Sidebar Sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

antenna lenghts

What I mean by normalized impedance is

z = Z/Zo

So normalizing 1.3-j0.3 --> R = 65 and X = -15
(multiply by the nominal impedance)

Most smith charts plot Zo as 1 + j0:

2CxG7DcIuL2.jpg


e5600a_smith_chart_util.gif


If you want to read about the mathematical derrivations for the smith chart:

http://murray.newcastle.edu.au/user...ct/finalreport/finalreport.html#_Toc422669170

When I talk to the engineers around here, I have to use a de-normalized impedance since the nominal impedance can be anything. It's not just a 50 Ohm world.

And I'm not entirely sure about the terminology here. I know the impedance is made up of a resitive component AND a reactive component, so when you rationalize the figure, do you still call it the impedance?

Maybe we should refer to one as the complex impedance and the other as simply the impedance, or magnitude. Symantics, really.
 
Oh yes, BTW, the simplified VSWR = Zl/Zo only applies when the load is real (resistive).

So, freecell, wise, huh? engineers, huh? yeah, 1.9 is the WRONG answer. try 33.8:1 VSWR at the load for 50 ohm line.

33.8 is also wrong, but so was 1.476.

The actual VSWR would be 34.2 using the following formula:

VSWR = [1+ (Zl-Z0/Zl+Z0)]/[1-(Zl-Z0/Zl+Zo)]


With the same load we had before, z = 1.3-j0.7, normalizing would be:

Z = 65 -j35

Using the simplified formula, the magnitude is 73.824

Then if we think we can use VSWR = Zl/Zo, the answer is

VSWR = 1.476, which is wrong!

The VSWR is actually 1.9, which is what was originally posted by the engineer.
 
these are the necessary calculations required to determine normalized impedance....z = Z/Zo = [1 + ρ]/[1 - ρ] is a function of ρ, and so its real and imaginary parts, z = r + jx, can be expressed in terms of the real and imaginary parts of ρ = u + jv.

the only way you or anyone can begin to understand any of this is to familarize yourselves as thoroughly as possible with the values represented by these expressed variables: z/d, Z, Zo, p, r, j, u. x and jv.

while most smith charts are plotted using Zo (Zo representing the characteristic impedance of the feedline) as 1 + j0, we're in the real world here and we're dealing with 50 ohm feedline. i suggest that you adjust your use of the chart accordingly.

don't waste my time copying and pasting partial clips of search engine references or partial formulas to miscellaneous pieces of a picture that you obviously don't grasp. i provided the correct answers to your questions based on a feedline Zo of 50 ohms and solved for Z and SWR based on the same when you made absolutely no mention of an alternate value of Zo. i naturally assumed that you adjusted the SC for Zo = 50 + j0. stupid me.

this thread was titled antenna length and the discussion was about resonance so as you can see we've wandered way off of the beaten path.

this entire exchange is going nowhere, absolutely pointless, incongruent and is not relevant to the subject under discussion. now quit wasting my time. if you want to make a contribution to the original topic (or just argue) then go ask your wise engineers to tell you how the resonance of an antenna is determined and don't forget to fill them in on all the details. if they're going to insist on using a smith chart for whatever reason then let them know that for our purposes here that Zo is equal to 50 ohms, (50 + j0) not 1 + j0.

in any tuned circuit (antenna) when Xl - Xc = 0 or X = 0 then resonance is established at the frequency where this occurs. it's a 50 ohm world here in the topic under discussion. what any of all of this that you've been talking about has to do with a 1/4 wave antenna with 50 ohm feedline and the occurrence of resonance in the antenna at specific frequencies and the physical lengths associated with them i have no clue. let me know if you find any feedline with a Zo of 1 ohm. if you want to pursue whatever it is you're trying to prove here i suggest that you start your own topic thread. don't forget to mention what the point is that you're trying to make and if you want to get into the math at least make an attempt to demonstrate your understanding by taking the readers step by step through all of it including the calculations, explaining the notation and proceeding in a logical and flowing manner.

thanks for all of the confusion.
 
The points were that the 246 constant you use is not the correct value for determining resonant length in a metal.

and your VSWR value of 33.8 is WRONG, regardless of it being related to the post topic.

Sometimes your responses are just unpleasant just for the sake of being so. I don't have the ability to waste your time anyway. You make that decision on your own accord. If you don't want to try and help someone, why don't you just stay out of it and don't be rude?

I cannot adjust my smith chart because it is 1) on laminated card stock, and 2) is fixed in the instrument software in which it is presented. So when I post images of a measurement trace on a smith chart, Z0 is represented as 1, which in the real world makes sense to me since transmission feed lines and sources do not all have a characteristic 50 ohm impedance.

Thanks for the confusion? It's not like your making things clear either. I'm sorry to not be as so enlightened as you, Mr. omnipotent, freecell, and that I don't present things to your liking, sir.

But if I can't teach you anything, I think Beetle could teach you a world's worth, whether he has any more technical knowledge or not.

"you obviously don't grasp."

Thanks for extending the credit, professor. It's likely I'm frustrated enough with it now that I probably never will.
 
"The points were that the 246 constant you use is not the correct value for determining resonant length in a metal."

so the smith chart was for what then exactly?

you're looney. i'm working with an actual model (real antenna) and i'm telling you that at 108" the antenna is resonant at a specific frequency in the citizens band. i know this because there are equal amounts of both types of reactance present at the frequency in question. when i shorten it to 103.3" resonance moves to a higher frequency further up the band. your comment is meaningless. you're posturing from your chair and i'm here looking at it and doing it. i told you once before that the VF of the metal is only one of many factors affecting the physical length required for resonance in an antenna. anyone who thinks about it long enough can come up with many more.

talk all you want. i'm finished with this thread.
 
What do you mean when you say "model" ?

Is that a real antenna or a computer model?

I'll have to go back and look to see exactly why the smith chart came up, but I'm sure it has something to do with resonance occuring when Xc and Xl being equal.

<stupid comments>And thank's. I like being looney. And it's probably better to posture on one's ass than to talk out of it (not saying anyone is, mind you).</stupid comments>
 
Oh yes, I brought up the smith chart after Beetle so eloquently provided the means for solving the impedance's magnitude:

"I thought it was the sum of squares, but was not completely sure. It makes a little sense considering that constant impedances plot circles on a smith chart."

p.s., I dint bother to edit the quote above, but yes, I should have said the square root of the sum of the squares, lest I be accused of not paying attention again.

I'm sure there is something else in that statement that you can pick on though.
 
Oh yes, and speaking of models, the point was that I was going to also make some actual measurements when I had time to put my antenna back on my car and get out the equipment.

But I'd like to ask then, and I did allude to this earlier, that there are other factors that affect this measurement.

Are you measureing resonance with 108" because that is the resonant length or becuase other factors are making it look that way?

That is what the tuner comment was about, in case you wondered.
 

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom
      Back