73, Pat W5THT & WD2XSH/6

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