|Ham Radio Deals
|VHF, and up, primer. 19/12/10
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|Author:||gw8asd [ Thu Dec 16, 2010 2:22 pm ]|
|Post subject:||VHF, and up, primer. 19/12/10|
Many thanks to Paul (GW8IZR) for getting things started and contributing 99% of the following.
More will be added as comes to mind, or questions are asked.
Amateur radio on VHF/UHF and SHF is much more than just simple local contacts. Line of sight contacts will occur with simple setups.
It is not unusual for a 25W 2m station with a modest antenna - for example a 9 element yagi to make contacts between 550Km and 650Km.
For many years a “normal” 2M/70cm station would be 10W or thereabouts, of AM (possibly FM on 70cm) and later SSB.
Many people regularly worked stations in excess of 500km with this sort of power and modest, 4 to 6element, homebrew, horizontally polarised, Yagis.
The overlying thing about these bands is patience and learning to understand what is happening.
There are many complicated propagation phenomena that can assist long distance contacts but when beginning to learn about VHF+ DXing the following modes of propagation are most commonly experienced.
Tropo: Tropospheric ducting, the typical "lift" conditions encountered on VHF and UHF. can extend range of signals beyond normal up to a couple of thousand Km for well equipped stations Often encountered after periods of high pressure and stable weather conditions.
Even modest low power stations can experience contacts from 600Km to 1500Km.
There are sea paths from some parts of the UK that have resulted in 3000Km to 4000Km contacts but these are fairy uncommon.
ES: Sporadic E: ionisation of the E layer some 120Km above the earths surface, by a combination of refraction and reflection radio signals can be returned to earth at very strong signal levels.
Distances of 1000 to 1500Km are not uncommon.
This propagation can be very selective allowing communications to occur in very small areas.
It is not uncommon for a station to be "in the propagation" enjoying very long distance QSO's whilst neighbours only a few Km away hear nothing.
This type of propagation tends to be seasonal and whilst it can occur at any time of the year the late spring and summer months offer more frequent openings.
MS: Meteor Scatter , tiny particles of meteor debris burning up in the earths atmosphere creates cylinders of ionisation, this ionised area occurs at similar heights above the earths surface to the E layer.(approximately 120Km)
Whilst these are often of very short duration they can be a few minutes in duration. Sometimes multiple occurrences can join together or overlap creating the appearance of several minutes ofionisation.
This ionisation can reflect VHF radio signals resulting in signals being heard at distances of 1000 to 1500Km or more.
Very short duration signals are commonly known as pings (p) and longer duration signals are known as bursts (b).
Very specialised operating methods are needed to make contacts using these short opportunities which may only be a few hundred mS in duration.
EME: Moon bounce or Earth Moon Earth communications relies on directing signals at the moon, a small proportion of the signal is reflected from the moon and returned to the earth.
If two stations on the earth can both see the moon at the same time then successful worldwide communications can occur.
The distance to and from the moons surface is great and the amount of signal reflected is small so many consider EME to be one of the greatest challenges to the VHF amateur.
Developments in digital signal processing (DSP) and coding methods has meant that modest but carefully optimised stations can now enjoy EME experimentation.
The use of CW for EME still requires large antennas, good receivers and high power but is considered by many to be the greatest challenge and the most satisfying when successful.
Backscatter: Often associated with ES it is the part of the signal that is scattered back in the direction it came from as opposed to forward scatter, which is the signal that travels forwards.
An example is a station in Ireland is beaming East working Poland on, say, 6M.
Part of his signal can be reflected back in his direction and heard, say, on the West side of Wales where a direct path to Ireland is difficult, because of the mountains.
The two stations can work via backscatter.
More to follow.
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