Merv's Brain Dump

TECHNICAL MANUAL COAXIAL CABLE ANTENNA TRAPS DESIGN, CONSTRUCTION, AND ALIGNMENT FOR AMATEUR RADIO MULTIBAND ANTENNA SYSTEMS

DOCUMENT NUMBER: TM-ANT-TRAP-001 REVISION: A DATE: 2026-04-24 CLASSIFICATION: UNCLASSIFIED

================================================================================ IMPORTANT NOTICE ================================================================================

This technical manual applies to coaxial cable antenna traps for amateur radio use covering 160 through 20 centimeters (1.8 MHz through 1296 MHz).

Transmitter power levels associated with amateur radio can produce RF voltages and currents sufficient to cause burns or RF interference with implanted medical devices. Review all safety precautions in Chapter 1 before beginning work.

All electrical values, winding data, and dimensions in this manual are derived from first-principles calculation. Verify all resonant frequencies with a vector network analyzer (VNA) or antenna analyzer before installing in an antenna system.

================================================================================ TABLE OF CONTENTS ================================================================================

CHAPTER 1 SAFETY AND PRECAUTIONS CHAPTER 2 GENERAL DESCRIPTION CHAPTER 3 THEORY OF OPERATION CHAPTER 4 TRAP SPECIFICATIONS AND WINDING DATA CHAPTER 5 MATERIALS AND TOOLS CHAPTER 6 CONSTRUCTION PROCEDURES CHAPTER 7 ALIGNMENT AND TESTING CHAPTER 8 INSTALLATION IN ANTENNA SYSTEMS CHAPTER 9 MAINTENANCE AND INSPECTION CHAPTER 10 TROUBLESHOOTING CHAPTER 11 REPAIR PROCEDURES APPENDIX A QUICK REFERENCE WINDING TABLES APPENDIX B NEC MODELING NOTES APPENDIX C OPENSCAD FORMER SPECIFICATIONS APPENDIX D CYD TRAP TESTER APPENDIX E GLOSSARY

================================================================================ CHAPTER 1 SAFETY AND PRECAUTIONS ================================================================================

1-1. SCOPE

1-1.1 This chapter defines safety precautions applicable to all procedures in this manual. The technician shall read and understand all precautions before beginning any procedure.

WARNING

HIGH VOLTAGE. RF voltages at transmitter output can exceed 300 VRMS at legal amateur power levels (1500 watts, 50 ohm feed). A trap positioned within 10% of the voltage maximum on an antenna element may develop several hundred volts across its terminals. Do not handle traps on a live antenna system. ——————————————————————————–

1-2. GENERAL SAFETY PRECAUTIONS

1-2.1 De-energize. Ensure the transmitter is off and the antenna switch or relay is in the BYPASS or DISCONNECT position before handling any antenna component.

1-2.2 Ground. Ground the feedline at the operating position before climbing to work on the antenna. This prevents static charge accumulation and provides a path for any induced RF from nearby transmitters.

1-2.3 At height. Follow local regulations for working at height. All trap assembly should be completed on the ground and the assembled antenna raised into position. Do not perform soldering, measurement, or adjustment at height.

1-2.4 Lightning. Do not work on or near antennas during electrical storms. Disconnect all coaxial feeds and connect antenna elements to ground through a direct short to the station ground system whenever the antenna is not in use.

1-2.5 Chemical exposure. Some RF compounds (silver-bearing solders, flux residues, dielectric greases) are skin and eye irritants. Use appropriate PPE.

1-2.6 Sharp edges. Cut coaxial cable ends are sharp. Wire ends after cutting. Coil former edges may be sharp after printing; deburr before handling.

================================================================================ CHAPTER 2 GENERAL DESCRIPTION ================================================================================

2-1. PURPOSE

2-1.1 A coaxial cable trap is a resonant circuit formed by winding a section of coaxial cable into a coil. The winding inductance and the distributed cable capacitance form a resonant L-C circuit without external components.

2-1.2 When installed in a dipole or vertical antenna element, traps act as frequency-selective impedance elements that isolate portions of the antenna at specific frequencies, making a single antenna system resonate on multiple amateur radio bands.

2-2. COVERAGE

2-2.1 This manual covers traps designed for the following amateur bands:

Band     Frequency Range      Typical Trap Coax
------   ------------------   -----------------
160M     1.800 - 2.000 MHz    RG-213, RG-8X
80M      3.500 - 4.000 MHz    RG-213, RG-8X
40M      7.000 - 7.300 MHz    RG-213, RG-8X, RG-58
30M     10.100 - 10.150 MHz   RG-213, RG-8X, RG-58
20M     14.000 - 14.350 MHz   RG-213, RG-8X, RG-58
17M     18.068 - 18.168 MHz   RG-8X, RG-58
15M     21.000 - 21.450 MHz   RG-8X, RG-58
12M     24.890 - 24.990 MHz   RG-58
10M     28.000 - 29.700 MHz   RG-58
6M      50.000 - 54.000 MHz   RG-58
2M     144.000 - 148.000 MHz  RG-58 (small)
1.25M  222.000 - 225.000 MHz  Coaxial stubs
70cm   420.000 - 450.000 MHz  Coaxial stubs
33cm   902.000 - 928.000 MHz  Coaxial stubs
23cm  1240.000 - 1300.000 MHz Coaxial stubs

NOTE: At frequencies above 144 MHz, wound coaxial traps become physically impractical. Coaxial stub traps are used instead (see paragraph 3-7).

2-3. TRAP TYPES

2-3.1 PARALLEL RESONANT TRAP (TYPE P). The most common trap type for multiband dipoles. At resonance, the parallel L-C circuit presents maximum impedance (high-Z). Current flow through the trap is blocked, effectively isolating the antenna element beyond the trap.

