given location should be flowing in the opposite direction to the current in the adjacent conductor,
and if the system is well balanced, the amplitudes of the two will be equal. Under these conditions,
the two sets of fields exactly cancel each other and very little radiation will result. If the two currents
are not equal or not in exact opposite phase, there will be radiation. Also, if the spacing between
lines is a considerable portion of the wavelength, radiation will occur. This is not a factor below VHF.
One final characteristic of transmission lines should be mentioned. The rf current flowing in the
line travels at a speed less than that of radiated power in a vacuum, or the speed of light, both
186,000 miles per second. This slowing is caused by the dielectric property of the medium through
which the field traverses. In coax cables it is polyethelene between inner and outer conductors, and
in parallel lines, it may be the plastic between the conductors, in the case of twin-lead type line, or the
air and plastic spacers in open wire types. The ratio of the speed in the line to the speed in a vacuum
(air is almost the same) is called the velocity factor of the cable. It is always less than unity. Because
of this slowing, the physical length of a transmission line is not the same as the electrical length. For
example, the wavelength in free space of a 30 MHz signal is exactly 10 meters. A transmission line 10
meters long will be one full wavelength only if the dielectric between the conductors is air. In the
case of coax cable with polyethelene dielectric, the velocity factor runs about 0.67. The same 10
meter length of cable will now appear electrically as an open wire or air dielectric cable 15 meters
long (10 divided by 0.67). This is equivalent to one and one half wavelengths. A polyethelene
type cable would only have to be 6.7 meters long to be one wavelength.
EFFECT OF TRANSMISSION LINE ON ANTENNA IMPEDANCE - As a result of all of the above, in
situations where we do not have a matched system throughout, and this is most of the time, the
impedance presented to the transmission line by the antenna sets up standing waves on the line.
These standing waves will alter the antenna impedance all along the line toward the transmitter.
What we really want to accomplish with the antenna tuner is to take whatever impedance that is
established at the transmitter end of the line and alter it to a 50 ohm resistance. Then the transmitter
will be happy, at least. The tuner will not affect the mismatch of antenna to line - only constructing
the antenna differently will do that - nor eliminate a standing wave on the transmission line. It will
eliminate a standing wave on the line between transmitter and tuner input, but not on the output
side of the tuner. A good antenna is still needed to "get out." If the antenna has a low resistance, the
tuner will transform it, along with the cable loss resistance, to 50 ohms. The full power will enter the
system, but it will be divided between radiation and cable heat loss. It is not uncommon that more
than half of the available power is wasted in cable losses, even with low loss cable. It just gets a bit
hotter. The split depends entirely on the ratio of radiation resistance to loss resistance.
What is the impedance established at the transmitter end of the line? It depends first on the
antenna impedance, which is then transformed by the line. This transformation is dependent on
frequency, electrical length of the line and the loss in the line. In an Amateur setup where many
different frequencies are used with the same antenna, there will be a multitude of impedances
presented to the tuner, so adjustment of the matching network will be required as frequency is
changed.
STANDING WAVE RATIO - A measure of how badly a system is mismatched is given by the
standing wave ratio (SWR) on the line. SWR is the ratio of the maximum voltage encountered along a
transmission line greater than one half wave-length long to the minimum voltage. It is also the ratio
of maximum to minimum current. The more nearly uniform the voltage distribution along the line,
the closer matched it is, and the ultimate is when the voltage is constant down the length of a lossless
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