Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers the design and construction of coaxial cable traps for multiband dipoles and verticals. A coaxial trap is a parallel resonant LC circuit wound using coaxial cable: the inductance is formed by the coax coil; the capacitance is the distributed capacitance of the coax itself. At resonance, the trap presents high impedance, electrically isolating the antenna element beyond the trap from the portion below it, enabling multiband operation.

Coax types covered: RG-58/U, RG-8X, RG-213. Bands: 160M through 23cm (1.8–1296 MHz) with band-specific former diameters and turn counts.

Chapter 2 — Theory of Operation

2-1 Resonant Circuit

The coil inductance follows Wheeler's formula for a short single-layer solenoid:

L (µH) = r² × N² / (9r + 10l)
  r = coil radius (inches), N = turns, l = coil length (inches)

The distributed capacitance of coaxial cable is approximately:

Resonant frequency:

f0 = 1 / (2π × sqrt(L × C))

Q factor: typically 100–200 for coax traps. Higher Q means sharper isolation (narrower stop band) and lower insertion loss in the pass band.

2-2 Effect on Antenna

Below the trap resonant frequency, the trap appears as a short inductor (low impedance) — the full antenna length is active. At the trap resonant frequency, high trap impedance isolates the antenna element beyond the trap. Above the resonant frequency, the trap appears capacitive and can shorten the resonant length of the inner portion of the antenna.

Chapter 3 — Equipment and Materials

Former material: PVC pipe, HDPE pipe, or 3D-printed ABS/PETG. Secure coax turns with UV-resistant cable ties or tape.

Chapter 4 — Construction

1. Cut the coax to the calculated length: L_coax = N × π × OD_former (one turn circumference times number of turns).
2. Wind N turns of coax tightly on the former. Leave 75 mm pigtails at each end for connection to the antenna element.
3. Strip 25 mm of jacket and braid from each pigtail end to expose only the center conductor and dielectric.
4. At each end, connect the center conductor to the antenna element wire and the braid to the antenna element wire (both center and braid connect to the same antenna conductor — this is correct for a trap; the trap is in parallel with a short section of element).
5. Use the trap_calculator.py script to verify the calculated resonant frequency before cutting. Input: coax type, former OD, number of turns.
6. Seal pigtail ends with self-amalgamating tape to prevent water ingress. Coat all exposed braid with liquid electrical tape.

Chapter 5 — Tuning and Adjustment

1. Measure the trap resonant frequency using the dip method: couple an RF signal source (TinySA or signal generator) to the trap through a 1-turn coupling loop; sweep the frequency while monitoring the S11 dip on the NanoVNA. The resonant frequency is the dip minimum.
2. If the measured frequency is too low: remove one turn (the trap is too long). If too high: add a turn or increase the coil pitch (spread turns slightly to reduce capacitance).
3. Target: f_trap within ±100 kHz of the desired band center frequency.
4. After tuning, secure turns with a bead of weatherproof silicone or UV-resistant cable tie at each turn.

Chapter 6 — Calibration

1. Measure trap Q: at resonance, measure the −3 dB bandwidth (BW) of the impedance peak. Q = f0 / BW. Target: Q ≥100.
2. Verify isolation: connect the trap in series with a test transmission line. Measure S21 at f_trap. Isolation must be ≥20 dB at f_trap for effective band isolation.
3. Measure insertion loss at off-resonant frequencies (other ham bands). Must be <0.5 dB at the bands the antenna is intended to operate on.

Chapter 7 — Verification and Acceptance

1. Resonant frequency within ±100 kHz of target band center.
2. Q ≥100 (BW ≤1% of center frequency).
3. Isolation ≥20 dB at resonant frequency.
4. Insertion loss <0.5 dB at all intended operating frequencies.
5. Weather seal: immerse trap in water for 10 minutes; verify no water ingress to coax ends.
6. Log: date, coax type, former OD, turns, measured f_trap, Q, isolation, operator.

Appendix A — Wheeler's Formula Worked Example

40M trap, RG-8X on 76 mm former, 7 turns:

Coil radius r = 76mm / 2 = 38mm = 1.496 inches
Coil length l ≈ 7 × 6.15mm (wire OD) = 43mm = 1.693 inches
L = r² × N² / (9r + 10l) = 1.496² × 49 / (9 × 1.496 + 10 × 1.693)
  = 2.238 × 49 / (13.464 + 16.93) = 109.7 / 30.39 = 3.61 μH

Length of 7 turns on 76mm former: 7 × π × 76mm / 1000 = 1.672 m RG-8X
C = 1.672 m × 78.7 pF/m = 131.6 pF

f0 = 1 / (2π × sqrt(3.61e-6 × 131.6e-12))
   = 1 / (2π × sqrt(4.75e-16))
   = 1 / (2π × 2.179e-8)
   = 7.30 MHz   (close to 40M; adjust turns to fine-tune)

Appendix B — Winding Table (RG-8X, Selected Bands)