[TowerTalk] 2 Meter Phasing Lines

[mikflathead at aol.com]

 By using the power divider you can use any length of cable as long as the 2 are the same. Not sure what you are asking. Mike

 

 

-----Original Message-----
From: Byron Tatum <bjtatum1 at att.net>
To: TowerTalk <TowerTalk at contesting.com>
Sent: Sat, Apr 12, 2014 3:42 pm
Subject: [TowerTalk] 2 Meter Phasing Lines


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
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