How to use directional coupler to provide reference signal in Easy Radio Astronomy software (suggestions by ChatGPT 27/12/2025)

December 27, 2025 Admin

The following are suggestions from ChatGPT (effectiveness not verified, 27/12/2025):

Easy Radio Astronomy (ezRA) expects a stable reference channel so it can compare amplitude and/or phase changes against the sky signal. A directional coupler is a good way to inject or extract such a reference without significantly disturbing the main RF path.

Below is a practical, radio-astronomy–oriented way to do it.

1. What the reference signal is doing in ERA

ERA uses the reference input to:

Track gain drift in the receiver
Provide a known amplitude (and optionally phase) comparison
Improve stability for long integrations (e.g. HI line, continuum drift scans)

The reference must be:

Stable in time
At or near the same frequency as the observed band
Much weaker than the main signal (typically −20 to −40 dB)

2. Where the directional coupler fits

You place the directional coupler in the RF chain before the receiver, usually:

Antenna → LNA → Directional Coupler → Receiver / SDR

The coupler has:

Through port → continues the sky signal normally
Coupled port → samples a small fraction of the RF power
Isolated port → terminated with 50 Ω

3. Two common ways to generate the reference

Method A — Injected noise diode (most common)

This is the standard radio astronomy approach.

Hardware setup

Broadband noise diode or noise source
Attenuator (so injection is small and known)
Directional coupler (reverse-injection preferred)

Noise source → Attenuator → Coupled port

Through path: Antenna → LNA → Receiver

How it works

The noise diode injects a known, repeatable noise step
ERA sees this as a calibration reference
You can switch the noise source on/off manually or electronically

ERA configuration

Reference input enabled
Use “calibration step” or “reference level” mode
Average over several seconds to reduce noise

Typical values

Coupling: −20 to −30 dB
Noise diode excess noise ratio (ENR): 5–15 dB
Injection duty cycle: a few seconds every minute

Method B — Continuous reference tone (simpler but less ideal)

Useful for demonstration or drift correction, not absolute calibration.

Hardware setup

Stable RF signal generator or crystal oscillator
Frequency offset slightly outside your spectral line of interest
Directional coupler injects a weak CW tone

Signal generator → Attenuator → Coupled port

Important

Keep the tone outside the hydrogen line if observing at 1420.405 MHz
Use very low level to avoid desensitising the receiver

ERA configuration

Reference channel set to monitor amplitude
Use it to normalise gain variations, not flux calibration

4. Connecting to Easy Radio Astronomy

ERA typically expects:

Main signal → receiver audio or SDR spectrum
Reference signal → either:
- Second SDR channel, or
- Time-multiplexed reference (noise on/off)

If using an SDR

Use dual-channel SDR if available
Or alternate noise injection and let ERA detect the step

Calibration workflow

Start ERA and let system stabilise
Enable reference input
Inject reference (noise on)
Record reference level
Turn noise off
ERA uses the delta as calibration

5. Practical tips (important)

Place the coupler after the first LNA, not before
(otherwise reference noise dominates system temperature)
Always terminate unused ports with 50 Ω
Use good shielding — reference leakage can mimic sky signals
Measure the coupling factor with a spectrum analyser if possible
Log temperature and time — ERA drift correction improves with metadata

6. Typical block diagram (text)

Antenna

↓

LNA

↓

Directional Coupler ──→ Noise Diode + Attenuator (reference)

↓

SDR / Receiver

↓

Easy Radio Astronomy

For 1420 MHz (HI line) work you want a coupler that is flat, well-matched, and predictable over roughly 1.35–1.45 GHz. Below are specific, proven values that work well in amateur and small-observatory radio astronomy.

1. Recommended coupling factor

Best general choice

−20 dB directional coupler

Why:

Injects a reference that is clearly detectable
Does not appreciably raise system temperature
Easy to attenuate further if needed

Alternatives

−30 dB → very gentle injection, excellent for high-gain LNAs
−10 dB → usually too strong unless heavily attenuated

Bottom line:
👉 Start with −20 dB, add attenuation if required.

2. Electrical performance targets at 1420 MHz

When selecting or building a coupler, aim for:

Parameter	Target
Coupling	−20 dB ±1 dB
Directivity	≥ 20 dB (25 dB preferred)
Insertion loss (through)	≤ 0.3 dB
VSWR	≤ 1.2 : 1
Frequency range	1–2 GHz minimum
Impedance	50 Ω

Directivity is more important than coupling accuracy for calibration stability.

3. Off-the-shelf couplers that work well

Mini-Circuits (excellent choice)

ZXDC-20-5-S+
- Coupling: −20 dB
- Freq: 0.5–5 GHz
- Directivity: ~25 dB at 1.4 GHz
- SMA connectors
ZXDC-20-4-S+
- Slightly narrower band but still fine at 1420 MHz

These are widely used in professional RF labs and behave very predictably.

Pasternack / Fairview (more expensive, very robust)

PE2217 (−20 dB, 1–2 GHz)
PE2218 (−30 dB, 1–2 GHz)

Good if your setup is outdoors or permanently mounted.

4. Injection level examples (real numbers)

Assume:

Noise diode output: −90 dBm/Hz
Coupler: −20 dB
Extra attenuator: −10 dB

Injected level at receiver ≈ −120 dBm/Hz

That produces a clear calibration step without compressing the LNA.

If the step is too large:

Add 3–10 dB more attenuation
If too small:
Reduce attenuation or move to −10 dB coupling

5. Where to place it (critical)

For HI work:

Antenna → LNA → Directional Coupler → Filter → SDR

Never inject before the first LNA unless the reference is extremely weak.

6. Termination and orientation

Terminate the isolated port with a 50 Ω load
Inject reference into the coupled port
Verify orientation: reverse injection if possible so noise travels toward the receiver, not the antenna

7. If you want to build one (stripline)

For a microstrip −20 dB coupler at 1420 MHz on FR-4 (not ideal but workable):

Line impedance: 50 Ω
Coupled section length: ~λ/4 ≈ 36–38 mm
Coupling gap: ~0.3–0.5 mm (depends heavily on PCB stackup)

astronomy.me.uk The Website of Lichfield Radio Astronomy Observatory (LRO)