2-3.2 SERIES RESONANT TRAP (TYPE S). Used in trap vertical antennas where a resonant circuit to ground shorts out antenna current at specific frequencies. At resonance, the series L-C circuit presents minimum impedance. Same physical construction as Type P; circuit topology differs by wiring arrangement.

2-4. COAX TYPES

2-4.1 RG-213/U. 50 ohm, 10.29 mm OD, 96.8 pF/m distributed capacitance, 0.66 velocity factor, solid polyethylene dielectric. Preferred for 160M through 40M traps where power handling is paramount. Can handle the full 1500W legal limit with adequate trap Q.

2-4.2 RG-8X. 50 ohm, 6.15 mm OD, 78.7 pF/m, 0.78 VF, foamed polyethylene. Good compromise between size and power handling for 80M through 20M. Not recommended above 30 MHz due to dielectric heating at high power.

2-4.3 RG-58/U. 50 ohm, 4.95 mm OD, 93.5 pF/m, 0.66 VF, solid polyethylene. Used for 40M and above traps at power levels up to 200W continuous. Adequate for most portable and base station applications at 100W operating levels.

================================================================================ CHAPTER 3 THEORY OF OPERATION ================================================================================

3-1. COAXIAL CABLE AS A DISTRIBUTED L-C ELEMENT

3-1.1 A section of coaxial cable wound into a coil has two simultaneous electromagnetic properties:

(a) INDUCTANCE: The spiral current path through the outer conductor braid
    creates a magnetic field equivalent to a single-layer air-core coil.
    The inner conductor carries equal and opposite current, but the
    contribution to net inductance is very small for tightly wound coils.

(b) CAPACITANCE: The coaxial structure maintains its distributed
    capacitance regardless of how the cable is routed or wound. Each meter
    of coaxial cable contributes a fixed capacitance between inner and outer
    conductors, determined by the dielectric constant and geometry.

3-1.2 When these two properties occur in the same physical structure, a resonant circuit is formed with no external components. The inductance and capacitance are not “lumped” — they are distributed along the cable length — but for frequencies below approximately 10% of the cable’s quarter-wave resonance, the lumped approximation is accurate within 2-5%.

3-2. INDUCTANCE CALCULATION (WHEELER’S FORMULA)

3-2.1 For a single-layer air-core coil, H.A. Wheeler’s formula gives inductance as:

    N^2 * r^2
L = ----------    (microhenries)
    9r + 10l

where:
    N = number of turns
    r = coil radius in inches
    l = winding length in inches

3-2.2 This formula is accurate within 1% for coils where l/d < 1 (short coils, which describes all coax traps). For coils where l/d > 1 use Nagaoka’s formula.

3-2.3 The winding length l = N * cable_OD for close-wound (touching turns) or l = N * (cable_OD + gap) for spaced windings. Close-winding is standard practice for coax traps.

3-3. CAPACITANCE CALCULATION

3-3.1 The total capacitance of the trap is the distributed capacitance of the coax winding plus any external trimmer capacitor added for fine tuning:

C_total = (L_cable * C_per_meter) + C_trimmer

where:
    L_cable    = total length of coax wound on former (meters)
    C_per_meter = distributed capacitance of coax type (pF/m):
                  RG-213:  96.8 pF/m
                  RG-8X:   78.7 pF/m
                  RG-58:   93.5 pF/m
    C_trimmer  = trimmer capacitor value (pF), 0 if not used

3-3.2 The coax length for N turns on a former of diameter D:

L_cable = N * pi * D    (meters, D in meters)

3-4. RESONANT FREQUENCY

3-4.1 The resonant frequency of the trap is:

       1
f = -------           (Hz)
    2*pi*sqrt(L*C)

where L is in henries and C is in farads. Converting to practical units:

         1000
f_MHz = -------
        2*pi*sqrt(L_uH * C_pF)

3-4.2 EXAMPLE. 40M trap, RG-213, 76mm former, 7.5 turns:

r = 1.496 inches (76/2 mm / 25.4)
l = 7.5 * 0.405 inches = 3.0375 inches  (RG-213 OD = 10.29 mm = 0.405")
L = (1.496^2 * 7.5^2) / (9*1.496 + 10*3.0375)
  = (2.238 * 56.25) / (13.464 + 30.375)
  = 125.9 / 43.84
  = 2.87 uH

Cable length = 7.5 * pi * 0.076 = 1.79 m
C = 1.79 * 96.8 = 173 pF

f = 1000 / (2 * pi * sqrt(2.87 * 173))
  = 1000 / (2 * pi * sqrt(496.5))
  = 1000 / (2 * pi * 22.28)
  = 1000 / 140.0
  = 7.14 MHz   (target: 7.15 MHz -- ACCEPT)

3-5. QUALITY FACTOR (Q) AND BANDWIDTH

3-5.1 The Q factor of a resonant circuit determines the sharpness of resonance and the efficiency (losses) of the trap. For a parallel resonant coax trap:

Q = R_p / (omega * L)

where R_p is the parallel equivalent resistance (losses).

3-5.2 Typical Q values for coax traps at various frequencies:

Band   Cable      Q (typical)   Q (good construction)
-----  ---------  -----------   ---------------------
160M   RG-213         80-130         150-200
80M    RG-213        100-150         160-220
40M    RG-213        100-130         140-180
20M    RG-213         60-90          80-120
10M    RG-58          40-70           60-90
6M     RG-58          25-45           40-60

3-5.3 Q degrades at higher frequencies due to: (a) Skin effect in braid conductor (resistance increases as sqrt(f)) (b) Dielectric losses in coax insulation (c) Proximity effect between adjacent turns at close winding

3-5.4 The -3 dB bandwidth of the trap resonance:

BW_3dB = f0 / Q    (same units as f0)

3-5.5 For a 40M trap at 7.15 MHz with Q=109:

BW = 7.15 MHz / 109 = 65.6 kHz

This means the trap provides significant isolation (>3 dB) only within
±33 kHz of the design frequency. Outside this range, trap impedance falls
and current flows through to the outer antenna element.

