Why the frequency shifts in the near field
When you measure or interact with an antenna in the near field, you are not only observing its radiated energy — you are also affecting its reactive field (electric and magnetic fields that store energy but do not radiate).
In the near field, the antenna’s reactance dominates, and this reactance is highly sensitive to nearby objects (like your hand, body, or even a nearby table).
What changes when you approach the antenna?
Capacitance changes: If you bring a conductor (or even your body) near the antenna, it changes the electric field distribution and increases the effective capacitance.
Inductance changes: The magnetic field can be distorted by nearby conductive materials, altering inductance.
Resonant frequency shifts because the antenna’s LC resonance changes.
This is why the frequency moves when you are in the near field.
Can aluminum foil help?
- Not in the way most people think.
Aluminum foil may act like a shield, but shielding the near field can be tricky and often makes things worse because:
It adds capacitance (because it’s a conductor close to the antenna)
It can create unwanted currents and detune the antenna even more
It may cause unpredictable reflections and standing waves
In many cases, foil will only make the tuning worse unless it is carefully designed and placed.
What actually helps
If you want the antenna to be stable in the near field, these methods are more effective:
- Keep the antenna away from objects
The simplest solution is to increase the distance from your body and nearby conductors.
- Use a proper ground plane or shielding
If your antenna is a monopole or quarter-wave type, using a proper ground plane reduces near-field sensitivity.
- Add a matching network
A small matching network can make the antenna less sensitive to small changes in reactance.
- Use a more stable antenna type
Some antenna types are less sensitive to near-field loading, for example:
Loop antennas
Folded dipoles
Balanced dipoles
These tend to be less affected by nearby objects.
Conclusion
The frequency shift you observe is caused by near-field reactive loading.
Aluminum foil is unlikely to fix the issue unless used as part of a carefully designed shield or ground plane. The best solution is to increase distance, use a proper ground plane, or add a matching network to stabilize the antenna.
