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Softrock Lite 6.2
Adventures in Electronics and Radio
Elecraft K2 and K3 Transceivers
The Curious Case of
Digiwave RG-58 Cable
Jeff, AC0C, sent me a length of Digiwave coaxial cable,
marked DGC-RG58-211000, to verify his measurements. Jeff purchased the cable
believing it was an RG-58 variant (foam dielectric, white jacket, foil and drain
wire shield) of 50 ohms nominal characteristic impedance.
After some odd results, Jeff measured the cable impedance,
(Zo) and found it to be nearly 100 ohms, not the 50 ohms one would expect for an
RG-58 type cable. I asked Jeff to send me a section for an independent
measurement and I've now confirmed that this is definitely not 50 ohm cable.
The first measurement looked at the impedance of the cable when terminated with
a 100 ohm trimpot. (I added a 20 ohm series resistor, not shown in the
When a transmission line is terminated with its
characteristic impedance, there is no reflection and the line looks like a pure
resistance. To determine the transmission line's Zo with this method, adjust the
trimpot for minimum phase angle while observing the impedance (magnitude and phase).
In theory, it should be possible to adjust the trim pot
for 0 phase angle
over a range of measured frequencies . This isn't actually the case, however, due to a couple of reasons.
One is that the trimpot and connecting wire has some residual reactive
component. For frequencies in the low MHz range, these imperfections are not too
But another factor can be at work. We are used to
thinking of Zo as being a constant value and resistive only, i.e., the
cable impedance is constant with frequency and has no reactive component. In
real cables, neither of these are true, and as we shall see, the Digiwave cable
is particularly poor in this regard.
The plot below shows the impedance magnitude (black) and phase (blue) over
the range 300 KHz - 30 MHz (log frequency scale), with the trimpot adjusted for
minimum phase variation. The phase excursion is 10 degrees over this frequency
range and the impedance range shows a minimum of 92 ohms and a maximum of
This data establishes that Zo is certainly not constant
over this frequency range and that Zo is not purely resistive.
The next step was to measure Zo directly. This is relatively
easy to do by measuring the cable impedance with the far end open
circuited (Zoc) and short circuited (Zsc).
impedance is then: Zo = Sqrt(Zoc*Zsc).
Zo, Zoc and Zsc are all vector quantities, i.e.,
they have magnitude and phase, or, if you prefer, a real and imaginary component. I'll generally
ignore the phase and treat the cables as having only a real component of
impedance. This assumption introduces negligible error with high quality cables
at normal amateur radio frequencies.
Short circuit and open circuit impedance measurements also
used the HP 8752B VNA. To verify my measurement procedure, I made similar
measurements on a short length of high quality RG-223 (double shielded, silver
plated 50 ohm coaxial cable) and a short length of cheap import RG-58 cable.
(These cables are about 9 feet long, and the Digiwave cable is approximately 59
Over the frequency range 300 KHz - 30 MHz, the Digiwave
cable Zo varies from 132 ohms to 88 ohms. The cheap RG-58 and the
RG-223 show much less variation over this frequency range.
There are two apparent anomalies here:
- Elevated Zo at lower frequencies
- At frequencies above 1 MHz, the impedance is nearly
twice the expected 50 ohms.
To study the cable impedance below 300 KHz, I ran Zoc, Zsc measurements between
10 KHz and 2.5 MHz with an HP 4192A LF Impedance meter. As a benchmark, I also
ran Zoc and Zsc measurements on a similar length of Belden RG-58 cable.
Elevated Zo at low frequencies
Let's consider the first apparent anomaly.
Both cables show a considerable increase in Zo at low
frequencies, with Zo in this frequency range being an excellent fit to a straight line relationship
when Zo and frequency are plotted on a log-log scale.
The difference between the two cables is where they
transition from low frequency frequency sensitive Zo to frequency insensitive
impedance. The Belden RG-58 makes this transition around 40 KHz, whilst the
Digiwave cable does not transition to a (semi) stable impedance until nearly 1
MHz. There is a clue in this plot.
Belden has two useful papers discussing the behavior of
coaxial cables at frequencies below those we normally use in amateur radio
The commonly stated relationship between a cable's
electrical parameters and Zo is:
L is the cable inductance and C is the capacitance as
determined in an infinitesimal length of cable.
This is a useful approximation for a high quality cable,
at normal amateur radio frequencies. However, the true relationship is:
R is the series resistance and G is the shunt conductance, representing leakage
in the cable dielectric.
If G <<ωC and ωL >> R then the R and G terms may be
neglected and the simplified form of the equation used.
With modern dielectrics (Teflon or polyethylene) G <<ωC is
almost always true, but it is not the case that ωL >> R at low frequencies. This
is the reason Zo increases at low frequencies; the numerator of the equation
increases due to the R component whilst the denominator remains constant.
(In fact, C has some change with frequency and L has a
greater relative change in frequency. This is due to skin and proximity effects
that alter the current distribution with frequency and hence change the flux
linkages and therefore the inductance. The normal consequence of this is that L
decreases slightly with frequency and therefore at frequencies where ωL >> R, Zo
will drop a bit as the frequency increases.)
To see if the resistive component of Zo explains the
Digiwave Zo impedance increase below 1 MHz or so, I measured the DC resistance
of the cable, and compared it with a sample of RG-223.
