How bad a problem is intermodulation of RF at 1418-1422 MHz in UK?

In the UK, true in-band transmissions at 1418–1422 MHz are essentially nonexistent because this sits inside the internationally protected 1400–1427 MHz passive band allocated to radio astronomy and Earth-exploration (passive). That means:

• There are no licensed transmitters operating there.
• Any direct signal in that range would normally be illegal or due to equipment malfunction.

However — and this is the important practical point — intermodulation products can still appear there, even though nothing is meant to transmit in-band.

Where intermod can come from

Just above the passive band, the UK (via Ofcom) allocates:

• 1427–1452 MHz – mobile/fixed services (supplemental downlink, LTE/5G)
• 1452–1492 MHz – additional mobile allocations

Strong base stations in 1427–1452 MHz are the most common nearby high-power sources.

If two strong signals at frequencies f₁ and f₂ mix in a non-linear device (corroded joints, cheap masthead amps, overloaded SDR front ends, etc.), third-order products of the form:

2f_1 - f_2 \quad \text{or} \quad 2f_2 - f_1

can land inside 1418–1422 MHz.

This does not mean the transmitters are misbehaving — it usually means something in your receive chain (or occasionally metalwork nearby) is acting non-linearly.

How serious is it in practice?

For UK amateur hydrogen-line work:

🔹 Rural sites

• Generally low risk
• Intermod rarely limits observations
• External interference usually below the system temperature

🔹 Suburban sites

• Moderate risk
• Nearby L-band 4G/5G masts can be strong enough to:
  • Drive LNAs into compression
  • Generate third-order products
  • Raise the baseline noise floor

🔹 Urban sites

• Can be a significant problem
• Particularly if:
  • You have line-of-sight to a 1427+ MHz base station
  • Your front end lacks sharp preselection
  • You’re using wideband LNAs before filtering

Important distinction

Inside 1400–1427 MHz, there are:

• ❌ No legal active services
• ❌ No routine transmitters
• ✅ Very strict protection under ITU Region 1 allocations

So if you see structured signals between 1418–1422 MHz in the UK, they are usually:

1. Front-end overload products
2. Mixer images
3. Poor-quality switch-mode PSU harmonics
4. Reflections or re-radiation from nearby strong services above 1427 MHz

—not genuine in-band transmissions.

How bad numerically?

In a well-filtered system (e.g., hydrogen-line SAW preselection before the LNA):

• Intermod is typically below the galactic background level
• Not the dominant limitation

In an unfiltered wideband LNA directly at the antenna:

• You can easily raise the system temperature by 10–100 K equivalent
• Or see false spectral spikes that mimic narrow features

Practical mitigation (for 1420 MHz observing)

1. Filter before the first active device
2. Use a narrow hydrogen-line SAW filter (~50–70 MHz wide)
3. Avoid high-gain wideband LNAs ahead of filtering
4. Check for overload by reducing gain — if “signals” disappear, it’s intermod

Bottom line

In the UK, intermod in 1418–1422 MHz is usually a local equipment problem, not a spectrum-allocation problem.

The national protection of 1400–1427 MHz is strong — but modern L-band mobile transmitters just above 1427 MHz can create overload conditions if your front end isn’t well filtered.