[Elecraft] Testing rigs, problems with coax to avoid

[George, W5YR]

Spoken like the man who knows and has been there!

Excellent, Stuart.

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437


Stuart Rohre wrote:
> 
> 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