At DC, the Digiwave cable has a total resistance 10 times that of RG-223. If the
inductance per unit length is similar in the two cables, the crossover frequency
at which ωL >> R will be 10 times greater. This is quite close to the
measured data. (R is not constant with frequency, as skin effect must be
considered. Hence agreement with a simple DC measurement will not be perfect.)
Why does the Digiwave cable demonstrate so much higher
The difference in shield resistance can be explained by
the cable construction; Digiwave uses a foil shield with a low coverage "drain"
braid. RG-223 has two dense shields, both of silver plated copper.
The center conductor resistance almost certainly
represents a cost savings measure. Jeff measured the center conductor diameter
as 0.5 mm, close to No. 24 AWG, (0.511 mm). No. 24 AWG copper wire has a
resistance of 25.7 ohms/1000 ft, or 26 mohms/ft. The measured resistance
is more than five times this value. Steel has an electrical resistance about 10
times that of copper, so my belief is that Digiwave's center conductor is soft
steel with a thin copper wash, not enough to make it to the copperweld
classification (40% conductivity usually, from what I can find).
Copper-clad center conductors are not unusual in coaxial
cable. RG-174 is constructed with it, as is nearly all 75 ohm CATV cable.
However, these use a thicker copper cladding than seems to be the case for
the Digiwave cable.
Zo is not 50 ohms above 1 MHz.
This is more puzzling. The most likely answer is the one
suggested by Jeff; that this cable was mis-manufactured. Either it was intended
to be 95 ohm cable (a standard value) and it was mis-labeled, or there was a
serious problem in manufacturing. My guess is the former is more probable than
Loss versus Frequency
The figure below show the loss for the Digiwave cable and
the similar length RG-58 over the range 300 KHz - 1 GHz. (59 feet each)
(Vertical scale is 2 dB/division) No adjustment is made for mismatch loss.
The results are interesting. Up to 100 MHz, real
RG-58 has lower loss, by 2 dB or so up to 10 MHz, but by 100 MHz, the two cables
have similar loss. (The ripples in the Digiwave data is due to the mismatch
between the 95 ohm cable impedance and the 50 ohm vector network analyzer source
and termination.) However, above 100 MHz, the Digiwave cable shows less loss
than real RG-58, amounting to nearly 6 dB less loss at 1 GHz. (Increasing cable
loss dampens the ripple amplitude as the frequency increases.)
Why is this?
At low frequencies, the Digiwave cable's high resistance
increases loss. But above 100 MHz, the difference in loss shifts to favor
Digiwave. Some of this loss may be related to the cable impedance; cables with
an impedance in the 60-75 ohm range have lower loss than 50 ohm cable. (See
http://www.epanorama.net/documents/wiring/cable_impedance.html for a useful
discussion of this concept.) Another factor may be related to the cable shield.
At VHF/UHF frequencies, the foil used in the Digiwave cable is more homogenous
than the woven braid in the classical RG-58, and has less potential for loss.
Another factor may be the foam dielectric used by Digiwave compared with
the solid polyethylene found in the Belden RG-58. Finally, to complicate things
even more, the Digiwave cable will exhibit increased loss due to the mismatch
with the 50 ohm VNA.
The additional loss caused by a woven braid over a uniform jacket (such as in
rigid line, or one with an extruded metallic jacket) is determined by the "braid
factor." The MIT Radiation Laboratory Series describes braid issues as follows:
p. 247, Microwave Transmission Circuits (MIT
Radiation Laboratories Series, Vol. 9), G. L. Raglan, ed., McGraw Hill, New
A brief explanation
of braid construction will serve to point out some of the aspects of the
design of flexible cables for a minimum braid factor. The first step in the
design of a braid is the choice of the wire that will produce a braid rugged
enough to minimize the contact resistance between individual wires. A number
of thin wires are combined to form a carrier that we might compare to a
single flat reed in a woven basket; a number of these carriers are woven in
and out to form the braid. Around 99 per cent coverage is required for a
braided conductor in order to avoid excessive loss by radiation and to
ensure proper shielding. This coverage is determined by the number of ends
per carrier, the number of carriers, and the number of “picks” per inch
which make up the braid. The number of picks per inch is the number of times
that a single carrier crosses over or under another carrier in an inch of
cable. Another term applied to this characteristic of the braid is the word
“ lay”; this is the length of cable required for the carrier to make one
complete revolution around it. Since the currents in a coaxial conductor are
always in the direction of propagation, a braid having a long lay will have
less attenuation than one having a short lay since less energy is dissipated
in contact resistance. Mechanical considerations, however, limit the lay of
the braid, and greater stability with flexing can be attained with a
shorter lay. The tightness with which the braid is woven is also important
in eliminating instability under flexing and in decreasing the contact
resistance between braid wires. It is also important that the individual
braid wires should not be embedded in the dielectric and that jacketing
material should not penetrate between them.
The statement "Around 99 per cent
coverage is required for a braided conductor in order to avoid excessive loss by
radiation and to ensure proper shielding" should be understood in the context -
the application discussed is microwave transmission in the range 1 GHz to 10 GHz
or more. A lower braid percentage braid coverage is satisfactory for frequencies
below 30 MHz.