Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers the construction and operation of a homebrew antenna analyzer covering 0.1 MHz to 1.3 GHz using a two-stage RF source architecture: an AD9851 DDS for HF (0.1–30 MHz) and an ADF4351 fractional-N PLL synthesizer for VHF/UHF (30 MHz–1.3 GHz). The measurement front-end is an AD8302 gain/phase detector providing simultaneous magnitude and phase readout. Output is displayed on a CYD (ESP32 + 2.8” touchscreen) with Smith chart, SWR plot, and R+jX numerical display.

Chapter 2 — Theory of Operation

2-1 AD8302 Gain/Phase Detector

The AD8302 accepts two RF signals on INPA and INPB (50Ω each) and produces two DC outputs:

• VMAG: Proportional to |V_A / V_B| in dB (30 mV/dB, range −30 to 0 dBm per input).
• VPHS: Proportional to phase difference (10 mV/°, 0° to 180° range).

INPA receives the reference (forward-coupled) signal; INPB receives the reflected signal. The ratio VREFL/VFWD and the phase shift Δφ together yield the complex reflection coefficient Γ:

Γ = |V_refl / V_fwd| ∠ Δφ
Z_ant = Z0 × (1 + Γ) / (1 − Γ)   (Z0 = 50 Ω)

2-2 Directional Coupler

A multiband RF coupler (TM-TOOL-002-SCH-004) separates the forward and reflected waves on the transmission line to the DUT (antenna). Coupling factor is −20 dB on the HF section and −20 dB on the VHF/UHF section. Directivity must be ≥20 dB across the band to keep reflected signal isolation adequate for accurate Γ measurement.

2-3 Frequency Sources

AD9851 DDS (HF, 0.1–30 MHz): 125 MHz reference clock, 32-bit frequency word, phase noise −100 dBc/Hz at 1 kHz offset. Controlled via 3-wire SPI from ESP32. Sweep rate: ~1000 points/second.

ADF4351 PLL (VHF/UHF, 30 MHz–1.3 GHz): Fractional-N synthesizer, 32.768 MHz TCXO reference, phase noise −90 dBc/Hz at 10 kHz. Integer-boundary spurs require spur-avoidance in sweep software when crossing integer multiples of the PFD reference frequency.

Chapter 3 — Equipment and Materials

Chapter 4 — Construction and Assembly

4-1 PCB Layout Notes

Route the RF signal paths (DDS/PLL output → coupler → DUT port) as 50Ω microstrip (width 2.9 mm on 1.6 mm FR4 with εr=4.6). Keep reference and reflected coupler outputs equal length to the AD8302 inputs. Length mismatch >5 mm introduces a phase error of approximately Δφ = 360° × ΔL / λ.

4-2 AD8302 Bias

The AD8302 requires ±5V dual supply. Derive from 5V USB with a MAX1044 charge pump for the −5V rail. Bypass each supply pin with 10 nF NP0 + 100 nF X5R within 3 mm of the device.

4-3 DDS Output Filtering

The AD9851 output contains harmonics and alias products. A 7-pole elliptic low-pass filter (cutoff 35 MHz) reduces spurious outputs to <−60 dBc before the coupler. Without this filter, harmonic reflections from the DUT appear as bearing errors at sub-harmonic frequencies.

Chapter 5 — Operating Procedures

5-1 SWR Sweep

1. Connect antenna to DUT port (SMA). Select band (HF or VHF/UHF).
2. Enter start/stop frequency and number of sweep points (101 or 201).
3. Press SWEEP. The display plots SWR vs. frequency in real time.
4. Identify the resonance (SWR minimum). Press MARKER; the instrument displays f_res, SWR, R, X at the marker frequency.

5-2 Smith Chart Display

1. Select SMITH mode. The sweep traces the impedance locus on the Smith chart.
2. Clockwise rotation with increasing frequency = capacitive reactance dominant (antenna too short). Counter-clockwise = inductive (antenna too long).
3. At resonance, the locus crosses the real axis; R at crossing = feedpoint resistance. Ideal dipole: 72Ω (free space), lower over ground.

Chapter 6 — Calibration

6-1 SOLT Calibration

Before measuring an antenna, perform a one-port SOLT calibration at the DUT SMA connector (not at the instrument chassis):

1. Connect SHORT (shorted SMA cap). Press CAL → SHORT.
2. Connect OPEN (SMA cap with no connection). Press CAL → OPEN.
3. Connect 50Ω LOAD (SMA terminator). Press CAL → LOAD.
4. Press CAL → DONE. Calibration plane is now at DUT connector.

After SOLT cal, short should read SWR >50:1, open should read SWR >50:1, and 50Ω load should read SWR <1.05:1 across the calibrated band.

Chapter 7 — Verification and Acceptance

1. Connect a precision 50Ω load (Pasternack PE6010 or calibrated terminator). Verify SWR <1.05:1 at all calibrated frequencies.
2. Connect a known 100Ω resistor (1%, non-inductive). Verify R reads 100 ± 5Ω, X reads 0 ± 5Ω at 1 MHz.
3. Connect a 50Ω + 50 nH load (resistor in series with known inductor). Verify jX reads within ±10% of calculated inductive reactance at 10 MHz.
4. If any check fails, repeat SOLT calibration. Persistent errors indicate coupler asymmetry or AD8302 bias fault.
5. Log: date, calibration kit used, frequency range, short SWR, open SWR, load SWR, 100Ω R/X readings.

Appendix A — Formulas

SWR = (1 + |Γ|) / (1 − |Γ|)
|Γ| = (SWR − 1) / (SWR + 1)
Return loss (dB) = −20 log10(|Γ|)
Z_ant = 50 × (1 + Γ) / (1 − Γ)    [complex arithmetic]

Appendix B — Worked Example

AD8302 reads VMAG = 2.10 V, VPHS = 0.75 V at 14.250 MHz.

Magnitude ratio (dB) = (2.10 − 1.80) / 0.030 = +10 dB  [AD8302 midpoint = 1.80V]
|Γ| = 10^(10/20) = 0.316  (not a valid interpretation here — see note)

Correct: VMAG represents |V_INPA / V_INPB|.
If VMAG midpoint (ratio = 1) = 900 mV, then:
  ratio_dB = (2100 − 900) / 30 = +40 dB  (INPA is 40 dB stronger than INPB)
  |Γ| = 10^(−40/20) = 0.01  → SWR = 1.02:1 (near-perfect match)