3-5.6 IMPACT OF Q ON ANTENNA EFFICIENCY. Trap losses manifest as heat in the coax resistance and reduce antenna gain. The efficiency degradation is:

Loss_dB ≈ 8.686 * (R_series / R_radiation)

For typical 40M traps (R_series ~1.2 ohm, R_radiation ~70 ohm):

Loss ≈ 8.686 * (1.2/70) = 0.15 dB per trap

A two-trap dipole (4 traps, 2 per element) loses approximately:

Total loss ≈ 4 * 0.15 = 0.6 dB at 40M

This is the standard accepted penalty for trap antenna convenience.

3-6. PARALLEL RESONANT TRAP BEHAVIOR

3-6.1 The impedance of a parallel L-C circuit at various frequencies relative to resonance is:

BELOW RESONANCE (f < f0): Circuit appears CAPACITIVE (C dominates)
AT RESONANCE (f = f0):    Maximum impedance, purely resistive (Z = R_p)
ABOVE RESONANCE (f > f0): Circuit appears INDUCTIVE (L dominates)

3-6.2 APPLICATION TO MULTIBAND DIPOLE. Consider a 40-20-10M trapped dipole with traps located outward from the feed:

INNER TRAPS (10M, 28.5 MHz):
- At 10M: high-Z. Inner elements only are active. Short dipole.
- At 20M: below resonance, trap looks capacitive. Capacitance
  combined with outer element wire allows 20M resonance at
  approximately the 20M half-wave length.
- At 40M: far below resonance, trap looks capacitive. Both inner
  and outer elements contribute to 40M resonance. Antenna is
  slightly shorter than full-size 40M dipole due to trap loading.

OUTER TRAPS (20M, 14.175 MHz):
- At 20M: high-Z. Isolates outer 40M elements.
- At 40M: below resonance, trap looks capacitive. Outer elements
  extend electrical length to 40M resonance.

3-6.3 SHORTENING FACTOR. The capacitive reactance of below-resonance traps shortens the apparent electrical length of the antenna element. This is why trap dipoles are physically shorter than full-size equivalent dipoles on their lowest design frequency.

Typical shortening: 15-25% on the lowest band of a 3-band trapped dipole.
A 40M dipole physical length for a 40-20-10M trap antenna is typically
18-19m (vs. 20.1m full-size).

3-7. COAXIAL STUB TRAPS (VHF/UHF)

3-7.1 Above approximately 144 MHz, wound coaxial traps become impractical: the inductance required is only nanohenries, and the physical dimensions of the winding become smaller than the coax cable OD.

3-7.2 STUB TRAP PRINCIPLE. A short-circuited quarter-wave coaxial stub presents infinite impedance at its design frequency (parallel resonance). An open-circuited half-wave stub is the equivalent open-circuit version.

3-7.3 QUARTER-WAVE STUB CALCULATION:

Physical length = (VF * lambda) / 4

where:
    VF     = velocity factor of coax (0.66 RG-58, 0.78 RG-8X)
    lambda = free-space wavelength = 300/f_MHz (meters)

3-7.4 STUB LENGTHS FOR VHF/UHF BANDS (short-circuited quarter-wave):

Band   Freq(MHz)   RG-58 QW Stub   RG-8X QW Stub
-----  ---------   -------------   -------------
2M       146.0        337.4 mm        399.3 mm
1.25M    222.0        222.0 mm        262.6 mm
70cm     440.0        112.1 mm        132.7 mm
33cm     902.0         54.7 mm         64.7 mm
23cm    1296.0         38.1 mm         45.0 mm

NOTE: Stub lengths must be trimmed after cutting. Start 10% long and trim to resonance with NanoVNA. At 70 cm and above, connector length affects stub electrical length significantly. Account for connector electrical length.

================================================================================ CHAPTER 4 TRAP SPECIFICATIONS AND WINDING DATA ================================================================================

4-1. DESIGN PARAMETERS

4-1.1 Trap resonant frequencies are set to the center of each amateur band. Construction tolerance and trimmer capacitor range permit adjustment to any point within the band.

4-1.2 Former diameters are selected for practical printability (fits 200 mm x 200 mm FDM print bed) and adequate Q.

4-1.3 Winding data was computed using the trap_calculator.py program (see Appendix C). Recalculate if different former diameters are required.

4-2. COMPLETE WINDING TABLE — PARALLEL RESONANT TRAPS

4-2.1 Table 4-1 gives winding data for all bands, all coax types.

TABLE 4-1. PARALLEL RESONANT COAXIAL TRAP WINDING DATA

 All former diameters are OD (outside of former tube, inside of coax winding)
 Turns: number of complete turns, close-wound
 L: computed inductance (microhenries)
 C: computed capacitance (picofarads)
 f: computed resonant frequency (MHz)
 Q: estimated Q factor
 Rp: parallel equivalent resistance at resonance (ohms)
 BW: -3 dB bandwidth (kHz)
 Len: coax length required (meters / feet)

TABLE 4-2. COAXIAL STUB TRAP LENGTHS (VHF/UHF)

QW short = quarter-wave short-circuit stub (choke trap) All lengths include connector body length. Trim to resonance. Connector electrical length (PL-259): approx 8-12mm equivalent Connector electrical length (N-type): approx 5-8mm equivalent Connector electrical length (SMA): approx 3-5mm equivalent

4-3. POWER HANDLING

4-3.1 Maximum continuous carrier power at key life locations:

Coax Type   1 MHz   7 MHz   14 MHz   28 MHz   50 MHz   Note
---------   -----   -----   ------   ------   ------   ----
RG-213      1500W   1200W    900W     600W     350W    Full legal limit
RG-8X        750W    600W    450W     300W     180W    100% duty OK
RG-58        300W    200W    150W     100W      60W    Reduce for RTTY

NOTE: Trap coils run hotter than straight coax at the same power level. Power handling figures above assume trap coils are in free air. Derate by 30% for traps enclosed in weatherproof housings without ventilation.

