Hello-
I thought I would post this for any comments that may help me to understand
what may be going on. What started it all is that years ago I stored away some
unused Plastoid RG-13A/U 75 ohm coax from a surplus source. The cable is old,
but it appears very nice physically with a clear dielectric and nice shiny
copper. I was thinking this cable would be worth saving because of the double
shield. My intentions are to use it for Q-sections for HF antennas and possibly
phasing lines for VHF antennas.
I measured off an exact 25' of the RG-13A/U and installed a PL-259
connector. I used the MFJ-259B in 2 different methods to measure the velocity
factor of the cable. I obtained a 0.67 and a 0.66 figure, instilling a little
re-assurance in me in the analyzer.
So, I thought I would measure some factory-built M2 phasing equipment for 2
meters to see how the MFJ-259B stacked up, as I am wanting to use the MFJ-259B
to measure the Q-sections and phasing lines of future projects.
Several years back I had a pair of M2 2M18XXX antennas stacked about 14
feet apart. I bought the power divider and phasing lines from M2. The system
worked fantastic but was up only about 1 year, as a spring storm bent the booms
on both antennas, so I took down the array and stored the phasing lines and
power divider. The power divider is the M2 2M2PTPD and the phasing lines are
Times Microwave LMR-400 with the silver plated crimp-on N-Male connectors.
There was no damage to the phasing lines and they were all sealed up real good.
I thought I would check out the phasing lines and power divider with my
MFJ-259B analyzer. Firstly, I connected each phasing line individually, with a
lab-grade 50 ohm load at far end, and measured the SWR, which showed 1.0 for
each phasing line, at every freq checked. I removed the terminations and
measured the cable loss, which was an identical 0.3 dB for each phasing line.
Next, I connected each phasing line individually, with a short at far
end, and measured the freq of zero reactance ("X" on analyzer) nearest 2
meters, each one came out to be much higher, at 162.2 Mhz. However, this
reading was very broad.
I used the MFJ-259B to measure the velocity factor of one phasing line and it
came out to be 0.87 (Times Microwave specs are 0.85). This aroused some
curiosity in me, as I measured each phasing line to be 287-1/4". Using the
published velocity factor of 0.85 for the LMR-400 cable a length of 287-1/4"
would not be a half wave multiple at 144.2 Mhz. However, using the measured VF
of 0.87, the 287-1/4" length is 8.05 half-waves.
Another thing I did was to connect identical lab-grade 50 ohm terminations to
the far end of each phasing line, and connect each phasing line to the power
divider. Then, I connected the power divider to the MFJ-259B and measured SWR.
Again, this reading was very broad, and my best reading was higher, up at 161.2
Mhz. However, the SWR was reading good(1.1-1.2) at 144.2 Mhz.
The last thing I did was to remove the phasing lines from the power divider
and leave the 2 ports un-terminated, then measure the freq of lowest reactance
("R"), this was above 2 meters at 158 Mhz.
So, am I doing something wrong or is the analyzer unreliable? This analyzer
is several years old but has seen very little use. Is there any "standard" that
I can check it against, or a test I can do?
Thanks, Byron WA5THJ
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|