Practical_Antenna_Handbook_0071639586

C h a p t e r 8 : M u l t i b a n d a n d T u n a b l e W i r e A n t e n n a s 225
of 300-Ω line or 34 ft of 450-Ω line. Thus, the antenna itself is shorter than a l/2 80-m
dipole but longer than a 40-m l/2 dipole. In short, the dipole itself is not a particularly
good match on 80, 40, 20, or 15 m, but the effect of the short matching section is to bring
the feedpoint impedance of the combination closer to a reasonable match for either
50-Ω or 75-Ω coaxial cable on those bands. It is not hard to see that the VSWR of the assembly
varies significantly across the HF spectrum, and the addition of amateur bands
at 12, 17, and 30 m has blunted the utility of the G5RV noticeably.
Of course, with a good ATU, the antenna can be matched throughout much, if not
all, of the HF spectrum but then there is no need for a specific length of 300-Ω or 450-Ω
line.
Longwire Antenna
If, instead of using a feedline, a single wire is brought from the end-fed antenna directly
into the radio room or to the ATU (wherever it may be located), the antenna is simply a
longwire antenna. In this case, it can be thought of as having a single-wire feedline, but
in truth the feedline is part of the antenna radiating system and should be analyzed or
modeled as such. Such an antenna configuration can work (the author used one about
30 ft high to workstations around the world with his 10W transmitter for the first four
years of his HF amateur radio activities), but the user must realize the most important
thing he or she can do to improve the antenna is to make sure there is an excellent RF
ground system (see Chap. 30) attached to the chassis of the ATU or transmitter where
the wire connects. Keep in mind that bringing the radiating portion of the antenna into
the radio room, coupled with an inadequate RF ground, may have some unintended
consequences: greater exposure to RF fields, greater likelihood of interference to audio
equipment and telephones, RF burns to the fingers caused by touching the metal chassis
of the transmitter, etc.
By longstanding convention, a “true” longwire is a full wavelength or longer at the
lowest frequency of operation. In Fig. 8.6 we see a longwire, or “random-length”, antenna
fed from a tuning unit. As the operating frequency is varied, the feedpoint impedance
of the long wire will also vary significantly, with a reactive component that can be
quite substantial. Again, use of a superior ATU is important for matching the generally
unpredictable feedpoint impedance of the antenna to the typical transmitter output
impedance of 50 Ω.
Off-Center-Fed Dipole
The radiated field from a half-wave dipole operated at its fundamental resonant frequency
is the result of a standing wave of current and voltage along the length of the
dipole. As we saw in an earlier chapter, the center of such a dipole is a point of maximum
current and minimum voltage. Thus, if we feed the l/2 dipole at its center, the
feedpoint impedance is 73 Ω in free space, and it oscillates between a few ohms and 100
Ω as the the antenna is brought closer to ground. If we feed the same dipole at one end
instead of in the center, the feedpoint impedance is quite high, perhaps between 3000 Ω
and 6000 Ω, depending on the effects of insulators and other objects near the ends of the
wire. By feeding the dipole at an intermediate location between the center and one end
it is often possible to find a more attractive match to a specific feedline’s characteristic
impedance.