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HF and VHF Bands

HF Band Aerials

HF Radio Signal Propagation

VHF Band Aerials

VHF Radio Signal Propagation

HF and VHF Bands

There are many radio frequency bands which UK Radio Amateurs may use.  The lowest frequency is 0.1357 MHz (135.7 kHz) and the highest is 250,000 MHz (250 GHz).  Each band has its own characteristics and devotees.

The following is an extreme generalisation and of limited accuracy but will give a rough idea of the Amateur Radio bands to those without technical knowledge. Using the frequency of 30 MHz as a reference point, bands below this are HF (High Frequency) and bands above this are VHF (Very High Frequency).

Another overstatement is that radio signals always travel in straight lines but can be reflected.  This is wrong.  They can be refracted (bent and curved) as well as reflected.  But it keeps things simple and the straight line concept is a nice one as it is a reminder that radio signals are a form of invisible light.

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HF Band Aerials

The effectiveness of any aerial depends on its physical size compared with the size it "should be".  The latter is often half of a radio wave.  As radio signals in air (and metal) travel at almost the speed of light, which is 300,000,000 metres per second, a radio wave length in metres is 300 divided by the frequency in MHz.

On the 3.5 MHz amateur band, a wavelength is therefore 300 / 3.5 = 85.7m and a half wave is 42.8m, or 141 feet.  Many people don't have a garden large enough for an efficient aerial, so they use the longest piece of wire which will fit.  If you have a 30 ft garden and a 141 ft piece of wire, it doesn't work if you just lie the remaining 111 ft of wire on the ground.  It really needs to be in a straight line.

Now consider the effectiveness of a 3.5 MHz aerial mounted on a car roof, as you want to be able to use HF amateur radio from your car.  Your car is probably not as long as 141 feet.  You must therefore mount the aerial vertically.  This will require a pretty strong car roof.

A radio transmitter always wants to work with a perfect aerial.  The solution, if the aerial is not the correct length in a straight line, is to use an adjustable electric circuit device to kid your HF transmitter that it is working into a proper aerial.  If the aerial isn't a half wave length, then it isn't very efficient.  The further away its length is from the correct length, the worse it will perform.  At least you can kid the transmitter that the aerial is a good one, even though it isn't.

For the 0.1357 MHz band, the aerial needs to be quite long and it probably won't be able to balance on your car roof.

Aerials are usually of the long wire type.  The higher they are above ground, the better they work.  The bands near 30 MHz can use beam aerials, which are seriously enormous versions of TV aerials.  A beam aerial concentrates its input power into one direction.  This results in a greater range in that direction and a reduction in all other directions.  Only being able to talk to people in one direction is a disadvantage, so a strong rotator motor is needed to move the heavy aerial to point in any direction.

A long wire aerial is directional to a much lesser degree.  Some people may have two long wires and selected the best for the situation.

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HF Radio Signal Propagation

An HF transmitter can be considered as sending out two types of radio signal.  One follows the surface of the earth in a straight line and soon fades away, as the earth is curved.  This is the ground wave.  The other is angled upwards and travels a long distance in a straight line until it gets reflected (by an invisible magic mirror?) back to earth.  This is the sky wave.

HF communications can therefore be via ground wave or sky wave.  If by ground wave, the range will be extremely close.  If by sky wave, it will be very far.  Between these points is a dead zone, called the skip distance.

Because the invisible magic mirror moves closer to or further from the earth on an hourly basis, the propagation range of the sky wave reduces or increases.  The skip distance also reduces or increases.  This effect results in being able to talk to people in a particular country for a while, then they fade out and you can only talk to people in a closer or more distant country.

The invisible magic mirror is formed by layers or shells around the earth, above the atmosphere.  Their ability to reflect radio signals varies in response to sun activity.

The general characteristic of an HF signal is noise.  The definition of noise is "unwanted signals", whether it refers to audio or radio.  Whatever signal you are receiving (e.g. speech or Morse Code), it often contains extra signals i.e. noise.  This could be hissing, squeaks, crackles or whistles.  Filters can reduce the noise, but if the noise frequencies are within the bandwidth of the wanted signal, they may not work well.  Modern computer software can process signals and remove most of the noise.