4-4. TRIMMER CAPACITOR SPECIFICATIONS

4-4.1 An optional trimmer capacitor connected in parallel with the trap coil permits fine-tuning of the resonant frequency.

Recommended trimmer range: 5-30 pF
Voltage rating minimum: 500V (HF traps), 250V (VHF traps)
Type: piston trimmer (NP0) or air variable
Preferred: Sprague-Goodman GJM, Murata TZB4, or Voltronics equivalent
Avoid: ceramic disc trimmers (microphonic, poor weather resistance)

4-4.2 Trimmer selection by band:

Band     Trimmer Range   Frequency Effect per pF
-----    ------------    -----------------------
160M     15-30 pF        ~12 kHz/pF
80M      10-25 pF        ~25 kHz/pF
40M      5-20 pF         ~60 kHz/pF
20M      5-15 pF        ~125 kHz/pF
10M      3-10 pF        ~280 kHz/pF
6M       2-8 pF         ~600 kHz/pF

================================================================================ CHAPTER 5 MATERIALS AND TOOLS ================================================================================

5-1. MATERIALS LIST

5-1.1 COAXIAL CABLE. Purchase coax in bulk spools. Required lengths per trap are given in Table 4-1. Purchase 20% extra for errors.

5-1.2 TRAP FORMERS. Print from designs in the openscad/ directory. Material: PETG or ASA Color: Black (UV stability) Temperature: PETG 235C/85C bed, ASA 255C/105C bed (enclosure required)

5-1.3 WEATHERPROOF ENCLOSURE HARDWARE. Qty Description 4 M4 x 20mm stainless steel hex bolts (per trap) 4 M4 stainless nuts 4 M4 stainless flat washers 1 Roll self-amalgamating tape, 25mm wide, 1-2m per trap 1 Bottle UV-curing adhesive or Loctite 5083 weatherseal 4 Stainless steel hose clamps (100-120mm) for mast mounting

5-1.4 ELECTRICAL CONNECTIONS. - PL-259 / SO-239 connectors: NOT used on trap coil itself. Trap coax pigtails are soldered directly to antenna element wire. - Solder: 60/40 tin-lead, non-acid flux. 63/37 preferred for joint quality. - Antenna element wire: #14 AWG hard-drawn copper or aluminium. - Heat shrink tubing: 13mm (before shrink) for covering solder joints.

5-2. TOOLS REQUIRED

Soldering station, 50W minimum
Wire strippers, coaxial
Cable knife for stripping coax jacket
Caliper (digital preferred, 0-150mm range)
NanoVNA or other vector network analyzer (mandatory for alignment)
Frequency counter (optional, useful for initial trap testing)
Drill with 4.5mm and 5mm bits
Deburring tool
FDM 3D printer (200mm x 200mm bed minimum, PETG/ASA capable)
Heat gun or lighter for heat shrink
Zip ties, stainless 200mm (for anchoring trap on former during cure)

================================================================================ CHAPTER 6 CONSTRUCTION PROCEDURES ================================================================================

6-1. PRINT THE FORMER

STEP 1. Open trap_former.scad in OpenSCAD.
STEP 2. Set former_od to the value from Table 4-1 for the desired band
        and coax type.
STEP 3. Set cable_od to: RG-213=10.29, RG-8X=6.15, RG-58=4.95
STEP 4. Set turns to the value from Table 4-1.
STEP 5. Check console output: "Fits 200x200 bed: YES/SPLIT REQUIRED"
STEP 6. If "SPLIT REQUIRED", enable split_former=true and print two halves.
STEP 7. Export STL and slice with 0.2mm layer height, 30% infill, 3 walls.
STEP 8. Print in PETG or ASA. Inspect for layer adhesion before proceeding.
STEP 9. Deburr all edges. The groove helix must be smooth and continuous.

NOTE: For 160M and 80M traps with RG-213, the former is large (120-90mm
diameter). These may require splitting depending on printer bed clearance
to gantry. Check total height (total_length in OpenSCAD console) against
available Z height.

6-2. PREPARE THE COAXIAL CABLE

STEP 1. Cut coax to the length given in Table 4-1, plus 150mm extra at
        each end for pigtail connections.
STEP 2. At one end, strip 50mm of outer jacket.
STEP 3. Push back braid 20mm to expose inner dielectric.
STEP 4. Do NOT strip the inner conductor at this time.
STEP 5. Tin the braid tail lightly with solder. Do not use excess solder.
STEP 6. Repeat for the other end.

CAUTION: When stripping coax for trap winding, ensure the braid is NOT
cut or opened along the wound section. A braid breach in the winding
will alter the trap capacitance and prevent resonance at the design
frequency.

6-3. WIND THE COIL

STEP 1. Route the first coax end through the cable port in the bottom
        flange. Leave 120mm pigtail below the flange.

STEP 2. Begin winding the coax into the first groove on the former.
        Wind in a consistent direction (clockwise when viewed from top).

STEP 3. Wind closely and firmly. Each turn should seat in the groove
        and contact adjacent turns. Do not allow gaps between turns.

STEP 4. After completing the specified number of turns, route the coax
        end through the port in the top flange. Leave 120mm pigtail.

STEP 5. Secure the winding at each end with 200mm stainless zip ties
        through the holes in the flanges. Tighten firmly.

NOTE: For half-turn increments (e.g. "7.5 turns"), route the end coax
through the flange at the 180-degree point (halfway around) rather than
at the same angular position as the start.

