[TowerTalk] Playing with my new VIA Analyst (long)
jimlux at earthlink.net
Fri Nov 11 19:06:51 EST 2005
At 10:50 AM 11/11/2005, Robert Chudek wrote:
>Merry Christmas to myself, early. I'm like a kid with a new toy using my
>new antenna analyzer!
>"You guys" pointed out I needed a 1/2 wavelength (electrical) run of coax
>to my 160m inverted vee to get true values on my meter. I took this as an
>excellent educational opportunity. So I bought a 500' roll of cheap RG-8x
>coax that I didn't mind hacking up into pieces. (btw, cheap = $60 for the
>I created an Excel spreadsheet which shows the steps I took to tune my 1/2
>wavelength feedline to 1830 KHz. I posted this spreadsheet to
>http://chudek.aberon.net in the "AEA VIA Analyst 160m Charts" folder. You
>can download it from there, if you are interested.
So what else did I learn? Well, I swept the feedline up to 54 MHz (the
limit of this model). I saw additional resonances or minimum VSWR's all
along the spectrum. I zoomed in to analyze the frequencies of interest on
other ham bands. I thought a 1830 KHz resonant feedline would be useful on
other harmonically related ham bands. For instance, I expected to see 3660
KHz as a resonant point. In reality, I measured a dip at 3718 KHz. My
spreadsheet contains information about all resonances that are within (or
close) to the other amateur bands.
>Another thing I discovered was the higher in frequency I went, the
>"shallower" the dip. For instance, on 10 meters the minimum VSWR only
>dropped to 1.30:1. I speculate this is due to cable attenuation or loss.
>But I do not know how to turn this number into a meaningful value, i.e. X
>dB per 100 feet at X MHz. Anyone?
There is an equation for this (improvement in VSWR with attenuation). I
don't remember it off hand (I work in dB return loss usually, where it's
>And I am puzzled by the original observation, why does the minimum VSWR
>frequencies "stretch out" and not hit on exact multiples of the
I'll speculate a bit.. your open end would ideally provide a perfect
reflection. However, it's very hard to make a precision open, because of
the fringing fields at the end (why calibrating a network analyzer with
Thru, Short, and Load is popular). You may have, in effect, a transmission
line connected to a reactive load (which itself varies with frequency).
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