[CQ-Contest] Correctly measuring coaxial phasing lines with an MFJ269
Tom W8JI
w8ji at w8ji.com
Sat Dec 3 17:50:58 PST 2011
Well I think I can guess at this. :-)
> It has been a few years since I have had to measure/cut a phasing line
> with the MFJ269
> and I can't find my earlier notes.
>
> I know the formula for length of one half-wavelength is 492/freq. x Vf.
> I have a 170' piece of Buryflex Foam 50 ohm Coax
>
> Questions:
>
> In order to find the Vf I believe that I need to do the following:
Read the 269 Manual, unless someone removed it or modified it, it should be
there. Someone did rewrite the manuals and mess things up, but they should
be back again. Try section 5.5.1
Using the distance to fault function. That is an Advanced mode function.
Just set Vf to 1, unless you want to know the physical length for some
reason.
Leave the other end open, because that is more accurate, although it will
also work on a short.
Tune for lowest resistance and lowest reactance, push the GATE button as the
display tells you.
Move frequency up or down to the next frequency for lowest reading.
Push the GATE button as the display tells you.
The display will show the electrical length in feet, and the electrical
length in degrees. The feet will vary with the analyzer's Vf setting, but
the degrees are naturally independent.
> 1) Connect one end (through a PL259) of the coiled coax into
> the MFJ269 and short the other end.
No. Shorts are not as reliable as opens.
> 2) Measure for lowest Resistance and X (Impedance).
Yes, but with cable open.
> Following this procedure, I get the following minimum values
> of R of 12 and X = 0 at
> 11.54 mhz
> 13.89 mhz
> 16.19 mhz
> 18.54 mhz
Well, you could do that, since impedance repeats at 1/2 waves, by
subtracting 11.54 from 13.89. That's 2.35 MHz for a 1/2 wave. 983.57/f is a
wavelength. 491.785 is a half wave. 491.785/2.35 = you have a 209.27 foot
long line electrically.
16.19-13.89 = 2.3
18.54-16.19 = 2.35
18.54-11.54 = 7 7/3 = 2.333
Using all the measurements it looks like it is about 2.333 MHz, or
491.785/2.333 = 210.795 feet electrical length.
Why not just let the 269 calculate the electrical length in feet, with
velocity factor set at zero? Just dial to the desired frequency after doing
the first bit, and the analyzer will display the electrical degrees for
those null points for any frequency. That's why it is in there, to make it
fast. I just did delay lines for 160, and they were within inches of a
Agilent VNA.
:-)
Or did they change something I don't know about? They do that at times.
73 Tom
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