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
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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