[TowerTalk] "Leaky" Coax, What is it?
Sat, 9 Aug 1997 23:51:47 -0400 (EDT)
To the forum:
I have seen several posts to this column regarding "leaky" coax. After
investing a significant amount of time and effort into attempting to
understand this phenomenon, I felt my findings might be helpful to the
community at large. I own my own words, and will gladly accept any
constructive comments on this issue.
I also do not want to be judgemental of those who expressed this concept, in
fact, it proves largely to be a matter of interpretation and degree. I am
not claiming "bad science" on anyone's part, but would hope that my comments
clarify this issue and stimulate further discussion.
When I first heard this concept being discussed, I bristled at the thought of
any significant amount of leakage either coming out of, or passing through
the braid of a coax operating into a purely resistive load at the coax's
characteristic impedence. The ARRL Antenna Handbook states flatly "...the
total electromagnetic field outside the coaxial line (as a result of currents
flowing inside) is always zero." (emphasis added) The reason for this is
that spacing in coaxial elements is extremely small at HF frequencies. The
handbook states that complete cancellation of currents occurs when conductors
are spaced within 1% of wavelength. That's 2and a half feet for 80M!!
Additionally, the wavelength is large relative to the gaps in the braid.
Essentially, the braid appears as a solid dielectric at HF frequencies.
I also recognized that statements of this nature beg to be challenged. I
constructed a chain of various grades of coax, the lowest quality being
marked "Sparkomatic", truly cheap piece of RG-58 coax apparently from a
discarded CB antenna. Even though my test equipment was relatively simple,
it was very sensitive, yet I could measure no leakage currents whatsoever on
any grade of coax into a 50 ohm resistive load. Based on several
calibrations and readings, I felt comfortable that any such leakage, if it
existed at all, was at least 60 dB down from the carrier signal.
These results were verified in discussions with a Belden engineer. He stated
that leakage does not occur in any appreciable amount at frequencies below
100 MHz. He said a leakage from a typical 95% braid coax was conservatively
60-70 db below the carrier, with the leakage decreasing significantly as the
frequency decreased. He stated that at HF frequencies, particularily the
lower end of the HF spectrum, there was no advantage whatsoever in using
hardline over braided coax. Mind you, 63db down from a Kilowatt is about one
milliwatt, a signal level that should have little effect on other receivers,
computers, or accessories in a muti-multi environment.
If "leaky" coax truly exists, what is really happening? I have heard it both
ways, claims that signal "leaks out" or that signals "leak in" to a braid.
Neither seems to be valid, as Belden Bulletin "Guide to Shield Performance
and Selection" indicates that "...coaxial shields were found to perform
equally well for both ingress and egress."
Perhaps the most famous "leaky coax" demonstration is taking a 100 ft piece
of inexpensive coax and terminating it in a 50 ohm load. In connecting the
opposite end to a receiver, signals can be noted that allegedly "leak through
the coax braid".
When a piece of hardline is substituted for the coax, the signal levels drop
Evidence would not point to the effectiveness of the braid, but rather to the
dielectric losses associated with inexpensive coax versus hardline. Consider
the coaxial trap example. A piece of coax is wound around a form, the
combination of inductance from the current flow on the outer braid, with the
capacitance of the inner conductor and inner braid, provides a resonant trap
whose bandwidth is only limited by the quality of the dielectric. The same
thing occurs with our coaxial "antenna". Even though grounded at one end,
the outer shield picks up signals like any other antenna would. The inner
shield and center conductor contain distributed resistive and capacitive
elements that provide a signal path to the receiver. The critical variable
becomes resistance in the form of the dielectric, a far more important
variable than the shield density at HF frequencies.
There are three possible explainations for the "leakage" being a significant
factor in a M/M environment: First, measured currents on the outside braid
of coax members are not leakage at all, but rather currents induced from
nearby antenna fields and should be handled by line isolators or current
chokes in the line. Second, a truly innapropriate or damaged piece of coax
may cause a localized problematic leak. Third, power levels in use are such
that 70 plus dB of isolation is not enough. I would like to believe this not
to be true!
Does this mean to get rid of the hardline and go back to coax? Not at all!
Hardline affords a tremendous flexibility in that you can use existing runs
of hardline for any antenna needs, using a remote switch to connect any
antenna from 160M to 2M.
As a solution to "leaky" coax, though, hardline appears clearly an expensive
overkill. Any 95% braid coax with respectable dielectric characteristics
should provide more than adequate performance in the HF range. Care should
be taken to avoid induced currents that can be far more destructive.
Let me close with an example of the law of unintended consequences. One
multiop setup uses 200' of good quality RG-8 to the antenna for their 80M
position, one uses 200' of hardline. Both stations use identical 1.5KW power
amps that have a problematic 28 MHz parasitic that can threaten 10M
operation. Which setup will have the biggest interference problem?
Ironically, the hardline will most efficiently pass the parasitic to the
antenna, despite the high SWR that the 28 MHz emission sees going into the
80M antenna. The 2-3 dB disadvantage that the RG-8 might have seen
transmitting into a perfect load becomes exponentially higher in this high
SWR environment. Consequently, the interference level at 28 MHz may be 10db
higher for the hardline-fed antenna. Not a knock on hardline, but something
to think about!
I hope these comments generates more light than heat. I am certainly willing
to listen to your comments and rethink the problem with you. I sincerely
appreciate Belden's assistance, and hold no stake or in interest that
73, Paul, KB8N
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