Balanced Line using Coax Results
I have summarized the many replies received to my question.
A very grateful thank you to everyone that replied.
Rich AA2MF
W4DG
I noticed a LARGE difference in the noise pickup between using the
traditional 450 ohm #14 commercial ladder line opposed to using the balanced
coax method. I concluded that having the shields interconnected and
grounded at the shack end was causing the improved noise factor!
Gene Bigham
Several years ago I put up a full size loop for 5 MHz and fed it with a pair
of rg59u coaxes using the center of each coax to either leg of the loop. The
shields were tied together out at the antenna but left floating/unconnected
at that point. In the shack each coax center was tied to the balanced output
of the tuner and the shields tied together to the case of the tuner. Along
the run of the twin-ax as I liked to call it I used nylon zips to keep the
two parallel to each other. I found this arrangement to provide a better
signal to noise ratio on the loop than either twin lead or coax alone. 1kw
was put through this line on several occasions with no ill effect and a lot
of PSK31 was done at low power on 30 meter band.
Dave, wa3gin
I have fed a 40m loop, tri-band Quad, mono-band beam and 160m dipole.
The loops were fed with the shields tied together at the antenna but not
connected. At the tuner side the shields were tied together and
connected to ground.
For feeding the beam, the driven element was a split dipole with hair
pin match. The connection at the tuner end was the same as with the
loops. The connection at the driven element was different. The center
conductors connected to one each of the dipole elements, as you would
expect. The shields were tied together and grounded to the boom. The
hair-pin match was adjusted to match the 104 +/- ohm coax line impedance.
One could hear the difference in common mode/static noise pick-up
between a single coax and the shielded pair line (the shielded pair was
very quiet).
For the 160m dipole the antenna side was the same as the 40m loop with
the exception that I added a pair of 2.5mh chokes that went from each
side of the dipole element to the bonded shields. The antenna was
strung between three 100' white oak trees and prior to going to twin
coax I had open wire line. However, the static charge build-up on those
wires during lightning storms would melt the feed-line right off dipole.
Some mornings I wake-up and see little white insulators scattered all
over the back yard. Problem solved with twin coax and chokes installed.
So, don't know what the book says or EZ, etc. just know that is the way
it worked for me.
Jim Lux
The losses of the dual coax will be comparable to that of a single coax run.
For lower frequencies, the ohmic losses dominate (skin effect,
predominantly) over the dielectric loss (at least until you get over 100 MHz
or so). Since the dual coax has twice the impedance of a single coax run, to
a first order, the current will be 70% less ( I1^2*Z0 = I2^2*2*Z0) so the
I^2*R losses will be halved (a rigorous treatment would look at whether it's
the AC resistance of the center conductor or the Rac of the shield that
dominates)
Jim Lux
Power rating wise, you need to consider both the thermal dissipation limit
(where, because the current is divided between two conductors, the
dissipation is half), and the breakdown voltage limit (where it should be
exactly the same.. the limit is center to shield, and with twice the
impedance, you'll have twice the voltage, divided equally between the two
runs of coax)
Jim Lux
Running a pair of coax as a balanced pair is just hooking two transmission
lines in series to make a line of impedance 2*Z0... It's no different than
hooking two lines in parallel to get Z0/2. If you don't connect the shields,
of course, then what you've got is two funky wires with a capacitor to
ground with some horribly indeterminate impedance. If you don't ground the
shields, but they are connected, then the shield will tend to float to the
common mode voltage. Impedance wise it will look ok, but you'll lose the
"shielded twin-lead" effect.
Jim Lux
Looking at loss as a dB/foot for the coax, you
divide the power into two pieces of coax, so the absolute loss (in watts)
will be half in each piece of coax, but you've got two coaxes, so the total
loss is exactly the same.
Jim Lux
Here's an interesting question.. Open wire line is reputed to have very low
loss compared to coax. Why is this? Is it because of the high Z, so low
currents? The dielectric loss in coax <30 MHz is pretty low. What's the
typical wire size on open wire line (RG-213 is AWG13)? Is the skin depth
different in a coaxial configuration compared to the twin-lead/quadro-line
configuration? Any EM fields folks want to take a crack at a rigorous
analytical description of the fields?
Mike, W4EF
If you don't connect the
shields together at both ends, then current can be
capacitively coupled from the center conductors through
the dielectric to the "outside" of the shields. In this case
the cable will look like unshielded twin-lead with "fat"
conductors of diameter equal to the OD of the coax
shields and impedance determined by a combination
of the spacing of the two coaxes and the dielectric
properties of the coax jacket material. If the coax
cables are physically close (as if taped together)
then this configuration will have a very low characteristic
impedance and hence be very lousy.
If you short the shields together on both ends, then the
electric field between the shields of the coaxes goes
to zero. In this case, the impedance of the shielded
twin-lead becomes 2*Zo, where Zo is the characteristic
impedance of the individual coax cable. I believe Jim,
W6RMK is incorrect that the insertion loss will be
approximately 1/2 that of the insertion loss of the
individual coax that makes up the twin-lead. Although
he is correct that the current is reduced by a factor
of 0.7 in each center conductor (as compared to
a single coax) there are still two center conductors,
so the net I^2*R loss is the same. Even if the loss
is due to I^2*R loss in the shield, the same logic
applies - the loss per foot is the same as that of
the individual coax cables.
Because the impedance of the shielded twin-lead
is 3 to 4 times lower than twin-lead or ladder line, this
type of line will generally be more lousy than twin-lead
or ladder line having the same center conductor
diameter. If you use matching transformers to keep
the VSWR low, the loss should be tolerable. If not, you
can still get decent loss performance by using very low
loss coax. CATV hard-line would be a good choice as
it would yield a net characteristic impedance of 150
ohms in a twin-lead configuration.
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