ChatGPT advice on how to look for cause of mal-performance on 1420MHz-tuned Yagi aerial 17/3/2025
1. Visual Inspection
Check for:
- Boom and elements: Ensure all elements (driven, reflector, directors) are securely mounted and straight.
- Feed point: Verify the driven element connection is intact, especially if it’s a gamma match or folded dipole.
- Coaxial cable: Look for damage, kinks, or water ingress, particularly at the feed point and connectors.
2. Continuity and Resistance Testing
Using a multimeter:
- Driven element connections: Check continuity from the coax center to one side and shield to the other — you should see 0 ohms or very low resistance.
- Center to shield: Should read infinite resistance (open circuit) to confirm no shorts.
- Boom grounding (if applicable): If the boom is grounded, ensure the elements are insulated properly (except for those meant to be grounded).
3. SWR Measurement
Use an SWR meter or antenna analyzer at 1420 MHz:
- Ideal SWR: Aim for 1.5:1 or lower.
- High SWR: Check for mismatched element lengths, poor feed point connections, or a bad balun if you’re using one.
- No reading: Indicates a broken feedline, bad connector, or disconnected driven element.
4. Feedline Loss Test
Coax loss is higher at this frequency — check for:
- Excessive attenuation: Compare signal strength with and without the coax using a power meter or analyzer.
- Suspected water ingress: Measure continuity from both ends of the cable. Water in the coax often shows up as higher-than-expected loss or a short between the shield and center conductor.
5. Impedance Check
Using an antenna analyzer:
- For a Yagi at 1420 MHz: Target impedance around 50 ohms resistive.
- Significant reactance (e.g., 50+j20) indicates tuning problems — check element lengths, spacing, or matching section.
6. Signal Test (On-Air)
- Transmit: Check power and SWR at low power to avoid damaging equipment if something’s off.
- Receive: Use a known 1420 MHz signal source (even a satellite beacon or hydrogen line signal if you’re doing radio astronomy) to compare reception against a working antenna.
- Noise floor: An unusually quiet antenna might suggest a bad feedline, poor connection, or a broken driven element.
Let’s go through a step-by-step process to check your 1420 MHz Yagi with a Vector Network Analyzer (VNA) for issues:
1. Setup and Safety
- Ensure the antenna is securely mounted and connected to the VNA via a suitable coaxial cable (e.g., low-loss RG-8 or LMR-400 for this frequency).
- Calibrate the VNA with an Open, Short, and Load (OSL) calibration kit to remove cable and adapter effects from the measurement.
- Set the frequency sweep range on the VNA to 1.4 – 1.44 GHz to cover the 1420 MHz region.
2. Check Return Loss (S11)
- S11 measures how much power is reflected back from the antenna.
- A good Yagi should have a return loss of -15 dB or better (lower is better) at 1420 MHz, meaning less than ~4% reflected power.
- If you see a high return loss (e.g., -5 dB or worse), check for:
- Incorrect element lengths or spacing
- Bad coaxial connections
- Balun or feed point issues
3. Verify VSWR
- The Voltage Standing Wave Ratio (VSWR) should ideally be 1.5:1 or less at 1420 MHz.
- A higher VSWR indicates impedance mismatch (e.g., 50Ω coax to a 75Ω antenna).
- Causes to check:
- Bent elements
- Loose feedpoint connections
- Corrosion or dirt on contacts
4. Measure Impedance
- A good Yagi for 1420 MHz should have an impedance near 50Ω at resonance.
- If the real part (R) is too low or too high (e.g., 20Ω or 80Ω), revisit element spacing and matching system (e.g., gamma match).
5. Check Smith Chart (Optional)
- The Smith chart plot should ideally converge near the center (50Ω).
- If it’s spiraling or skewed:
- Coaxial cable length might be altering the measurement (recheck calibration).
- Balun or choke issues could cause unexpected reactance.
6. Scan Gain Pattern (If Equipment Available)
- If you have a signal generator and power meter, you can measure forward gain and front-to-back ratio.
- The main lobe should be sharp and strong, with minimal side lobes.
Let me put together some simulated VNA plots showing different types of problems — like impedance mismatch, bad return loss, or feedpoint issues.
Here are two simulated plots showing how different problems look on a VNA:
Left Plot: Return Loss (S11)
- Green (Ideal Yagi): Return loss around -20 dB at 1420 MHz — good performance with minimal reflected power.
- Red (Impedance Mismatch): Poor return loss around -5 dB — strong reflection, indicating mismatch or wrong element tuning.
- Orange (Feedpoint Issue): Moderate return loss (~-8 dB) with uneven ripple — suggests a loose connection, bad balun, or partial short.
Right Plot: VSWR
- Green (Ideal Yagi): VSWR around 1.2:1 — excellent match.
- Red (Impedance Mismatch): VSWR spikes to 3.6:1 — severe mismatch.
- Orange (Feedpoint Issue): VSWR around 2.5:1, fluctuating — often from bad feedpoint or coax problems.
Would you like me to simulate something more specific, like a broken director, bent element, or wrong coax length?
Let’s dive into S21 Logmag — which measures the forward transmission (how much power gets through the system, from port 1 to port 2). It’s useful for checking how well your Yagi radiates or receives signals. Here’s how different problems affect it:
1. Ideal Yagi Antenna
- S21 Logmag should show a strong peak near 1420 MHz, indicating maximum power transfer at resonance.
- A typical value might be around -1 to -2 dB, meaning most power is transmitted, with only minor coaxial or connector losses.
2. Impedance Mismatch
- If the feedpoint impedance isn’t 50Ω, the mismatch causes more power to reflect (seen in S11) — less power is radiated or received.
- S21 Logmag drops significantly, maybe -10 dB or worse at 1420 MHz, showing poor transmission.
3. Feedpoint Issues (e.g., loose connection or bad balun)
- A partial connection or a broken balun causes unpredictable power loss.
- S21 Logmag may fluctuate with a wavy or erratic curve, showing dips and peaks due to partial reflections and resonances at the feedpoint.
4. Coaxial Cable Problems
- High-loss coax (e.g., using RG-58 instead of LMR-400) leads to steady power loss across all frequencies — the whole curve shifts lower.
- Water ingress or damaged coax causes inconsistent losses, often resulting in a jagged or “noisy” S21 trace.
Would you like a simulated plot showing how these problems affect S21 Logmag too? I can visualize that next!
Here’s a simulated S21 Logmag plot showing how different issues affect your Yagi’s performance:
- Green (Ideal Yagi): Strong, clean peak at -1 dB around 1420 MHz — most power is transmitted or received.
- Red (Impedance Mismatch): Peak drops to -10 dB — significant power loss due to reflections.
- Orange (Feedpoint Issue): Fluctuating curve — partial reflection causes power to vary unpredictably.
- Blue (Bad Coax Cable): Gradual loss across the range — typical of lossy or water-damaged coax.
