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  #31  
Old 02-14-2022, 02:44 AM
Cumulo Cumulo is offline
 
Join Date: Apr 2015
Location: KHMT
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Quote:
Originally Posted by TASEsq View Post
Here is the plot of the analysis - if anyone knows what the second graph means please elaborate!

https://drive.google.com/file/d/1h5q...w?usp=drivesdk
OK, elaboration follows: ;-)

The instrument your Ham friend used looks like a somewhat upscale NanoVNA, good device for this application.

The three traces of the 2nd plot look like impedance, resistance, reactance.
Impedance is the vector sum of the reactance(not good) and resistance(good).

This is the basic info from which VSWR is calculated. This resistance BTW is not ohmic resistance but rather the effect of RF power radiated to or from the antenna. (Antennas are completely reciprocal.)
Edit add:
The lower trace in the 2nd plot I believe, is the reactance and from what I see it is capacitive (-ohms) across the band, so increasing the series capacitance will tend to reduce that reactance and drop the VSWR a tad. Sounds backward, I know, but increasing the amount of series capacitance will lower the cap reactance. Antenna length is also in there, so 2 variables.

The glass composite wingtip when installed will cause some dielectric loading of the antenna which may tend to lower the tuning a bit. I wonder if some one has already measured for any significant effect? In any case, your numbers are good to go. Any improvement from your less than 2.0 VSWR would be hard to detect.
Good work,

Ron

Last edited by Cumulo : 02-14-2022 at 03:38 AM. Reason: add info
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  #32  
Old 02-14-2022, 01:43 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
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I’ll add a simple bit too. The reason your ‘too long’ capacitor needed no adjustment is because it was also 20% ‘too thick’ (spacing between plates). Increasing spacing decreases capacitance, increasing plate area increases it. You lucked out!
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  #33  
Old 02-14-2022, 03:24 PM
TASEsq TASEsq is offline
 
Join Date: Aug 2017
Location: Melbourne, Australia
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Quote:
Originally Posted by BobTurner View Post
I’ll add a simple bit too. The reason your ‘too long’ capacitor needed no adjustment is because it was also 20% ‘too thick’ (spacing between plates). Increasing spacing decreases capacitance, increasing plate area increases it. You lucked out!
Sounds like i did!!

Better to be lucky than good.

When we were seeing 1.5-2 on the VSWR we were surprised - but it made it an easy task.

Quote:
Originally Posted by Cumulo View Post
OK, elaboration follows: ;-)
Thank you!

Up in the very top right corner i noticed a tiny R X [Z] - the green is the X, which is reactance - you were right. All a bit of a mystery to me how the capacitor works - a little bit of magic i think.

Shame it will be a few years before i can try the antenna out but it was a fun project - learnt a lot about antennas.
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  #34  
Old 03-03-2022, 08:32 PM
TASEsq TASEsq is offline
 
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Location: Melbourne, Australia
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Here’s the full write up on the build and tuning of the antenna if anyone has 10 mins to kill!

https://tasrv14.blogspot.com/2022/03...tenna.html?m=1
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  #35  
Old 03-04-2022, 02:03 AM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by TASEsq View Post
All a bit of a mystery to me how the capacitor works - a little bit of magic i think.
.
The function of the capacitor is to change the relative phase between the current and voltage, to match the phase difference found in the standing waves in the antenna.
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