NOTE: Do not substitute electrical tape for zip ties. Tape loosens over
time and allows the winding to unwind slightly, shifting resonant frequency.

6-4. SOLDER THE CONNECTIONS

6-4.1 Parallel resonant trap (Type P) connection:

The inner conductor of the coax at both ends is connected to the center
of the antenna element wire passing through the trap. The outer braid
at both ends connects to the SAME conductor (inner or outer, per
antenna design). See Figure 6-1 (dipole connection).

DIPOLE TRAP WIRING (inner element side = feed side):

     Element wire (feed side) ----+-----+---- Element wire (outer side)
                                  |     |
                        Coax inner|     |Coax inner
                        End A     |     | End B
                                  [Trap Coil]
                        Coax braid|     |Coax braid
                        End A ----+-----+---- End B

Both inner conductors connect to element wire on respective sides.
Both braid ends connect together (and may optionally connect to element
wire through the trap -- see note below).

NOTE: In some commercial trap designs, the braid is NOT connected to
the element wire; the trap acts as a pure series insert. In others,
the braid is connected to the element wire, creating a parallel
connection. Both configurations are valid; performance is nearly
identical. This manual recommends connecting braid to element wire
on both sides for mechanical security and reduced voltage stress on
the coax pigtails.

6-4.2 SOLDER PROCEDURE:

STEP 1. Cut element wire to length (leave 30mm extra for trimming).
STEP 2. Strip 15mm of insulation from element wire at trap connection.
STEP 3. Pre-tin element wire end.
STEP 4. Pre-tin coax pigtail inner conductor (after stripping back 15mm).
STEP 5. Twist element wire and coax inner conductor together.
STEP 6. Apply heat and solder to create a full mechanical and electrical
        joint. Do not leave a cold joint.
STEP 7. Allow to cool without movement (15 seconds minimum).
STEP 8. Repeat for outer braid connection to element wire.
STEP 9. Cover with 40mm heat shrink. Heat uniformly from center outward.
STEP 10.Cover joint and heat shrink with self-amalgamating tape, minimum
         2 layers, extending 50mm past the shrink on each side.

6-5. FINAL ASSEMBLY

STEP 1. Install completed coil assembly in lower enclosure half.
STEP 2. Route cable pigtails through strain relief ports.
STEP 3. Verify pigtails protrude at least 80mm from enclosure ends.
STEP 4. Seat upper enclosure half. Verify no cable is pinched.
STEP 5. Install M4 bolts. Tighten to finger-tight plus 1/4 turn.
        Do not over-tighten; ASA/PETG will crack under excessive torque.
STEP 6. Apply bead of Loctite 5083 or similar to clamshell joint.
STEP 7. Wrap entire enclosure with 2 layers self-amalgamating tape
        over and around cable entry points.
STEP 8. Verify drain holes are clear and oriented downward in final
        antenna installation.

================================================================================ CHAPTER 7 ALIGNMENT AND TESTING ================================================================================

7-1. EQUIPMENT REQUIRED

7-1.1  NanoVNA v2 (4GHz), NanoVNA-H, or equivalent vector network analyzer
       with S11 measurement capability (return loss / SWR mode).
7-1.2  Coupling loop (3 turns RG-58, 25mm diameter) for non-contact testing.
7-1.3  Optional: CYD Trap Tester (see Appendix D).

7-2. PRE-ALIGNMENT CHECK

STEP 1. Before connecting the VNA, visually inspect the completed trap:
        (a) Winding is fully seated and consistent
        (b) No gaps or crossovers in the coil
        (c) Cable jacket is not kinked or compressed
        (d) Solder joints are clean and fully flowed
        (e) All zip ties are tight

STEP 2. With a capacitance meter set to the 200-pF range, measure the
        capacitance between inner and outer conductors of one pigtail.
        Expected value is within 10% of the C value in Table 4-1.
        A value significantly different indicates a coax measurement error
        or unintended braid break.

7-3. VNA MEASUREMENT PROCEDURE

STEP 1. Calibrate VNA with SOLT (Short-Open-Load-Through) at the
        measurement port. Use the calibration standards provided with
        the VNA or a calibration kit appropriate for the test frequency.

STEP 2. Connect a coupling loop to port 1 of the VNA. Do NOT connect
        the trap directly to the VNA — the coil must be tested in its
        operating environment (as a resonant circuit in free space).

STEP 3. Place the coupling loop 5-10mm from the trap winding. Maintain
        consistent spacing throughout the measurement.

STEP 4. Set VNA sweep range to ±20% of the design frequency.
        Example for 40M trap (7.15 MHz): sweep 5.7 - 8.6 MHz.

STEP 5. Enable S11 log magnitude display.

STEP 6. Observe the resonance dip. The trap resonance appears as a
        sharp dip (minimum) in S11 (or peak in return loss) at the
        resonant frequency.

STEP 7. Record:
        (a) Measured resonant frequency (f_measured)
        (b) Depth of dip (dB below off-resonance level)
        (c) Marker bandwidth at -3 dB from dip

STEP 8. Calculate Q:

        Q = f_measured / BW_3dB

STEP 9. Compare with Table 4-1 values. Acceptable tolerances:

        Frequency: ±2% of design value
        Q:         ≥70% of estimated value in Table 4-1

7-4. TRIMMING PROCEDURES

7-4.1 FREQUENCY TOO HIGH (f_measured > f_target * 1.02):

Option A: Add a trimmer capacitor in parallel with the trap.
          Mount on the access port in the former. Start at maximum
          capacitance and reduce while monitoring VNA.

Option B: Add one half-turn to the winding (increase N by 0.5).
          Unwind enough cable from the end to add the additional half-turn.
          Re-secure with zip tie.

7-4.2 FREQUENCY TOO LOW (f_measured < f_target * 0.98):

Option A: Remove a half-turn from the winding. Unwind one pass from
          the end of the coil. Re-secure.

