given l cati n sh uld be fl wing in the pp site directi n t the current in the adjacent c nduct r,
and if the system is well balanced, the amplitudes f the tw will be equal. Under these c nditi ns,
the tw sets f fields exactly cancel each ther and very little radiati n will result. If the tw currents
are n t equal r n t in exact pp site phase, there will be radiati n. Als , if the spacing between
lines is a c nsiderable p rti n f the wavelength, radiati n will ccur. This is n t a fact r bel w VHF.
One final characteristic f transmissi n lines sh uld be menti ned. The rf current fl wing in the
line travels at a speed less than that f radiated p wer in a vacuum, r the speed f light, b th
186,000 miles per sec nd. This sl wing is caused by the dielectric pr perty f the medium thr ugh
which the field traverses. In c ax cables it is p lyethelene between inner and uter c nduct rs, and
in parallel lines, it may be the plastic between the c nduct rs, in the case f twin-lead type line, r the
air and plastic spacers in pen wire types. The rati f the speed in the line t the speed in a vacuum
(air is alm st the same) is called the vel city fact r f the cable. It is always less than unity. Because
f this sl wing, the physical length f a transmissi n line is n t the same as the electrical length. F r
example, the wavelength in free space f a 30 MHz signal is exactly 10 meters. A transmissi n line 10
meters l ng will be ne full wavelength nly if the dielectric between the c nduct rs is air. In the
case f c ax cable with p lyethelene dielectric, the vel city fact r runs ab ut 0.67. The same 10
meter length f cable will n w appear electrically as an pen wire r air dielectric cable 15 meters
l ng (10 divided by 0.67). This is equivalent t ne and ne half wavelengths. A p lyethelene
type cable w uld nly have t be 6.7 meters l ng t be ne wavelength.
EFFECT OF TRANSMISSION LINE ON ANTENNA IMPEDANCE - As a result f all f the ab ve, in
situati ns where we d n t have a matched system thr ugh ut, and this is m st f the time, the
impedance presented t the transmissi n line by the antenna sets up standing waves n the line.
These standing waves will alter the antenna impedance all al ng the line t ward the transmitter.
What we really want t acc mplish with the antenna tuner is t take whatever impedance that is
established at the transmitter end f the line and alter it t a 50 hm resistance. Then the transmitter
will be happy, at least. The tuner will n t affect the mismatch f antenna t line - nly c nstructing
the antenna differently will d that - n r eliminate a standing wave n the transmissi n line. It will
eliminate a standing wave n the line between transmitter and tuner input, but n t n the utput
side f the tuner. A g d antenna is still needed t "get ut." If the antenna has a l w resistance, the
tuner will transf rm it, al ng with the cable l ss resistance, t 50 hms. The full p wer will enter the
system, but it will be divided between radiati n and cable heat l ss. It is n t unc mm n that m re
than half f the available p wer is wasted in cable l sses, even with l w l ss cable. It just gets a bit
h tter. The split depends entirely n the rati f radiati n resistance t l ss resistance.
What is the impedance established at the transmitter end f the line? It depends first n the
antenna impedance, which is then transf rmed by the line. This transf rmati n is dependent n
frequency, electrical length f the line and the l ss in the line. In an Amateur setup where many
different frequencies are used with the same antenna, there will be a multitude f impedances
presented t the tuner, s adjustment f the matching netw rk will be required as frequency is
changed.
STANDING WAVE RATIO - A measure f h w badly a system is mismatched is given by the
standing wave rati (SWR) n the line. SWR is the rati f the maximum v ltage enc untered al ng a
transmissi n line greater than ne half wave-length l ng t the minimum v ltage. It is als the rati
f maximum t minimum current. The m re nearly unif rm the v ltage distributi n al ng the line,
the cl ser matched it is, and the ultimate is when the v ltage is c nstant d wn the length f a l ssless
