[TowerTalk] Playing with my new VIA Analyst (long)

[Rob Frohne]

Hi Bob,

I haven't been back to read the posts that directed you that you need
1/2 wavelength of coax to measure the correct VSWR, but it isn't true if
you have any loss in your coax.  The signal is attenuated as it goes
down the line and attenuated again as it returns, making the forward
power you measure OK, but the reflected power is attenuated by a trip
down the line and back assuming you measure the VSWR at the
transmitter.  

The thing that does happen at half wavelength intervals along a lossless
line is that the impedance repeats itself.

73,

Rob, KL7NA
On Fri, 2005-11-11 at 12:50 -0600, 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 roll).
> 
> 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.
> 
> Here's what I did and some new questions I have.
> 
> >From the L=(246/F)*2 formula I calculated 215 feet would be needed using coax with a velocity factor of 80 percent. I whacked off a 222 foot chunk, installed a PL-259 on one end, and started my "education".
> 
> The tuning setup is simple. You install a T connector at one end of the feedline, put a 50 ohm load on the second leg of the T, and attach the analyzer to the third leg. The opposite end of the feedline is left "open". You then run a frequency sweep to locate the VSWR minimum.
> 
> My first scan showed a nice, sharp VSWR dip at 1643 KHz. This squelched my apprehension I might find the resonance at 1900 KHz! I was 187 KHz lower than my target frequency of 1830. My next step was to determine what length to take off the feedline without overshooting. I crunched a few numbers on the calculator and decided 10 feet would be very safe. Ten feet would also be a nice number to use for extrapolating additional cuts. I cut the ten feet and reswept the feedline. The center frequency moved up precisely 80 KHz to 1723.
> 
> My spreadsheet documents the three additional cuts and the results of this tuning at each step along the way.
> 
> 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?
> 
> And I am puzzled by the original observation, why does the minimum VSWR frequencies "stretch out" and not hit on exact multiples of the fundamental frequency?
> 
> My next step is to take my brand new 1830 KHz tuned RG-8x feedline up the tower and connect it to my almost brand new 160m inverted vee. Today is nice an sunny, so see you guys later!
> 
> 73 de Bob - K0RC
> 
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