Hams often postulate that certain HF antennas are “low noise’ antennas.
There are many possible explanations for why an antenna captures less noise power than another, this article discusses the distribution of electric and magnetic fields (E and H) very near to a radiator, and the power captured by antennas that respond more to E or H fields.
Electromagnetic radiation consists of both and E field and a H field, and they are in the ratio of η0=µ0*c0Ω, the so-called impedance of free space, often approximated to 120πΩ or 377Ω. Close to a radiator there are components of E and H additional to the radiation components, the ratio of E/H is not simply 377Ω.
Fig 1 shows the magnitude of the ratio E/H near a quarter wave vertical over average ground at 3.6MHz. |E/H| depends on location near the antenna, and with increasing distance it converges on 377Ω.
Whilst the chart is of an intentional radiator, the effect occurs around any radiator, be it house wiring or antenna. Near to high current parts of a radiator, H field is relatively high, and near charge parts of a radiator, E field is relatively high.
Figure 2 shows the error in dB calculating power flux density as S=E^2/η0. The error in calculating flux density as S=H^2*η0 would be the negative of the value in the chart.
An alternative way of interpreting the chart is that an antenna responding principally to H field (eg a small loop) at ground level 5m from the radiator would capture 6dB more power than you would expect given the actual power density at that point.
Fig 3 is a map of power flux density near the antenna for 1W input to the antenna. Taking a check point, at 46.5m out and 10m height, S=-40dBW/m^2.
Fig 4 shows the expected power density ignoring ground attenuation, it is about 0.4dB higher than the NEC model (which does capture ground attenuation), so the model reconciles.
Now in the real world, antennas aren’t tiny and E or H dominated, nor are radiators (intentional or unintentional) tiny, they typically sustain standing waves and in regions of current antinodes (maxima) H field will be higher, and in charge antinodes (maxima) E field will be higher.
Whilst it might be observed that some antenna in some particular location seems less sensitive to noise from local sources at some frequency, it is unlikely that the effect can be exploited generally (different locations, radiators, frequencies).
Fig 3 shows that power flux density decays very quickly near the radiator, a small increase in distance can result in a large reduction in power flux density and captured power from that source.
The simple facts are:
- some antennas may be dominated by E or H fields, more so for small antennas;
- the ratio E/H is fixed in the far field (more than say a half wavelength from a radiator);
- near field of radiators may contain a higher or lower E/H ratio at some locations on some frequencies;
- separating receive antennas from noise radiators will reduce local noise pickup;
- effective measures to eliminate feed line common mode current is a measure towards separating receive antennas from noise radiators.