Antenna radiation hazard
In addition to Radio Frequency (RF) burns caused by touching
the antenna element and/ or connections at the ATU and other insulators,
there is a hazard from the RF radiation if you stay in the proximity of the
antenna, when signals are being transmitted, for too long a period of
time. The amount of time you can stay close to a transmitting antenna
without harm depends on the power-output being used and the
transmitting frequency. Antenna-safe distances, where known, should
be indicated at the antenna and at the normal operating position. In the
absence of any information in your equipment manual you should, at
least, stay out of arm's reach (i.e. more than 2ftl60cm) of the transmitting
antenna on a SSB radio.MF /HF Radio Wave Propagation
Your SSB radio transmits on Medium Frequency (MF) and High
Frequency (HF) (see Appendix IV). The transmission is a form of
electromagnetic (combined electrical and magnetic) energy - just like
visible light - and performs in a particular way when presented to an
antenna. Using the vertical whip antenna as the example, the radio wave
energy is radiated equally all around the antenna. The electrical energy
is radiated in the same plane as the antenna (i.e. the antenna is vertical,
so the electrical energy is also vertical, see figure 7.5).Basic rules for ground wave propagation
1 The lower the frequency, the greater the ground wave - so MF (2MHz
band) signals will travel much further across the ground than HF
Signals (4, 6, 8, 12, 16,22 and 26MHz bands) from the same transmitter.
2 The higher the power, the further the ground wave (e.g. for a SSB
voice transmission 03E) - 150 watts at 2182kHz might get you around
130-150 miles; 300 watts at 2182kHz might get you up to around 200
miles - efficiency going down as power is increased)
3 At high frequency (4MHz and above), ground wave range decreases
to not much better than VHF and the higher the frequency the shorter
the ground wave. So we tend to use only the 2MHz band for vessels
within ground wave range, otherwise VHF will probably suffice.Basic rules for sky wave propagation
1 The ionosphere stretches from about 40 miles to 300 miles above the
earth's surface when fully energised (during the afternoon) and refracts
(bends) HF radio waves back towards the earth's surface.
2 Higher frequencies penetrate further into the ionosphere before being
refracted - higher frequencies therefore achieve greater range.
150 THE NAUTICAL INSTITUTE