Actually, it depends on whether or not you connect the
shields together at both ends. If you don't connect the
shields together at both ends, then current can be
capacitively coupled from the center conductors thru
the dielectric to the "outside" of the shields. In this case
the cable will look like unshielded twinlead with "fat"
conductors of diameter equal to the O.D. 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 lossy.
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
twinlead 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 twinlead. 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 twinlead
is 3 to 4 times lower than twinlead or ladder line, this
type of line will generally be more lossy than twinlead
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 hardline would be a good choice as
it would yield a net characteristic impedance of 150
ohms in a twinlead configuration.
73 de Mike, W4EF............................................
----- Original Message -----
From: "Frank Mayer" <domino@worldlynx.net>
To: "Towertalk" <Towertalk@contesting.com>
Sent: Tuesday, July 29, 2003 2:39 AM
Subject: Re: [TowerTalk] Balanced Line using Coax ?
> The dual coax setup will have no more loss appreciably than the twin lead.
> It essiencially becomes shielded twin lead. Remember that the shield from
> the 2 coaxes is NOT connected at the antenna end. That makes all the
> difference.
> Frank, WY3D
>
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