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Distance to fault
This short article appeared as message 22664 On the Tower-pro mailing list and is a must-read for anyone using sweep techniques to test antennas. The text has been moderately edited for spelling but is otherwise as written by its author. JH
DTF is a calculation. Not a measurement!!!!!!
I implore everyone to take the time to read this.
You can not use DTF for pass / fail
DTF takes an average of the frequency swept VSWR, puts that into an equation that also takes the default value for the attenuation of a feeder cable, somehow the default velocity percent gets thrown in and it redistributes this information over distance.
First, the attenuation values stored in the machine are based on a manufacture's catalog specs which are typically spec'd at an ambient temperature of 68 degrees F. If it is not exactly that temperature outside when you test the line, you have already introduced a variable into the equation. The velocity percent shown in catalogs is also plus or minus a certain tolerance and is not exact. Another variable.
If you have a 70 foot length 7/8" feeder, and say for example, a 15 foot 1/2" Superflex jumper at the bottom, and a 10 foot regular 1/2" jumper at the top, you are actually applying the loss of the 7/8" feeder cable to the 25 feet worth of jumpers which have a higher loss than the feeder - but the DTF calculation can not account for this. Another variable.
Real life examples:
I visited a site where the installer had a DTF spike in feeder cable about 35 feet from the top jumper and could not find anything wrong. The spike was above the operators "spec" so the line was deemed faulty. At the end of the day, we switched the bottom jumper, 275 feet away from the so-called fault in the cable, and the spike went away. Why? The jumpers "electrical lengths" were different and they were in a different phase. Changing the bottom jumper had no affect on the overall system performance but the DTF plot looked better. Basically, the installer had lost time chasing ghosts.
I received a DTF plot of system (with antenna) that showed a spike in the middle of a feeder run. Site could not be passed. A DTF plot of just the cable showed no spike in the feeder. The phase relationship of the feeder and antenna together created a spurious spike in the cable. Both the antenna and feeder were good. The system sweep with both together was stellar. But, the installer had wasted time chasing ghosts to satisfy his customer's "spec"
Had a guy in Canada saying his cable was bad because he was getting a 1.05 DTF on his connectors. He insisted the cable was making the connectors look bad because his connector attachment visually looked great. His system sweep of the transmission line was -31 db. He replaced the cable, used the same connectors, got 1.02 DTF numbers and called me to tell me he was right. His system sweep was now -26 dB. But hey, the DTF looked nice, guess it doesn't matter his first lines actually performed better. Another example of wasted time.
I know for a fact that the following scenario happens. A line is
installed, system sweep exceeds customer's spec, but a DTF spike at 2
connectors is above the DTF "spec." Installer loosens the connector
until the DTF "looks good" and does a balancing act between th DTF
and system sweeps, making the DTF look good enough while having a
negative affect in the system sweep, but keeps the system sweep
barely passing. At the end of the day, the operator, by setting specs
DTF specs, now has a line that does not perform as good as it could
and has a line that is set up for premature failure because the
connectors are not tightened the manufacture's specs.
Actually, I am stern advocate for DTF's "intended" use, which is not pass fail.
You sweep a line, it passes, and you run a DTF and save it. A year from now, the line is not performing as it did at commissioning for some reason. You run a new DTF plot, compare it to the old, and look for something that has changed so that you are not troubleshooting blindly. This saves time on troubleshooting. Pass /Fail on DTF wastes time.
You sweep a line, and it does not pass. You use DTF to look for
something that is grossly wrong. A 1.04 versus and 1.05 versus a 1.07
on a connection is meaningless. If you have a 1.35, then you may have
something to fix. Where is the line versus what is a good connection
versus a poor connection? There isn't. It is indefinable because of
all the factors / variables that go into it. Operators have set DTF
specs on what is "typically" seen in DTF. Something above "typical"
is not necessarily something bad. I have had operators tell me they set DTF specs to ensure that the
installers are doing a good job regardless of what the system sweep
results are. Little do they know.
If the line passes the system sweep, you have a quality installation with quality products, regardless of what the DTF plot looks like.
Best Regards
Dan Druszkowski
Product Analyst
Cable Products Group
Andrew Corporation