Practical_Antenna_Handbook_0071639586

218 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
lel resonant traps are used in combination with shortened segments of wire or aluminum
tubing to provide the equivalent of quarter-wavelength half-elements on each
band of interest. The technique can be applied to monopoles or to both sides of a dipole
or other balanced antenna. Typically, each trap is parallel resonant at one of the desired
operating frequency ranges. The high impedance associated with a parallel-resonant
circuit allows very little radiofrequency (RF) energy at the trap frequency to pass from
one side of the trap to the other. At frequencies other than the trap design frequency,
the RF excitation of the element passes through the trap relatively unattenuated. Below
the trap design frequency, the trap appears as a net inductive component and above the
design frequency it appears as a net capacitive component.
Let’s look at how one pair of traps can provide two-band operation with low SWR
for a simple dipole. In the example of Fig. 8.1A, the trap on each side of the center insulator
is parallel resonant on 10 m. Because of the high impedance of the trap on that
band, very little 10-m energy gets beyond the traps, and only the sections of wire labeled
“A” have any appreciable RF in them. If the length of each section A is approximately
one quarter-wavelength (or about 8 ft long on the 10-m band), the antenna will
function as a resonant 10-m dipole on that band.
If a transmitted signal on a lower frequency (say, 15 m) is applied across the center
of this antenna, the reactance of the trap capacitor will increase but the reactance of
the inductor will decrease. Since the capacitor and the inductor are in parallel, the
capacitor will not have a major impact on the operation of the antenna on 15 m, but
the inductor will provide a modest amount of lumped-element “loading”. As a result,
the sum of the lengths of A and B will be less than a full l/4 (the natural nontrap
length) on 15 m.
In general, trap dipoles are shorter than nontrap dipoles cut for the same band. The
actual amount of shortening depends upon the values of the components in the traps,
so consult the manufacturer’s data for each model of trap purchased.
Some trap antennas employ multiple traps on each side of the center insulator to
cover three or more bands with a single feedline. The most popular combinations are
probably 20-15-10 and 80-40-20, employing two separate parallel-resonant traps on
each side of the insulator. The principle of operation is as previously described, except
that a third wire segment is usually found between the two sets of traps on a side.
Where more than one pair of traps is used in the antenna, make sure they are of the
same brand and are intended to work together.
On all bands except the highest one, the radiation efficiency of the trap antenna will
be somewhat less than that of a full l/4 monopole or l/2 dipole for the same band.
That is because a small portion of the radiating element has been converted to a nonradiating
lumped inductance. However, the effect is small, usually resulting in a net reduction
in radiated field strength of 0.5 dB or so, depending on the exact design of the
traps and their distance from the feedpoint.
A disadvantage of trap antennas is that they provide less harmonic rejection than an
antenna designed for a single band. The antenna has no idea what band the transmitter
“thinks” it’s transmitting on, so any harmonic energy that falls within any of the design
bands of the trap antenna will be radiated with the same efficiency. As a result, users of
multiband antennas need to take every reasonable precaution to be sure that harmonics
and other out-of-band spurious emissions from their transmitters or transceivers and
associated amplifiers are as low as possible.