Radio observation in hydrogen line (1420MHz)

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.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.