Option B: Reduce trimmer capacitor value.

CAUTION: Removing turns reduces total coax length, slightly decreasing
available pigtail length. Verify adequate pigtail length remains after
any turn count change.

7-4.3 Q TOO LOW (Q < 0.70 * estimated):

(a) Inspect braid for break or poor continuity. Measure DC resistance
    between braid at both ends: should be <0.5 ohm.
(b) Check for turn-to-turn shorts (inner conductor touching braid
    at a tight bend). Inspect winding visually.
(c) Verify former material is PETG or ASA, not ABS or PLA.
(d) Verify no metallic foreign objects near winding (clamps, screws).
(e) For 10M and 6M traps: coiling causes proximity effect losses.
    Space turns by one cable OD if Q is inadequate. Recalculate
    resonant frequency (will shift high) and re-trim.

7-5. INSTALLED ANTENNA TESTING

7-5.1 After traps are installed in the antenna:

STEP 1. Connect NanoVNA between feedline and antenna (or use
        antenna analyzer at the feedpoint).
STEP 2. Sweep each band. For a trapped dipole, observe SWR minimum
        on each design band.
STEP 3. Acceptable SWR at design frequency: <2.0:1 without a tuner,
        <1.5:1 preferred.
STEP 4. If resonant frequency is off, adjust the corresponding element
        length (see NEC model notes for which element controls which band).

7-5.2 PATTERN AND GAIN VERIFICATION.

The NEC models (nec_models/ directory) provide predicted patterns and
gain values. Field verification is practical only with calibrated
test equipment. For routine installation, SWR verification on all bands
is sufficient evidence of correct installation.

================================================================================ CHAPTER 8 INSTALLATION IN ANTENNA SYSTEMS ================================================================================

8-1. TRAPPED DIPOLE

8-1.1 STANDARD 40-20-10M DIPOLE.

Element half-lengths (one side from center):
    Feed to 10M trap:  2.64m (inner element, 10M resonant)
    10M trap to 20M trap: 2.62m
    20M trap to tip:    4.79m
    TOTAL half-length: 10.05m

Feedline: 50 ohm coax, RG-213 or LMR-400 for permanent installations.
Feed impedance: approximately 65-75 ohm on 40M, 50-65 ohm on 20M,
                40-60 ohm on 10M. SWR below 2:1 on all bands without tuner.

BALUN: Install a 1:1 current balun (choke balun) at the feedpoint.
       Minimum: 10 turns RG-213 on FT-240-43 toroid core.
       This prevents common-mode current on feedline and pattern distortion.

8-1.2 STANDARD 5-BAND DIPOLE (80-40-20-15-10M).

Element half-lengths (one side from center):
    Feed to 10M trap:  2.64m
    10M trap to 15M trap: 0.90m
    15M trap to 20M trap: 1.72m
    20M trap to 40M trap: 4.79m
    40M trap to tip:    10.05m
    TOTAL half-length: 20.10m

Height: Minimum 10m AGL for HF performance. 15-18m preferred.
Note: 80M performance is marginal on this antenna; efficiency
      is reduced approximately 3-5 dBd due to antenna shortening.
      Dedicated 80M elements with 80M/40M trapped dipole are superior.

8-1.3 TRAP PLACEMENT CONSIDERATIONS.

(a) Traps must be in the antenna element, not in the feedline.
(b) Trap bodies must not contact mast, support structure, or
    other conductive objects. Minimum clearance: 50mm.
(c) Orient trap drain holes downward. Traps that collect water
    develop dielectric losses and shift resonant frequency.
(d) Secure antenna element to trap flanges with 3mm stainless
    wire wrapped twice through flange holes and twisted tight.
    Do not rely on solder joints alone to support mechanical load.

8-2. TRAP VERTICAL

8-2.1 TRAP VERTICAL INSTALLATION.

(a) Mount the vertical element on a non-conductive support or
    directly on a metal mast through a base insulator.
(b) All three base-of-antenna requirements apply:
    - Install minimum 32 radials, at least 8m long, on or above ground.
    - Ground rod at base: 1.8m copper-clad steel minimum.
    - Bond all radials to ground rod and to antenna ground point.
(c) Trap bodies add weight at the balance point. Use a tapered
    element (aluminum tubing, large diameter at base) if possible.

8-3. MAST MOUNTING

8-3.1 Traps may be mounted on a fiberglass or wooden support mast.

(a) Use the saddle slot in the enclosure top with a 100mm stainless
    hose clamp. Tighten until the enclosure does not rotate.
(b) Do not mount traps directly on conductive mast without insulator.
    A conductive mast within 50mm of the trap winding will detune it.
(c) For portable installations, mast-mount using UV-resistant nylon
    cable ties through flange mounting holes.

================================================================================ CHAPTER 9 MAINTENANCE AND INSPECTION ================================================================================

9-1. INSPECTION INTERVALS

Annual: Full visual inspection and VNA measurement
After severe weather: Visual inspection
After lightning event: Full electrical check and VNA measurement

9-2. ANNUAL INSPECTION PROCEDURE

STEP 1. Visually inspect enclosure for cracking, UV degradation,
        or mechanical damage.
STEP 2. Inspect cable pigtails for jacket cracking or corrosion at
        solder connections.
STEP 3. Inspect antenna element-to-trap connections for corrosion.
        Clean with fine-grit emery cloth if oxidized. Re-solder if
        connection is mechanically loose.
STEP 4. Measure resonant frequency of each trap with VNA or CYD tester.
        Compare with Table 4-1 values. Accept if within ±3%.
STEP 5. Measure Q of each trap. Accept if Q ≥ 60% of Table 4-1 value.
        Replace trap if Q has degraded significantly from original value.
STEP 6. Inspect drain holes. Clear with 3mm drill if plugged.
STEP 7. Re-wrap cable entry points with fresh self-amalgamating tape
        if existing tape shows cracking.

