[Antennas] Re: Twin lead 450 Ohm

[Barry L. Ornitz]

Honest Charlie's Used Cars <chas_east@yahoo.com> 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     ornitz@tricon.net