>From Cebik: http://cebik.com/content/a10/trans/spcoax.html
There are two ways of connecting coax cable to create open wire lines- series
and parallel.
In series-connected coaxial cable, we use the center conductors of two coaxial
cables that run side-by side and are of the same length. We can separate the
lines, since the currents and voltages on each leg of the line are functions of
the center conductor and the outer cylinder. By connecting the outer cylinders
(or braids) together at the top and bottom, each line will have comparable
current magnitudes in opposite phases at each measurement point along the cable
length. The characteristic impedance of a series connected coaxial cable is
twice the impedance of a single cable of the same type. However, the power
rating will be about the same as for a single cable, since the current-
carrying ability of the center conductors has not changed. However, in higher
impedance situations--for which series cables are often used--the current level
may be naturally lower than when the same cable is used in a well matched
situation. On the other hand, the series
cable may be used in highly mismatched situations, resulting in high peak
currents resulting from the high SWR level.
In parallel-connected coax, the two center wires are soldered together at both
ends and the two braids are soldered together at both ends. The use of
parallel-connected coax cables of the same length normally involves cases where
we need a cable of 1/2 the Zo of a single line of the same cable. Paralleled
cables will provide roughly twice the power handling capacity of a single cable
in the HF region as the surface area of the center wires is doubled.
Rudy N2WQ
--- On Thu, 11/5/09, Rick Karlquist <richard@karlquist.com> wrote:
From: Rick Karlquist <richard@karlquist.com>
Subject: Re: Topband: Open wire line vs. Paralleled coaxials.
To: "Rudy Bakalov" <r_bakalov@yahoo.com>
Cc: "topband@contesting.com" <topband@contesting.com>
Date: Thursday, November 5, 2009, 10:18 PM
Rudy Bakalov wrote:
> the impedance of a single cable of the same type. However, the power
> rating will be about the same as for a single cable, since the current-
> carrying ability of the center conductors has not changed. However, in
No. The power handling capability is doubled. The insertion loss
is unchanged.
> The losses on a coax are because of dielectric losses. But here you are
Dielectric losses in coax are neglgible on 160 meters. Nearly all
loss is conductor loss.
> not using the shield as RF conductor, so the PE of foam dielectric is
> working in a different manner. The return goes on the other center
> conductor and not the shield. Loss figures are between those of coax and
> those of "real" open wire lines; they are closer to the open wire and not
Again, the dB/100 ft loss of two coax lines in a balanced configuration
is exactly the same as one coax line in an unbalanced configuration,
assuming the shields are tied together, and assuming a matched load.
If unmatched, losses in general tend to go up.
>
> Rudy N2WQ
Rick N6RK
_______________________________________________
160 meters is a serious band, it should be treated with respect. - TF4M
|