[Antennas] Balanced line optimum conductor spacing?

[Frank J. Jershe]

Jim,
If I can put it other terminology, other than antennas, it would go like
this. Gene looked at the comment that the sun is always hot on earth and it
is the winter snow that cools the earth in winter. Both are true
statements, but not entirely true for any given day of the year. To say the
"sun is always hot on earth" is true at some location on the earth, but the
sun is not always as hot at every earth location because of the tilting of
the earth (in relation to the sun) as it orbits the sun in any given year.
In the northern hemisphere, the greatest heat of the sun is felt during the
"summer months" with the longest day (daylight produced by the sun)
occurring in June. In the southern hemisphere, this is the same day that
they have the shortest day. Same is true in December, when we, in the
northern hemisphere, have the shortest day (of daylight), and those in the
southern hemisphere have the longest day. The reason for the cold is not
the snow in winter, but the fact that the sun is not heating that "half of
the earth" for a long enough period each day. That allows water to turn
into snow and stay in that form until enough heat is applied to melt it.
The snow cover on the ground usually does keep the temperature at that spot
on the earth from going above 55 degrees because of the cooling effect of
the evaporating snow as it is being heated from the sun (usually in the
spring time in Minnesota), but it is not the reason for cold temperatures
in the winter months! Gene was merely saying that the answer was mostly
correct, but not exactly for the reasons stated. I don't know Gene, but I
totally understand what he was saying. If you've never got into the deep
math/engineering/physics, then his answer may seem "off the wall", "too
technical", or even "offensive". He was merely trying to correct the
"reason why" and was not trying to offend anyone, nor change the "bottom
line" answer. I hope I didn't confuse you any more with my "sun" example.
73
Frank

At 02:46 PM 01/05/2002 -0600, you wrote:
>
>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 <dshrader@earthlink.net>
>>Reply-To: antennas@mailman.qth.net
>>To: antennas@mailman.qth.net
>>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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