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)
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)
----- Original Message -----
From: "RCARIELLO" <RCARIELLO@si.rr.com>
To: "Towertalk" <Towertalk@contesting.com>
Sent: Monday, July 28, 2003 6:03 PM
Subject: [TowerTalk] Balanced Line using Coax ?
> Hello to all,
>
> In the ARRL antenna handbook they show where you can use two coax lines as
a
> balanced line. Only the center conductors of the Coax are used. The
article
> covers the new impedance of the lines but skips what the losses are. Also
> what is the power rating of this type of line. It is mentioned the Coax
> shielding provides easier installation and better noise rejection then
true
> Open Wire Line.
>
> I am setting up a new station with a local radio club. The best location
for
> the tower is some distance from the station. We plan to put a large
Tri-Band
> antenna on the tower with a single feed system for 10-15-20 meters. The
> total distance from the antenna feed point to radio input will be about
800
> feet.
>
> To keep transmission line losses and cost to a minimum the thought was to
> use Open Wire Line. This is very doable at this location for most of the
> transmission line run. At the tower and into the station building a Coax
> line would need to be used.
>
> Any help would be appreciated,
> Rich AA2MF
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
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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