[Antennas] Twin lead 450 Ohm

[George, W5YR]

----- Original Message -----
From: "Chris BONDE" <ve7hcb@rac.ca>
To: <antennas@mailman.qth.net>
Sent: Wednesday, December 11, 2002 1:02 PM
Subject: Re: [Antennas] Twin lead 450 Ohm


> I donot know too much about transmission lines, I had started
studying such
> but a health problem came along. (So much for an intro)
>
> One of the things that twiged my attention was the use of two
parallel coax
> lines as twin lead.  The centres were used as the two twin lead
lines, the
> braid was joined at either end and grounded.  The formula for
calculating
> the resultant characteristic impedance was not that difficult.  Some
people
> have used short runs 10ft or so from the outside of the shack to the
tuner
> (of two coax lines) joined directly to the twin lead.  The physical
> placement of the two coax lines doesnot seem that  onerous.  So I
think
> that it should be looked at.
>
> Any comments from those who have used and know?

Almost correct, Chris - I use just that arrangement to bring
ladderline into the house and tuner. I use parallel RG-213 with the
shields connected at both ends.

BUT, and this is a big one, the braid is connected to the "station
ground" in the shack - usually the tuner chassis - and left floating
at the other end. Do not ground both ends - the reasons are many and
complex, so just don't do it!   <:}

The two coax lines can be placed anywhere as long as they are the same
length. They need not maintain any particular spacing.
>
> Second point seems to me is that, yes, anything near the twin lead
will set
> up a disconuity hence a reflection hence changes.  However, reading
what
> some pople have done, eg run the line over coax, over pipe, held
down by a
> tool box,  it seems as if as long as the disconuity is equal on both
sides
> the reflection doesnot upset the ham stuff that much, ei can tune it
out
> and the conjugate match is great.
> A minor point:  donot the papers say that as long as the probable
> interfering objects is 2 to 3 time the spacing between the wires
away from
> the centre of the wires there is little interference?

The effects are usually small enough to ignore. Running ladderline at
right angles to anything metallic is a good idea and keeping it spaced
a few inches from any mass of metal is another one.
>
> Again any comments?

Nope - just don't ground those shields at both ends but connect them
together at both ends.

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489  IC-765 #2349 IC-756 PRO  #2121 IC-756 PRO2 #03235