Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers the Kmr-400 Compact Magnetic Loop Antenna for operation on 2M (144 MHz), 1.25M (222 MHz), 70cm (432 MHz), 33cm (902 MHz), 20cm (1296 MHz). The loop topology provides an omnidirectional azimuth pattern (for vertical orientation) or a bidirectional pattern (for horizontal orientation), with inherent advantage over dipoles in low-band noise floor due to reduced sensitivity to vertically polarized man-made noise.
1-2. APPLICABLE REFERENCES
- ARRL Antenna Book — Chapter on Loop Antennas
- NEC2 model: kmr_400_magnetic_loop.nec (in antenna directory)
- ON4UN’s Low Band DXing — Loop antenna chapters
- FCC OET Bulletin 65 — RF Exposure Evaluation
1-3. SAFETY PRECAUTIONS
CHAPTER 2 — THEORY OF OPERATION
2-1. RADIATION PHYSICS
Small-diameter magnetic loop with tap-switched inductance per band; high-q resonant loop. A full-wave loop fed at any point resonates when the total conductor length equals one free-space wavelength: λ = 300/fMHz meters. The feed-point impedance of a square loop at resonance is approximately 100–150 Ω (real), varying with shape and height above ground. A circular loop has slightly higher radiation resistance than a square loop of the same perimeter.
2-2. RADIATION PATTERN
A horizontal full-wave loop at 0.5λ height produces a low-angle radiation pattern in the plane of the loop (edge-fire). Gain over a dipole at the same height is approximately 1.5–2 dB because the current distribution around the loop produces additive broadside contributions. Vertical loops produce an omnidirectional azimuth pattern with vertically polarized radiation; horizontal loops are horizontally polarized. Estimated gain: 2–5 dBi per band.
2-3. IMPEDANCE AND MATCHING
Feed impedance: 50 Ω. The loop impedance at resonance is typically 100–150 Ω, requiring a 2:1 balun (or direct feed to 75 Ω coax) for a reasonable match to 50 Ω systems. Off-resonance the loop can be operated through an ATU. Harmonic resonances occur at odd and even multiples of the fundamental frequency, extending multiband coverage without retuning.
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. BILL OF MATERIALS
| Qty | Item | Specification |
|---|---|---|
| 1 length | Loop conductor | #14–#12 AWG copper wire; length = 1008/fMHz feet (full wavelength) |
| 2–4 | Corner insulators / spreaders | UV-stable plastic or PVC; one per loop corner |
| 1 | Feed-point insulator with connector | SO-239 or N-type; weatherproof |
| 1 | 2:1 current balun | Mix-31 or Mix-61 ferrite; handles operating power |
| 1 | Support structure | PVC mast, fiber-glass pole, or tree; non-conductive preferred |
3-2. PERIMETER FORMULA
CHAPTER 4 — ASSEMBLY PROCEDURES
- Calculate loop perimeter per formula above. Cut conductor 5% long for trimming.
- Install corner supports at spacing equal to side length. Attach conductor to corner insulators with UV-stable lacing or binding wire.
- Close the loop — solder or crimp conductor ends to feedpoint connector terminals. Install 2:1 balun at feedpoint before connecting coax.
- Raise loop to operating height. Horizontal polarization: loop plane horizontal. Vertical polarization: loop plane vertical, feed at bottom.
- Route feed line downward from feedpoint, maintaining 90° departure from loop plane for at least 0.5λ to minimize coupling.
- Perform SWR sweep per Chapter 5.
CHAPTER 5 — CALIBRATION PROCEDURE
- SOLT calibrate NanoVNA at antenna end of feed line (or at feedpoint balun output).
- Set sweep: fundamental resonance ±20%.
- Verify fres where X = 0. Record R and SWR.
- Expected: R = 100–150 Ω at resonance before matching. After 2:1 balun: R = 50–75 Ω at 50 Ω coax.
- Trim loop perimeter in 6-inch increments to shift resonance to design frequency. Do not cut — fold wire back on itself until correct length found, then cut cleanly.
- Check harmonic resonances at 2f0, 3f0; verify SWR acceptable for multiband use.
CHAPTER 6 — TUNING AND ADJUSTMENT
Trim loop perimeter to shift resonance. Each 1-ft change in total perimeter shifts resonance approximately 10–20 kHz at 40M. An ATU at the feed line allows operation across a 2:1 frequency range from a single cut length. For delta loops, adjusting the feed point position (base corner vs. apex) changes the feed impedance but not the resonant frequency.
CHAPTER 7 — VERIFICATION
| Parameter | Requirement | Pass/Fail |
|---|---|---|
| SWR at resonance | < 1.5:1 | ____ |
| Resonant frequency | Within ±1% of design | ____ |
| R at resonance | 100–150 Ω (before matching) | ____ |
| Gain (NEC2) | 2–5 dBi per band | ____ |
| Efficiency | 80–93% per band | ____ |
APPENDIX A — CALCULATIONS AND FORMULAS
APPENDIX B — EXAMPLE RESULTS
| Band | fres (MHz) | SWR | R (Ω) | X (Ω) | Notes |
|---|---|---|---|---|---|
| 40M | 7.150 | 1.4:1 | 120 | +5 | After 2:1 balun: SWR 1.6:1 |
| 20M | 14.175 | 1.5:1 | 110 | −8 | Harmonic resonance |