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Author Topic: Help with cable testing  (Read 1115 times)
W9KDX
Member

Posts: 770




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« on: October 29, 2011, 10:25:09 AM »

I have an FT-950 connected to an Ameritron 811H and then to a Daiwa meter and the antenna.  The two jumpers connected between the FT and amp and amp to meter are the issue.  I noticed an increase on SWR (measured at the meter) on 10 meters.  It went from the usual 1.2:1 up to 1.7:1.  I did the usual things with tuning and no change.  I then swapped the cables and the problem went away.  Both cables tested out, as well as I can test with a low cost tester, and the only thing I can guess as different is the amount of watts carried for each cable, depending where they are installed.  Transceiver to amp is 50 watts and amp to meter is 600 watts.

I plan to replace the problem cable next time I place another parts order; the question is this: given the limited testing equipment most of us on the low end have, how are we ever to tell if a cable is a problem if resistance and continuity testing show no problems?  The cables are both fine now with what they are conducting, but other than swapping every cable into a high wattage connection, one would never know it.

So, is there some other test method of testing I can use with simple equipment and, is the cable I have now, which is working fine, OK for a while until I get a new order in?

Thanks

Should have mentioned, the antenna is an OCF dipole with a Guanella 4 to 1 balun.
« Last Edit: October 30, 2011, 05:17:22 PM by KD0PLD » Logged

Sam
W9KDX
WB6BYU
Member

Posts: 13288




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« Reply #1 on: October 29, 2011, 11:45:55 AM »

Wait - something doesn't make sense here.

You measured higher SWR on your meter.  But neither of the cables in question would affect
the SWR measured on the meter
, because they are between the rig and the meter, not
between the meter and the antenna.

Only things PAST the meter affect the SWR it sees.  (The exception would be if there was a change
in the harmonic output of the amplifier, since the SWR for the harmonic frequency would be higher
than for the fundamental.)

Question:  did you observe the higher SWR only when using the amp, or did you measure the same
high SWR with just the rig when the amp was bypassed?

Extra credit question:  With the amp in line and the output power from the radio reduced to where
the amp is putting out 100 watts, do you see the same SWR as with the rig alone operating at that
output power?  Does the SWR change as you increase the output power up to maximum?


One possible explanation is that there is an intermittent problem where the main feedline connects
to the SWR meter - in the process of swapping cables, you moved the meter to unscrew the cable,
and this may have flexed it back to working properly.  (This is not uncommon.)  To check this,
measure the SWR at low output power (10W or less if possible) and wiggle all the coax joints
in the shack, especially the the main feedline.  It is sometimes to difficult to wiggle one feedline
without also moving the meter and the feedline on the other side, but if you see the SWR jump
then look into it further.  I had a recent case where I replaced several crimp-on connectors that
appeared to be intermittent before discovering the problem was that the inner pin of the connector
on my SWR meter was splayed out a bit and didn't make reliable contact except on full-length center
pins.

There may be other explanations, but I wouldn't blame it on the patch cables because they weren't
in the part of the circuit that the SWR meter was measuring.
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W9KDX
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Posts: 770




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« Reply #2 on: October 29, 2011, 12:10:39 PM »

SWR ,measured at the meter means just as I said, at the meter not at the transceiver.  With the amp out of the circuit, the SWR at the transceiver is also low no matter where the cables are which leads me to think that the problem is only with the higher wattage provided by the amp.

If you are, in fact saying that none of the cables prior to the meter/antenna can affect SWR, then I really don't have a clue as to what is going on as I have not touched the antenna cable at all.

Thanks for the idea about the change fixing something with some flexing.  I thought of this but as things are working for now I don't yet want to swap back and see if I can get the problem back.

« Last Edit: October 29, 2011, 12:15:08 PM by KD0PLD » Logged

Sam
W9KDX
K1CJS
Member

Posts: 6034




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« Reply #3 on: October 30, 2011, 04:54:18 AM »

Low cost testers will tell you if there is a connection, they will not tell you how good that connection may be.

For example, if one or two strands of a coax shield were connected and the rest were floating (don't laugh, it can happen with the flexing of the cable) a low cost tester would tell you there was continuity.  However, there may well be signal leakage at that point--or more likely an insufficient connection to transfer the RF energy.  Either could show you an increase of SWR.  Likewise, a poor solder joint at the center conductor could read as good on cheap test equipment--but would be noticeable in the antenna circuit.  Don't forget the fact that cables that have been overheated could have had their specs changed by the overheating and could throw a noticeable SWR 'bump' into the line--but cheap test equipment will not show it!

That is why if you make your own jumpers, to be sure you start with good, clean surfaces, use a quality solder and a good soldering iron of sufficient wattage with a clean tip.  Also, don't hold the soldering iron to the connection longer than to flow the solder and make a good, clean joint.

That brings up two points.  #1 is...  Practice, practice, practice.  Get some scrap wire and practice soldering.  If you use an iron, practice with it until you're comfortable using it to make connections.  For RF connectors such as the PL 259, make sure the iron is of sufficient wattage to flow the solder onto the barrel to shield connection.  Clean up some older PL259 connectors and, using some old cable, practice some more.  To quote one of my old instructors, "Let's repeat making these solder joints until we're proficient at doing it, gentlemen."

