[Antennas] Balanced line optimum conductor spacing?

Sat, 05 Jan 2002 14:46:22 -0600

Thanks for the information provided on transmission lines.  I found it to be 
lucid as well as informative.  Don't know what the comments provided by Gene 
Mason were attempting to prove.  Please don't let such criticism inhibit you 
in replying to antenna questions.

Sincerely,

Jim, wd4air

>From: Dave Shrader <[email protected]>
>Reply-To: [email protected]
>To: [email protected]
>Subject: Re: [Antennas] Balanced line optimum conductor spacing?
>Date: Sat, 05 Jan 2002 07:32:41 -0500
>
>Gene Mason wrote:
> >
> > Comment..  Is this a case of the 'proof is left up to the student' ?
> > btw, the last time I checked, Maxwells equations didn't discriminate
> > between positive and negative currents.
>
>You are correct in that Maxwell's Equations don't discriminate based on
>current polarity; but, the resulting EM fields are vectors that have
>magnitude and direction [a 100 V/M field at 0 degrees at a point and a
>100V/M field at the same point from near the same source but at -180
>degrees relative phase shift produce a 0 V/M field.
>
> >
> > In a transmission line, frequency doesn't enter into the
> > calculations, until the line losses become excessive at the upper
> > frequency limit.
>
>Disagree. Attenuation in a transmission line is present at all
>frequencies. It is measured in Nepers/Wavelength. Frequency is very much
>part of all transmission line calculations [reciprocal/wavelength].
>Current is not constant in the transmission line. It decreases along the
>length of the line as e^-((alpha + jBeta)*L) where alpha is the
>attenuation in Nepers/wavelength and Beta is the phase velocity in
>radians/unit-length and L is line length. So, from the point of Physics
>the vector sum of the fields from an open wire line, using a method of
>moments calculation, contains more energy from the source portion of the
>line than the load portion of the line. From an engineering perspective
>line losses are part of the system power budget.
>
> > The technical data on designing and constructing open
> > wire feeders, suggests that the wire is not insulated and low loss
> > spreaders are used. To do otherwise will produce undesireable
> > results.
>
>Agree. But the question dealt with line to line spacing in open wire
>line and trying to determine 'optimum' spacing. If you are a Physicist
>the optimum spacing is as close as you can make it to minimize pattern
>distortions from the collinear axis of the line. If you are an engineer,
>a small variation in a pattern is accepted as the result of a series of
>compromises including wire strength, wind loads, ice loads, practical
>characteristic impedance levels, construction details such as spreader
>losses, precipitation effects, etc.
>
>Driven antenna arrays, Collinear or Broadside, are examples of open
>transmission lines, 1/4 to 1/2 WL long, where the line to line spacing
>is a significant portion of a wavelength producing relative phase angles
>in the resulting EM fields to achieve gain! This gain is the
>'distortion' I'm trying to address. As long as the relative spacing
>between the wires is a small percentage of a wavelength the
>'distortion', radiation from the line, is minimal.
>
> > At very high frequencies, there exist alternative
> > transmission devices, these are called waveguide at microwave
> > frequencies.
>
>Agree, but it was not a question of waveguides! It was about spacing of
>the wires in open line construction. Propagation in waveguides is a
>totally different mode from transmission lines. In waveguides the energy
>is contained in the fields within the guides, not the conducting
>surfaces. In a TL the energy is contained in the conductors.
>
>[BTW, my center fed doublet uses #16 stranded and insulated wire at a
>nominal 600 ohms characteristic impedance.]
>
>Let's let it end here before this becomes an honest dispute between
>Physics and Engineering.
>
>73, Dave, W1MCE, BSEE
>Manager, Textron Antenna Range, retired
>
>PS
>
>W8JK, John Kraus, has written extensively, McGraw-Hill Publishing, in
>his University Text Books on Electromagnetics and Antennas on this
>subject. It's been 43 years since I plowed through the math, but I
>really learned the lesson!
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