9-3. EXPECTED SERVICE LIFE

Properly installed coax traps have an expected service life of 10-15 years
before Q degradation requires replacement. Factors that reduce service life:

(a) Continuous operation above rated power levels
(b) Trapped moisture (degraded dielectric constant of coax)
(c) UV degradation of outer jacket
(d) Physical damage from ice loading or mechanical stress
(e) Repeated thermal cycling (high-power operation, daily temperature swings)

================================================================================ CHAPTER 10 TROUBLESHOOTING ================================================================================

TABLE 10-1. COAXIAL TRAP TROUBLESHOOTING

Resonant frequency 5-20% high Too few turns wound Re-count turns vs. Turn gap too large Table 4-1. Wind tighter. Wrong former diameter Verify with caliper. Capacitance lower than Measure coax cap with expected LCR meter.

Resonant frequency 5-20% low Too many turns Reduce one half-turn. Former diameter too large Verify former OD. Braid shorted turn-to-turn Inspect for bridge solder

Q lower than expected Wrong coax type Verify coax marking. Proximity effect Space turns 1x OD. Metallic objects near coil Clear 50mm around coil. PL-259 connectors on coil Never connect PL-259 to pigtails under load trap pigtails under power.

SWR high on one band, OK others Trap on that band off-freq Re-align trap. Element length incorrect Trim element wire.

SWR high all bands Feedline common mode Install choke balun. Feedpoint corrosion Clean and re-solder. Open in antenna element Check continuity.

Trap runs hot during operation Over-power condition Reduce power. Use Wrong coax for power level RG-213 for high power. Trapped moisture in coax Replace coax section.

Resonant frequency drifts Loose winding Re-secure zip ties. (changes seasonally) Temperature coefficient Normal; ±1% acceptable. Moisture intrusion Improve weathersealing.

================================================================================ CHAPTER 11 REPAIR PROCEDURES ================================================================================

11-1. COAXIAL CABLE REPLACEMENT

11-1.1 If coax cable in a trap has a braid break, dielectric failure, or significant degradation, replace the coax entirely. Removing individual turns from a damaged winding is not practical.

STEP 1. Remove trap from antenna installation.
STEP 2. Remove bolts from enclosure. Separate halves.
STEP 3. Cut all zip ties. Unwind coax from former.
STEP 4. Cut coax pigtails at solder joints.
STEP 5. Inspect former for damage. Clean groove surfaces.
STEP 6. Re-wind with new coax per Chapter 6 procedures.
STEP 7. Verify resonant frequency before re-installation.

11-2. FORMER REPLACEMENT

11-2.1 If the former is cracked or deformed:

STEP 1. Print replacement former (see Chapter 6, Step 1-9).
STEP 2. Transfer coax winding to replacement former. This requires
        temporary coiling of the coax between formers; do not un-wind
        completely as maintaining coil "memory" aids re-winding.
STEP 3. Verify resonant frequency after re-winding.

11-3. ENCLOSURE REPLACEMENT

11-3.1 Print replacement enclosure halves from trap_enclosure.scad. The enclosure is field-replaceable without re-winding the trap coil.

================================================================================ APPENDIX A QUICK REFERENCE WINDING TABLES ================================================================================

A-1. PARALLEL RESONANT TRAPS — SHORT FORM

Note: For full specifications including Q, Rp, BW, see Table 4-1. Values for RG-213 unless noted.

Band Target Former Turns Coax Len Former MHz mm meters OD —– ——- —— —– ——– —— 160M 1.850 120 14 5.35 120mm 80M 3.750 90 11 3.05 90mm 40M 7.150 76 7.5 1.79 76mm 30M 10.125 60 7.5 1.38 60mm 20M 14.175 50 7 1.07 50mm 17M 18.118 45 6 0.88 45mm 15M 21.225 40 6 0.78 40mm 12M 24.940 35 6.5 0.70 35mm 10M 28.500 30 7 0.65 30mm 6M 51.000 25 5 0.40 25mm

A-2. MULTI-BAND DIPOLE ELEMENT DIMENSIONS

A-3. COAX STUB TRAP LENGTHS (SHORT-CIRCUIT QW)

================================================================================ APPENDIX B NEC MODELING NOTES ================================================================================

B-1. FILES PROVIDED

nec_models/trapped_dipole_40_20_10.nec — 3-band dipole nec_models/trapped_dipole_80_40_20_15_10.nec — 5-band dipole nec_models/trap_vertical_40_20_10.nec — 3-band vertical with radials nec_models/reference_dipole_untrapped.nec — reference dipoles for comparison

B-2. SOFTWARE

Recommended: EZNEC (Windows), xnec2c (Linux/Mac), 4nec2 (Windows)
Format: NEC-2 compatible
For best accuracy with traps: EZNEC Pro/4 with NEC-4.2 kernel

B-3. TRAP MODELING LIMITATIONS

B-3.1 Traps are modeled as lumped LD loads at the trap insertion points. The LD card specifies series R, L, C values. These are the series EQUIVALENT of the parallel RLC trap at or near resonance.

B-3.2 At non-design frequencies, the series equivalent changes with frequency. The NEC model files use fixed L,C values (frequency-independent approximation). For multi-frequency comparative analysis, use EZNEC’s built-in trap model which accounts for frequency-dependent impedance.

B-3.3 Trap body length (100-200mm) is not modeled. The physical trap occupies 100-200mm of element length. The NEC model treats the trap as a point element at the insertion position. This introduces error of approximately ±5% in resonant frequency prediction.

B-4. RUNNING THE MODELS

(1) Ensure proper ground model (GE card). The models include real earth
    (medium ground, ep=13, sigma=0.005 S/m).
(2) Run separate frequency sweeps for each design band.
(3) For the 5-band model, sweep each band separately (10 bands, 5 sweeps).
(4) Compare gain and pattern vs. the reference dipole to quantify trap loss.

