[Antennas] Re: Twin lead 450 Ohm

2002年12月11日 17:07:39 -0700

Thanks for proving my point! Too much theory, not enough practical
experimentation! It's "Amateur Radio". The guy on the other end of the
QSO still won't hear the difference! HAVE MORE FUN!!!
73's de Jerry, K�EJF
On 2002年12月11日 18:54:17 -0500 "Barry L. Ornitz" <[email protected]>
writes:
> Honest Charlie's Used Cars <[email protected]> asked:
>> > Opinions Please...If 450 later line was suspended in
> > the center of a ten" PVC pipe and buried would it have
> > the properties of one suspended in Air?
>> If the spacing from the wires to the pipe are at least two to 
> three times the spacing between the line conductors, the 
> effect will be minimal. So with commercial "window line" with 
> its relatively small spacing, the line should show little 
> extra loss or change in velocity factor. However, with true 
> open-wire line having an air dielectric, you would see more of 
> an effect as the spacing between the wires will be larger with 
> most realistic wire sizes (for example, a 450 ohm ladder line 
> would have a spacing of 1.75 inches for 12 gauge wire).
>> A simple thought experiment will show why this is true. The 
> impedance of the line is determined by the distributed 
> capacitance between the conductors and inductance of the 
> conductors. If the line is near another conductor, the line 
> capacitance will also include the effects of the capacitance 
> of each line conductor to this other conductor. In this case, 
> consider the PVC pipe to be buried in perfectly conducting 
> soil. The capacitance between the inside and the outside of 
> the PVC pipe will be something over 1000 pF per foot if the 
> wall thickness is 0.25 inch.. But the capacitance between one 
> of the line conductors and the wall of the pipe will be at 
> most a few picofarads per foot. Since the two capacitors are 
> in series, the resulting capacity will still be dominated by 
> the capacitance of the line to the wall. But as the spacing 
> between the line conductors and the wall become less, their 
> capacitance to the wall increases and the effect of the wall 
> becomes more.
>> George, W5YR, noted that when running ladderline that it is 
> best to keep it perpendicular to anything metallic. If it has 
> to be run parallel to another conductor, it is important to 
> keep both wires an equal distance from that conductor to 
> maintain balance. Actually it is better to twist the line 
> slightly over long runs. Much like twisted pairs at lower 
> frequencies, this insures that both conductors will see equal 
> interfering signals induced in them. I always twist my ladder 
> lines with a half-twist per foot for most installations.
>> As for running two sections of coaxial cable in parallel 
> (using the two inner conductors as a balanced line), the 
> impedance is simply twice the coaxial cable impedance. I 
> wouldn't call this a complicated formula! Naturally both 
> cables should be exactly the same length. Bonding the shields 
> together at both ends is optional, but with real cables it 
> insures slightly better balance. Again as George notes, the 
> shields should be grounded to earth at one end only - usually 
> the transmitter end.
>> A question has also come up about the effect of "mist" in the 
> vicinity of the ladder line. Someone else suggested that line 
> losses might be affected by humidity changes. Actually as 
> long as the mist is isolated water droplets in humid air and 
> the line is not actually wet with liquid water, the effect 
> will be minimal. The dielectric constants of gases, even 
> polar gases such as water vapor, are very close to unity. The 
> main effect that humidity would have on the loss of ladder 
> line would be the moisture absorbed in the line dielectric. 
> With a polyethylene dielectric, the maximum moisture 
> absorption is less than 0.01 percent making the change in 
> dielectric constant less than about a third of a percent in 
> worse case conditions. The effect of humidity in the air 
> makes a much larger difference in radio propagation at 
> microwave frequencies, but even here you are talking about 
> decibels of loss per kilometer, not centimeters.
>> However surface oxidized polyethylene is wettable and liquid 
> water will cling to this surface. A thin coating of liquid 
> water will increase the cable losses and lower the 
> characteristic impedance. The effect is strongly dependent on 
> the thickness of the water. Treating the surface of ladder 
> line with a water repellant will therefore help in wet 
> weather. Silicone oils are generally to be preferred over 
> conventional organic waxes (i.e. Turtle Wax or Armor-All is 
> better than Carnuba wax).
>> 73, Dr. Barry L. Ornitz WA4VZQ [email protected]
>> - - - 
>> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
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>>
E. Jerold (Jerry) Forwood
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