Antennas/Antennas_CD-ROM_Walnut_Creek_September_1996.iso/articles/trapant/trapants.txt

Text File | 1996-06-30 | 7KB | 170 lines

DESIGN YOUR OWN TRAP ANTENNA

R. P. Haviland, W4Mb


Even though the trap antenna seems to be sim-
pie, designing one -- on paper or by cut-and-try --
can be tedious. It's possible, however, to take the
tedium out of the process by using a home computer
or a programmable calculator. The antenna described
in this article should perform acceptably on first use,
and should be easy to adjust.

 The basic concept underlying the trap vertical, di-
pole, or beam antenna is simple: the bottom (or half
center) section is simply 1/4 wavelength long at the
highest frequency. A parallel-resonant trap tuned to
this operating frequency effectively isolates the rest
of the antenna. On the next lower band, the bottom
section will be less than 1/4 wavelength long (physical-
ly). This length is electrically extended by the loading
coil effect of the below resonance trap (looks like an
inductance), and the combination must be brought to
90 degrees effective (electrical) length by a second sec-
tion of tubing. A trap resonant at this lower frequen-
cy can now be added if more bands are needed, or
the antenna simply "stopped." Each additional band
requires a trap, plus an extended metal section. At low
frequencies, a loading coil can also be added to reduce
section length.

 The challenge of trap antenna design lies in the fact
that conversion from the physical length of sections
to electrical length depends on the ratio of section di-
ameter to wave length, so that the values must be re-
calculated for each band. Also, the trap-to-loading coil
effect must be redone for each band, with conversion
from reactance to effective length also considered.
Unless an organized approach is used, it's easy to get
lost.

 For Amateur Radio purposes, the key to this or-
ganized approach is a series of articles by Boyer,
WGUIH (Ref. 1). The following summarizes the calculation
steps involved and the equations used. (For explana-
tions of the individual steps, refer to the Boyer series.)

INPUT the design parameters:
  Number of bands
  Operating frequencies, fn, (highest first, in order)
  Section diameters, Dn
  Trap capacity or inductance, Cn or Ln

CALCULATE for specified frequencies:
  Trap inductance or capacitance, and reactance, Xn
  Section characteristic impedance, Zo
CALCULATE by looping:
  Set first section length h to 90 degrees
Loop for each band and for each trap
Calculate the wave length =11808/f inches
Calculate the section impedance
  Zn = 60 (Ln*LAMDAn/Dn, - 1)
Calculate the ratio of trap frequency to operating
frequency fn/f'n =m
  Calculate effective section length h' = mh
  Calculate normalized trap reactance X'L = XL (1/m
  - m)/Zn
Add reactances X' = X'L + X
Convert to length h = tan^-1* X'
  Determine section length when all traps and sec-
tions have been considered.

S = 90" - h

Coax can be used for Traps

 There is a special feature used in the program. This
is a routine to calculate the number of turns of RG-58U
coaxial cable required to resonate as a trap, as de-
scribed by Johns, W3JIP, Ref.2, and to calculate the induc-
tance. If other coil diameters are needed, some expe-
rimental coils should be constructed and their frequen-
cy measured. The number of turns required:

 N = 135.7f^-0.91 on 7/8 inch form
 N = 68.86f^-0.86 on 1.5 inch form

The inductance is calculated from Wheeler's frequen,
        L = ((D + 0.2)^2* N2)/(18*(D + 0.2) + 8*N

where the 0.2 is the diameter of RG-58. These rela-
tions must all be changed if coax of another diameter
and capacity per foot is used.

 The final step in the program is conversion of the
dimensions and printing the results. The sketch of Fig.
1 should aid in keeping the quantities organized. (In
the above equations, a prime is equivalent to chang-
ing a subscript).

Program Listing

 The BASIC program for the calculation is given in
the section, W4MB antennas. Each section of the program
is set off by an REM statement to correspond to the
program outline. The original program
was in Sinclair BASIC, but should run on any common
home computer with minor changes.

 Fig. 3 shows a sample run. A five-band version
using only one-inch tubing would be difficult to build,
mechanically, but could be done by using glass-fiber
lines for support. A three-band version using two traps
is very practical. The writer, as experimental station
KK2XJM, used a six-band version covering 10, 12, 15,
18, 20, and 30 meters.

 The main calculation routine is also available for use
on the HP 67/97/41 series of calculators.(Ref.3)

Construction Hints

 Note that these programs can be used for dipoles
by entering wire diameter and by considering that the
base section is measured from the center of the dipole
to the first trap. When designing antennas using these
programs, it is usually best to set the capacitance,
since it is most difficult to change. Typical values for
dipoles would be 25, 35, 50, and 100 pF for the 10-40
meter traps.

 The ARRL Antenna Handbook has some hints on
construction, and there have been many articles on
construction of both integral traps for beams and dis-
crete ones for dipoles. Lately, the author has used only
the coaxial trap design for HF. Trap tuning in all cases
is by changing coil turn spacing. Sections should be
built to allow some length adjustment - about four
inches. If necessary, sections can be cut shorter, but
the need for this should be rare.

 To avoid making up new sections if they must be
lengthened, a form of "capacity hat" may be used.
This can be two lengths of small tubing clamped to
the section to be lengthened at 90 degrees to each
other. Tuning is accomplished by moving the "hat"
toward or away from the top end of the section. The
total length of each added section of tubing should
be about twice the added length of section needed.

 If the section lengths are excessive, the required
lengths can be reduced by using the "capacity hat,"
a loading coil, or both. To calculate the effect of a
loading coil, introduce a dummy frequency, a few per
cent lower than the value of the next higher band fre-
quency, and use a trap inductance equal to the value
of the loading coil contemplated. It will probably be
necessary to make several trials to arrive at reasonable
values (see sidebar).

 Note that these programs are also usable for reso-
nant single-band antennas because the length of these
is equivalent to the length of the first section of the
trap antenna. Boyer's articles provide information that
allows calculating the SWR versus frequency for these
trap antennas.