I'm sure I can get alot of comments on this one. VHF
IS lin-of-sight and so is HF if you only consider the ground wave of the transmitted signal. What makes HF useful for long range communications
is that it is much more easily reflected or refracted from the different layers of the inosphere than VHF
. HOWEVER, VHF has its enhanced propagation modes also. Infact, all frequencies have enhanced propagation modes up into the microwaves which includes RADAR
. Admittedly, the enhanced modes of propagation occure much less often the higher you go in frequency.
Enhanced modes of propagation on VHF and above are called "Sporadic E", "Aurora", "Ducting" and "TE". You can also get strange reflections from heating
of the surface of the water
. Sporadic-E is an ionized cloud of the E-layer of the ionospere which can cause reflection of VHF and higher frequencies over several hundred miles much the same way that HF frequencies are propagated. Aurora is reflection of VHF signals off of the Aurora Borialis. These signals are very weak and may not be detectable on FM, but may be dectected on the upper HF frequencies. Often the reflected signal sounds like a whisper. Ducting can cause long distance communications
(100's of miles) with little or no loss of signal strength. TE or transequatorial propagation occurs between stations located approximately the same distance from the equator (north and south) and usually occurs in the VHF range.
Using FM modulation on VHF would make these modes of propagation more difficult to detect, but they can happen. Ducting of a RADAR
signal might give a flase reflection, but the odds of it happening over any length of time would be rare. However, it could happen.
For those using the Amateur Radio
frequencies on VHF and above with SSB
or CW instead of FM the oportunity for use of these modes of propagation become much more of a possibility.
Use of the numerous satelites available for Amateur Radio
communications using SSB
, CW, FM or digital communications becomes a real source for communications over large portions of the earth. Most of the frequencies available for use by satelites are in the VHF and above frequency range. This means that only a Technician license
is required as long as you use the right equipment
, which at times may only have to be a VHF/UHF (dual band) FM handheld unit. Satelites available for this type of communications are called LEO or Low Earth Orbit satelites and are only available for short periods of time. Other satelites give hours of available communications but require high gain antennas and the ability to track the satelite.
It is wise to remember that all frequencies have enhanced propagation modes, but these modes become less of a concern the higher in frequency you go. On HF, the enhanced modes (reflection from the ionosphere) are absolutly necessary for reliable long distance communications, but if those modes are not available, as changes occur in the ionosphere between day and night, certain HF frequencies are only good for Line-of-sight.