#2 is...  If you're in doubt of the connection, do it again.  Even the best of us have inadventantly made bad solder connections on cables, and have only found problems after putting things together.  Even the best of us have had to sometimes pull out hair out looking for the bad connection in a line.  After all, we're only human!   Grin
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WB6BYU
Member

Posts: 13288




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« Reply #4 on: October 30, 2011, 06:23:29 AM »

Quote from: KD0PLD
...which leads me to think that the problem is only with the higher wattage provided by the amp.

SWR shouldn't change with power level.  If it does, then it usually means one of three things:

(1) harmonic or other spurious output at a frequency where the antenna isn't matched

(2) the meter isn't accurate at one of the power levels (commonly they read low SWR when the power
is barely adequate to drive the meter to full scale.)

(3) something is arcing at the higher output level.  It might be the end of the antenna wire, or a stray
strand of shield wire inside a connector.  If this is the case you should see the SWR suddenly start
to jump around as you raise the power to a certain level, and it will often be erratic.

If you crank down the output power on the amp to match that from the transceiver and still see
the problem, it won't be (2) or (3).


Quote
If you are, in fact saying that none of the cables prior to the meter/antenna can affect SWR, then I really don't have a clue as to what is going on as I have not touched the antenna cable at all.

That is what I am saying - the meter reads the SWR on the antenna and coax past where it is inserted
in the feedline.  Changes in the coax jumpers may affect the amount of power arriving at that point, but
not the SWR seen by the meter.

You may not have intentionally wiggled the main coax feedline, but with most SWR meters I've seen
it would be difficult to swap the cables on one side without flexing the other side.  (Unless the sensor is
securely mounted to the wall, etc.)


Quote
Thanks for the idea about the change fixing something with some flexing...

I recently spent some time on the bench at work with just such a problem - I was trying to take
measurements on a circuit board and they would jump back and forth between two values.  I
wiggled everything and swapped out the coax cables, attenuator, etc. before putting a second board
in place and the problem went away.  Apparently the first board had an intermittent contact in or
around the output connector.

A common cause of this in ham equipment is a PL-259 center pin with too much solder on it
that spreads the contacts on the SO-259 jack.  Then contact can be intermittent when a plug
without the extra solder on the pin is inserted.
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W8JI
Member

Posts: 9296


WWW

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« Reply #5 on: October 30, 2011, 08:43:14 AM »

I have an FT-950 connected to an Ameritron 811H and then to a Daiwa meter and the antenna.  The two jumpers connected between the FT and amp and amp to meter are the issue.  I noticed an increase on SWR (measured at the meter) on 10 meters.  It went from the usual 1.2:1 up to 1.7:1.  I did the usual things with tuning and no change.  I then swapped the cables and the problem went away.  Both cables tested out, as well as I can test with a low cost tester, and the only thing I can guess as different is the amount of watts carried for each cable, depending where they are installed.  Transceiver to amp is 50 watts and amp to meter is 600 watts.

I plan to replace the problem cable next time I place another parts order; the question is this: given the limited testing equipment most of us on the low end have, how are we ever to tell if a cable is a problem if resistance and continuity testing show no problems?  The cables are both fine now with what they are conducting, but other than swapping every cable into a high wattage connection, one would never know it.

So, is there some other test method of testing I can use with simple equipment and, is the cable I have now, which is working fine, OK for a while until I get a new order in?

Thanks

Based on what you say it could be common mode on the feeline from a poor or broken antenna design (you never did mention the antenna).

Since you swapped the cables, with no other changes, and since the cables were on the amplifier side of the SWR meter, it pretty much has to be in the antenna system after the amplifier or it could be instability.

Some radios are also unstable driving some loads. I have an IC751A that oscillates out of band with some loads on the test bench. When I change cable lengths or power supply voltage, the 751A settles down!!

It creates an effect exactly like you describe.

73 Tom
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WB6BYU
Member

Posts: 13288




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« Reply #6 on: October 30, 2011, 12:37:08 PM »

Quote from: KD0PLD
So, is there some other test method of testing I can use with simple equipment...


There are two tests with simple equipment that will tell you the condition of your coax.

First, install the coax between your SWR meter and a dummy load.  Check the SWR.
Wiggle the coax around, especially flexing it near the ends, and see if the SWR jumps.
If you suspect the cable is failing at high power, repeat the process at maximum output
from the amplifier.  A good cable should show low SWR for this test. 

However, a cable with high losses will usually still show a low SWR, so you need to
run a second test for that.  One approach is check the power going into the cable
(the indicated forward power on an SWR meter is adequate) then move the meter to the
far end of the cable between it and the dummy load and measure it again.  From that
you can calculate the cable loss and compare it to the expected value.  You can also
measure the SWR with the far end open or shorted - the lower the SWR, the higher
the cable loss.  I use this as a quick check of an installed cable when I know that my
antenna has a high SWR on some band - I just check that the SWR reads high.

There are other methods of measuring the loss, but those are the basics:  make
sure it shows a low SWR when terminated with a dummy load, and that the loss
is as expected.
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W9KDX
Member

Posts: 770




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« Reply #7 on: October 30, 2011, 05:23:27 PM »

I worked the radio quite a bit this weekend and things seemed just fine so I am inclined to think I had a weird connection or possibly just an odd part of the frequency where the SWR is higher.  I have noticed that my SWR increases as I go up the bands.  In any case, the CQ Worldwide DX Contest this weekend got me into 35 new countries with no problems so things are better than I would ever have expected.  I even made it to Japan which is on the wrong end of my dipole.  Up to now I had figured I would just not have the reach with my bad location.  Ain't propagation wonderful!!
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Sam
W9KDX
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