On 9/10/2012 9:23 AM, Jim Hoge wrote:
Let's do some quick math.... 234/1.82=128.57 gives us the length in feet of a
quarter wave at 1.820 mHz. Multiply that by a velocity factor ( say 85% for
LMR-400) and you get a length of 109.29 feet.
That math is a bit too simple, because Vf VARIES as function of
frequency. The PUBLISHED Vf is for VHF, but the Vf is a few percent
lower at 2 MHz. This happens with ALL transmission lines, and is
predicted by Maxwell's Equations. For more on this, including measured
data for coax similar to LMR400, see
http://audiosystemsgroup.com/Coax-Stubs.pdf
Bottom line -- you MUST measure the coax AT THE OPERATING FREQUENCY to
hit an exact half wave. An easy way to do this is with an analyzer like
the MFJ259, which drives the line with a small manually tuned signal
generator, short the far end, and tune the generator until you see the
sharp null of the short. With your AIM, you need to short the far end
and look for the null in the impedance around 160M. I don't know
anything about the AIM unit you have, but you need a setting that
provides the greatest detail around 2 MHz.
BTW -- there's a great piece of FREE software by AC6LA called zplots
that takes S1P or S2P files from analyzers like the AIM units and
computes plots all the fundamental characteristics of a transmission
line that you have measured. It's an Excel spreadsheet. I just used it
with the output of my DG8SAQ VNWA to plot Vf, Zo, and attenuation of
some CATV hard line I inherited from a neighbor SK, and some vintage
Times triax that I found at a hamfest. See the link below for a
complete description and a link to download the spreadsheet. .
http://ac6la.com/zplots.html
73, Jim K9YC
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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