This article will, of necessity, be VERY basic…
One of my favourite subjects has always been HF (High Frequency) radio communications. Because it can be fairly demanding I have found, over the years, that people do not want to be bothered with it. The antennas require knowledge to erect and maintain, some knowledge of how frequencies react at certain times of the day and, particularly in the case of mobile installations, some knowledge of the principles of earthing and potential differences. Very few people have believed in HF and if they did not have someone on hand who could look after it the installations that were put in would be neglected and the equipment blamed for all manner of reasons.
I talk about HF in several of my articles and I thought perhaps I should devote a short post to outlining some of the basics of how HF radio works – this is very basic so, all you technical types, please don’t confuse the issue with complex discussion around the subject.
HF (High Frequency) radio is normally accepted as being intended for long range communications beyond the reach of conventional VHF (Very High Frequency) radio. Long range can be anything from a few hundred kilometres to halfway round the world and the construction, and type, of antenna plays a big part in HF communications.
The most common HF antenna in use – and that was used in the Rhodesian Army – is the half-wave dipole that was normally erected in a T configuration (an inverted, upside down, V configuration can be used where a space for a normal dipole is constrained) (see graphics). I know that these antenna work exceptionally well for short and medium distance HF communication – and in some cases, properly erected, around the world.
One of the VERY BASIC antenna fundamentals we were taught on our operators’ courses was – the greater the distance you want to cover on HF the higher the antenna needs to be. Conversely an antenna that was comparatively low to the ground (as ours generally were), would tend to have a much SHORTER skip distance (bearing in mind that the area covered when the signals came back to earth was not a SPOT but actually a very large (almost) omni-directional footprint. Remember – Rhodesia has a comparatively small land area less than 900 kilometres at its widest point.
So, although with our VHF communications we were always trying to get high ground for our relay stations, the rationale in respect of HF was a bit different.
Skip distance is the distance from where the effective ground coverage (the area of direct communication over the ground from the base to an outlying station) ends and the first radio waves refracted from the ionosphere return to earth (see graphic). This is because a dipole antenna is primarily designed for long range communication and makes use of sending the signal to the ionosphere where it is refracted (bounced if you like) back to earth. The sharper the angle at which the signal reaches the ionosphere the closer to the base station it will return to earth. The reverse is true and is achieved by adjusting the height of the antenna from the ground to control the angle of the radio wave.
Think of looking into a mirror. If two people stand a metre apart in front of a mirror the angle at which they see each other is very small. If they move away from each other so as to still be able to see each other in the mirror, the angle at which they are looking into the mirror becomes greater and greater as they move further apart.
Besides the height of the antenna two other main factors affect how the radio waves react to the ionosphere. The frequency in use and the power of the transmitter. Too much power can be as bad as too little and the wrong frequency for the time of day will result in poor, or no, communications. I will resist going into too much detail and, for those who are interested, much information is available on the web. I know what I was taught by very good instructors a lifetime ago and what I have learned because I so enjoyed the subject – but it is practical stuff that I will post about in other ANTENNA articles – about my experience of the use and misuse of antenna in both the military and as a civilian!