================================================================================ APPENDIX C OPENSCAD FORMER SPECIFICATIONS ================================================================================

C-1. FILES PROVIDED

openscad/trap_former.scad — Single-band winding former with flanges openscad/trap_enclosure.scad — Two-piece weatherproof enclosure openscad/trap_assembly.scad — Visual assembly preview (not for printing)

C-2. KEY PARAMETERS

trap_former.scad: former_od — Set from Table 4-1 (former OD column) cable_od — Set to coax OD: RG-213=10.29, RG-8X=6.15, RG-58=4.95 turns — Set from Table 4-1 (Turns column) split_former — Set true if former > ~175mm diameter (160M/80M RG-213)

trap_enclosure.scad: trap_od — Set to former_od + 2*former_wall + cable_od + 3 (clearance) trap_length — Set to total_length from trap_former.scad console output cable_od — Match coax type

C-3. SLICING PARAMETERS

Profile: 0.20mm layer height Infill: 30% (former), 40% (enclosure) Pattern: Gyroid infill Walls: 3 perimeters (former), 4 perimeters (enclosure) Top/bottom: 4 layers Supports: None required Brim: 5mm brim for ASA enclosures (adhesion) Material: PETG or ASA. Do NOT use PLA or ABS.

================================================================================ APPENDIX D CYD TRAP TESTER ================================================================================

D-1. DESCRIPTION

The CYD Trap Tester is an Arduino sketch for the ESP32-2432S028 (CYD) module. It displays measured trap resonant frequency, Q factor, and PASS/FAIL status against table target values for all amateur bands 160M through 2M.

D-2. HARDWARE REQUIRED

ESP32-2432S028 (CYD) — 2.8" ILI9341 TFT with resistive touch
Si5351A breakout module — signal source, I2C address 0x60
Coupling loop — 3 turns RG-58, 25mm ID, with low-pass filter

D-3. CONNECTIONS

Si5351A SDA: GPIO21
Si5351A SCL: GPIO22
ADC input:   GPIO39 through 100 kohm resistor to GND + 100pF to GND

D-4. USAGE

1. Press UP/DN to select target band.
2. Hold coupling loop near trap winding (5-10mm clearance).
3. Press SCAN. Display shows frequency, Q, and PASS/FAIL.
4. Adjust trap (trimmer, turns) and re-scan until PASS.

D-5. ACCURACY

Frequency: ±0.5% (limited by Si5351A crystal accuracy)
Q:         ±20% (limited by 12-bit ADC dynamic range)
Range:     500 kHz to 160 MHz

D-6. FILE LOCATION

cyd/trap_tester_cyd.ino

================================================================================ APPENDIX E GLOSSARY ================================================================================

BALUN Balanced-to-unbalanced transformer. Placed at antenna feedpoint to prevent common-mode current on coaxial feedline outer shield.

BANDWIDTH (-3 dB) Frequency range over which a resonant circuit’s response is within 3 dB of maximum. For a trap, indicates the range of effective isolation. BW = f0 / Q.

CHOKE TRAP See PARALLEL RESONANT TRAP.

COAXIAL STUB A section of coaxial cable, one end shorted or open, used as a resonant or reactive element. Quarter-wave short stub = high impedance at design frequency.

CLOSE-WOUND Winding style where adjacent turns touch without spacing. Standard for coax traps. Spacing increases Q slightly but shifts resonant frequency.

DISTRIBUTED CAPACITANCE Capacitance that exists along the full length of a coaxial cable between inner conductor and outer braid, as opposed to a lumped capacitor.

GDO Grid Dip Oscillator (or Gate Dip Oscillator). Instrument that detects resonance of a nearby circuit by measuring power absorbed from a sweep oscillator. The CYD Trap Tester implements GDO principles.

NEC Numerical Electromagnetics Code. Software for computing antenna patterns and impedances by the method of moments. NEC-2 is public domain.

PARALLEL RESONANT TRAP L-C circuit where inductor and capacitor are in parallel. At resonance, presents maximum (high) impedance. Used as current choke in antenna elements.

PROXIMITY EFFECT Increased conductor resistance caused by interaction of magnetic fields between adjacent conductors. In close-wound coils, proximity effect reduces Q at higher frequencies.

Q FACTOR Quality factor of a resonant circuit. Q = f0 / BW. Higher Q = sharper resonance, lower losses, narrower bandwidth.

RG-58 50 ohm coaxial cable, 4.95mm OD, 93.5 pF/m. Standard lightweight HF coax.

RG-8X 50 ohm coaxial cable, 6.15mm OD, 78.7 pF/m, foam dielectric (0.78 VF). Lower loss than RG-58 for same OD.

RG-213 50 ohm coaxial cable, 10.29mm OD, 96.8 pF/m. Low-loss HF cable, full legal power rating.

SERIES RESONANT TRAP L-C circuit where inductor and capacitor are in series. At resonance, presents minimum (low) impedance. Used as short circuit to ground in trap vertical antennas.

SOLT Short-Open-Load-Through. Standard VNA calibration procedure using four known reference standards.

VELOCITY FACTOR (VF) Ratio of signal propagation speed in a cable to speed of light in vacuum. Affects electrical length calculations for stub traps.

WHEELER’S FORMULA Mathematical expression for inductance of a short single-layer solenoid. L = r^2 * N^2 / (9r + 10l), result in microhenries (inch units).

WINDING TABLE Tabulation of coil winding parameters (turns, former diameter, coax type, resulting L, C, resonant frequency) for each design. See Table 4-1 and trap_winding_tables.ods.

================================================================================ END OF TECHNICAL MANUAL TM-ANT-TRAP-001 REV A ================================================================================