Someone get the duct tape my head is about to explode!
This sounds like a good way to have a balanced feed system with no
baluns to go zap...but...it looks like the verdict is still out.
I have one situation where this might be interesting to me personally...
feeding a 160 meter dipole/inverted vee which is suspended pretty high
up - the though of having to keep open wire line away from the tower is
a nightmare... I have been feeding it with coax without a balun at the
top and wonder if two runs of relatively inexpensive coax like RG8X
paired up would be an improvement. I think the 8X should handle 1500W
the okay and if I place a tuner at the base of the tower I can have a
short coax run into the shack/six pack.
Is this worth the effort? I like the idea of minimizing noise pickup
whenever possible and with several broadcast sites near the station I am
wondering if this would help.
IF YOU HAVE DONE THIS, let's hear form you... the theorists are giving
me a headache! Lets talk actual results!
Thanks,
Jim, K4OJ
RCARIELLO wrote:
> 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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> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
> questions and ask for Sherman, W2FLA.
>
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