[Elecraft] Testing rigs, problems with coax to avoid

[Stuart Rohre]

Having run some labs for 30 years, and seen and built many a coax cable with
various connectors, it has come to my attention that as hams we often fail
to inspect and test our cables before doing experiments, or using them for
vital hook ups like Field Day, etc.  Or we trust our test hook up cables too
much when building a new project or kit.

One of the areas of problems is quality of the coax itself.  Some of the
cheap CB vintage coaxes were truly terrible, and Radio Shack has greatly
improved its line of coax to overcome the problem commonly seen, low shield
density, which could leak power out of the shield.

Normally, all coax current should be on the inside of the shield even if
there is SWR.  If the shield has a low wire count, it could be an imperfect
shield.  Such cable should not be relied upon for standard half wave
sections and test sections.  Always try for cable with 95 percent or more
shield coverage.

Double shielded cables were used for thick Ethernet and are commonly
available as give aways as Computer Centers convert to twisted pair Cat. 5
networking.  Especially, if they have some old reels of tag ends around.
This has no ham measurable coax leakage.

The other failure prone part of the coax system are the short jumpers
between rig and tuner, and rig and antenna selector switch.  Poor soldering
of UHF ground shell to shield is the usual failure point here, although I
have seen some poorly soldered center pins as well.  Pre tinning helps when
getting a good joint, and adequate wetting of solder to flow through out the
end of the pin, or under the PL 259 back shell to tightly bond to braid.
You should be able to count the braid wires after soldering, if you totally
flowed solder to the braid, and did not pile up excess.  There should be a
nice fillet at the side of each hole in the PL 259 shell, going all around
it.

In making various antenna tests, SWR measurement, and Antenna Analyzer hook
ups, make sure your cables are above reproach before trusting that you have
the right answer.  One proactive pretest is to occasionally take your DMM on
low ohms and check the coax shield, connector to connector, of all jumpers,
and center pin to center pin of all center conductors.  Keep a log of such
measurements for future reference.  If you have many cables, attach labels
with numbers to keep them sorted to your record.

I have had short bad jumpers reappear at Field Days when I was lax and did
not pretest all coax to be jumpers.

The same can be said for adapters, Tees for BNC use, have a habit of one
part coming unthreaded and you lose one center pin.  Unless you glance at
all three ports before using the tee, you will miss this and spend quite a
bit of time trying to find your open circuit.

The other thing that can upset antenna measurements is that over time coax
plugs will work themselves loose, and create a high impedance joint.  Not
running a plug up all the way may create a spot of leakage of cable power.

Antenna joints that are bolted need to be checked for tightness, and
oxidation over time.  Conductive compounds need to be used with aluminum
which oxidizes easily.

Old coax can become hard and brittle as glass up on the antenna tower in
sunny climates.  It will then crack in sections allowing moisture to enter
and condense.  Pigtails of coax may allow moisture to enter and flow down
the cable to exit at loose shield to connector points.  We have even seen
hardline drain of moisture for two days, after being removed from a long
time installation where it was vertical on a tower.  Apparently a poorly
sealed upper coax fitting allowed water ingress.

Running coax has its own problems.  IF it is in the field of a wire antenna
and not dressed with even spacing, such as being closer to one side of the
antenna than another, it may suffer induced currents on the outside of the
shield.  This is not from SWR, but coupling to one side of the antenna more
than the other.  The outside the shield INDUCED currents are the ones that
may radiate and affect the antenna pattern in ways you do not want.  Foam
coaxes can lose constancy of impedance if bent in too tight a radius, as the
center wire can migrate toward one side of the shield changing the spacing
and impedance.

When you build kit rigs and accessories, you should get a set of tested
known good coax cables to assist in kit tests, to not lose valuable time to
a suspected problem that turns out to be bad test leads.  The same can be
said for DMM leads, clip lead jumpers for power or any connection to the
outside world from your kit.  When complex problems happen, a good place to
start is remove all external wires and methodically check them as you
reconnect.  Loose power connections are a common late night happening.
Always check your power, then all the other elements of connection to solve
mysterious test readings.
72, Stuart K5KVH