Bands are often chosen for the distance required.  A band which is good for world wide communications is usually poor for talking within the UK.  Similarly, a band which is good for use within the UK is probably of little use for another continent.

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VHF Band Aerials

Compared with HF, VHF aerials are usually very efficient.  As the frequency goes up, the length of the radio wave goes down.  If the "correct length" for an aerial is half of a radio wave, they fit into gardens or onto cars quite easily.

On the 50 MHz Amateur Radio band, the aerial will be 300 / 50 = 6m for a full wave and 3m for a half wave.  On the 145 MHz Amateur Radio band, the aerial will be 300 / 145 = 2.1m for a full wave and 1m (39 inches) for a half wave.  On the 435 MHz Amateur Radio band, the aerial will be 300 / 435 = 0.7m for a full wave and 0.34m (13.6 inches) for a half wave.

Because of the relatively small physical size of these aerials, we can join several together to form a single aerial with high gain.  A beam aerial, as used for most TV reception, is directional and normally gives a high gain, at the expense of its directivity.  A directional aerial on  a moving car is of little use, so to keep an omnidirectional capability, aerials are joined end to end with a coil to form a single aerial.  This gives lower gain but transmits and receives in all directions at the same time.

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VHF Radio Signal Propagation

Using the simplification of "radio signals only travel in straight lines", means that at VHF, your aerial has to be able to see the other aerial or communications can't take place.  It isn't too bad a statement as all the basics are there.  A better way of putting it could be: If the aerials can see each other, then comms. will probably work.  If they can't, then comms. may or may not work.

The loss of signal strength when the signal passes through air is quite low.  There is more loss when the signal passes through other materials e.g. people, bricks, wood, metal, soil, rock.  We can talk of a large building blocking a VHF radio signal but it doesn't really block it to the extent of killing it completely.  The radio signal passes through the building and comes out of the other side with a loss of power.

If we ignore the loss of signal strength due to it passing through air, whether communications are possible or not depends on what else the signal passes through and how much of it.  One tree trunk won't matter.  A few tree trunks won't matter.  If you are in the middle of a forest with 10,000 trees, it probably will matter, unless you can get your aerial higher than the trees.

The higher the frequency of the radio signal, the more signal strength it loses when it passes through objects.  If they start off at equal power or signal strength, a 50 MHz signal passing through a building will come out of the other side stronger than a 435 MHz signal.  This indicates that a lower frequency band has a larger distance range than a higher frequency band.  In practical terms, this is complicated by the fact that a 435 MHz aerial is probably of higher gain than a 50 MHz aerial.  As aerial gains work on both transmit and receive, if the 435 MHz radio link has high gain aerials at each end, it could provide a better radio link.

Radio signals can bounce (reflect) off flat surfaces.  The higher the frequency, the better it bounces.

In open country and with most things being equal, a lower frequency will have a much greater range than a higher frequency.  On the face of it, the 50 MHz band will be much better at covering a large town or city because it will suffer less loss when passing through buildings.  In real life, the 435 MHz band may be just as good as it can bounce through the streets by being reflected off walls.

Middlesbrough is at quite a low height (30 ft above sea level).  Local Radio Amateurs could often use the 145 MHz band and 20 Watts of transmit power to talk to people in Barnoldswick (Lancashire), Duns (Berwickshire), Douglas (Isle of Man) and Forfar (Angus).  These distances are 59, 96, 134 and 160 miles.

The above radio paths are lucky ones.  The terrain profile allows VHF signals to get through.  There are many places within 5 miles of Middlesbrough which can't be reached.

The general characteristic of a VHF signal is clarity and lack of noise.  There are exceptions, such as when your aerial is close to VDUs, computers, GPS receivers and other devices which weren't designed to transmit wide band very short range VHF signals, but do so.

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Page updated on 09 January 2017

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