Coax Cable Interconnections

[Edward A. Dauer]

A week or two ago there was a thread here on the permissible bending radius of various types of coax, occasioned by a discussion of “ugly balun” chokes.  The problem, I gather, is that with a small radius turn the inner conductor can migrate through the dialectric.  I filed the point away but didn’t think much about it at the time since I don’t (as of this moment) use coax chokes.  But today, as I was doing some tidying up of the rat’s nest of cables behind my desk’s misleadingly neat façade, I noticed something.  The coax – RG8 – connecting the KPA500 to the KAT500 sitting atop takes a very sharp bend.  The same is true of the cable coming out of the K3, making a sharp right turn, travelling under the desk and then making another sharp turn to connect to the amp.  The same is true of the K2 and its tuner on which it is perched – a short, sharp turn.

I am going to guess that any change to the characteristic impedence, if the center conductors in the interconnects do migrate, would not itself be a problem over very short lengths.  But the possibility of a dialectric breakdown caused by a severe migration is more troubling.  So a question for those who know about such things:  In the zeal for tidyness and the shortest possible cables, is there a significant risk?  How fast does migration occur – should these sharp-U-turn interconnecting cables be replaced on some regular basis?

Ted, KN1CBR
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[Don Wilhelm-4]

Ted,

I would suggest RG-8X or RG-58 for interstation connections.  Those
'jumpers' are usually only a few feet long.
Once you exit the station, you will likely have longer coax runs to the
antenna where the larger coax makes sense.
The coax cable loss is proportional to the length and the inherent loss
of the connecting cables.
Smaller and short coax cables will have insignificant loss at HF.
Smaller coax diameters will have smaller bending radius requirements
than larger coax.

73,
Don W3FPR

On 2/20/2016 9:34 PM, Dauer, Edward wrote:

> A week or two ago there was a thread here on the permissible bending radius of various types of coax, occasioned by a discussion of “ugly balun” chokes.  The problem, I gather, is that with a small radius turn the inner conductor can migrate through the dialectric.  I filed the point away but didn’t think much about it at the time since I don’t (as of this moment) use coax chokes.  But today, as I was doing some tidying up of the rat’s nest of cables behind my desk’s misleadingly neat façade, I noticed something.  The coax – RG8 – connecting the KPA500 to the KAT500 sitting atop takes a very sharp bend.  The same is true of the cable coming out of the K3, making a sharp right turn, travelling under the desk and then making another sharp turn to connect to the amp.  The same is true of the K2 and its tuner on which it is perched – a short, sharp turn.
>
> I am going to guess that any change to the characteristic impedence, if the center conductors in the interconnects do migrate, would not itself be a problem over very short lengths.  But the possibility of a dialectric breakdown caused by a severe migration is more troubling.  So a question for those who know about such things:  In the zeal for tidyness and the shortest possible cables, is there a significant risk?  How fast does migration occur – should these sharp-U-turn interconnecting cables be replaced on some regular basis?
>
> Ted, KN1CBR
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[Jim Brown-10]

On Sat,2/20/2016 8:19 PM, Ron D'Eau Claire wrote:
> I use right-angle adapters to minimize the radius of the turns if I'm restricted to RG-8 size coax, but almost all of my inter-equipment connections are RG-58. It's FB all the way above 1 KW as long as it's not subjected to very high SWR's at high power. (Actually RG-58 can handle 2 or 3 KW at an SWR of 1:1).

Power handling is only one property of coax. Another is SHIELDING, and
one measure of shielding is the Transfer Impedance, which is the ratio
of the voltage between center and shield as a result of shield current.
The lower the transfer impedance, the better the shielding. Transfer
impedance is the result of shield quality -- density and uniformity --
and its resistance at the frequency of interest. The lower limit on
transfer impedance is the RESISTANCE, so a shield with lower resistance
will provide better shielding. Since shield resistance is largely a
function of skin effect, larger diameter coax generally provides better
shielding than a smaller one. THAT'S why all the coax in my station is
RG8, specifically Davis Buryflex.

What's the big deal about shielding? Well, for several years, I've been
working on my station to minimize issues with harmonics when I'm running
SO2R on harmonically related bands. I have very good bandpass filters,
and I've added double stub networks to kill harmonics on several bands.
But I wasn't getting as much suppression as I expected (and measured on
the networks on the bench), so I suspected issues with coax. Last
summer, I replaced all the inside cable with hand-soldered Amphenol
83-1SP connectors on cables cut to length. The result was an improvement
of about 10 dB in harmonic suppression! The lesson -- when you're
looking more than about 50 dB down, little stuff really matters.

73, Jim K9YC
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[Edward R Cole]

Jim and all:

Coax quality -- absolutely makes a difference.

I inherited some "previously driven" (aka used) coax from work
thinking to save money -ha!  It dates from before I began working at
my last employer as comm tech else would never have been
purchased.  I found in preparing for installing connectors the shield
density so poor one could not make good connections -- only useful
buried in ground as radial.

I ran into the coax shield quality issue in a different situation but
is the same isolation or "radiation" problem.  When I increased power
from 600w to 1400w on 2m eme I suddenly started blowing preamps.

There were multiple "suspects":  My TR relays were not rated for
1200w nor had the Rx port isolation needed.  I try to keep RF into
the preamp < 0 dBm (1mw) and 1200w = +61 dBm.  My relays were only
spec at 45-dB isolation.  Also turned out only good to 800w.  I
replaced all after melting down three of them!!  Now using NARDA
sem123N rated 1200w & 80 dB isolation.

But to the point the high emf of being installed at the antenna
subjects connecting coax jumpers to high RF fields so shielding is
critical.  I was using RG-58.  I swapped for RG-142 double-shielded
coax.  I also fed 12v to the preamp thru shielded power wiring (RG-6
or RG-58).  Preamps has feedthru bypass caps so that path should not
as critical.

So changing three things means I do not know which was the "cure" but
no longer blow up preamps.  Double-shielded coax is commonly used as
jumpers with repeater duplexers for the same reason cited by Jim.

I wonder if better coax jumper in the shack would improve isolation
from wifi type RFI?  Probably couldn't hurt.

73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
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