simply RED
May 16th, 2006, 01:43 PM
10-16 MHz strip is the most useful part of HF. It offers all-over-the world range espacially at day hours.
1. Used Apparature
Usually good HF transciever is impossible to be made at home.
If I am wrong I welcome everybody to offer schematics!
Anyway, (usually expensive) models like this : DX-77T
http://www.alinco.com/usa.html
Are available on the market.
2.Modulation
Amplitude Modulation (AM)
AM is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of the modulating signal. In AM you have carrier weave (F), carrier plus modulating (F+m) and carrier minus modulating (F-m).
Example:
Carrier wave 10MHz, modulating - 3~5 kHz (for sound signal) .
== (10000 - 3)kHz; 10000kHz; (10000 + 3)kHz
AM is a standard for short waves broadcast.
Single-sideband Modulation (SSB)
One method of producing an SSB signal is to remove one of the sidebands via
filtering, leaving only either the upper sideband (USB) or less commonly the lower sideband (LSB). Most often, the carrier is reduced (suppressed) or removed entirely.
With other words (10000 + 3)kHz is left.
Assuming both sidebands are symmetric, no information is lost in the process. Since the final RF amplification is now concentrated in a single sideband, the effective power output is greater than in normal AM (the carrier and redundant sideband account for well over half of the power output of an AM transmitter). Though SSB uses substantially less bandwidth and power, it cannot be demodulated by a simple envelope detector like standard AM.
SSB is the modulation - most often used for short waves radio telephony.
Most modern transcievers work on SSB.
Frequency Modulation (FM)
In FM, frequency of the carrier wave is varied by the amplitude of the modulating signal. FM is not used for short waves. Wherever used for VHF the depth of the modulation usually varies from 5 to 100kHz.
On FM, AM or SSB it is possible to encode digital signals - which is an interesting alternative of radio telephony.
3. Antennas
As a rule - the antennas for 10-16 used by non-government enthusiasts are home-made. Yaggies are possible choice for long distance work. Nadenenko dipole has wide spectrum (O,78l < length of the wave < 2l) (l = dipole length) but is quite easily spotted (your neighbours are going to "dislike" such weird thing on the roof :) ).
Half-wave or 1/4 wave dipols are often used.
Practically the length of the antenna could be less than 1/4 wave, for adjustment of the resistance in this case - a coil is placed in the base of the antenna - (2-4 cm in diameter - 10-50 winds). (The number of the winds is adjusted experimentally).
Anyway, do not expect very much from such antenna...
HF antennas are generally horizontal because horizontal dipols have directed emission - very useful for ionosphere reflection.
HF communication is very difficult to be untracable. Size of the antennas, powers used for reliable work (no less than 50-100W) and the short spectrum are majour drawbacks. Anyway, if very directed antanna is used - which does not emit towards the ground - it will be impossible (in theory) to catch without airborn assistance...
1. Used Apparature
Usually good HF transciever is impossible to be made at home.
If I am wrong I welcome everybody to offer schematics!
Anyway, (usually expensive) models like this : DX-77T
http://www.alinco.com/usa.html
Are available on the market.
2.Modulation
Amplitude Modulation (AM)
AM is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of the modulating signal. In AM you have carrier weave (F), carrier plus modulating (F+m) and carrier minus modulating (F-m).
Example:
Carrier wave 10MHz, modulating - 3~5 kHz (for sound signal) .
== (10000 - 3)kHz; 10000kHz; (10000 + 3)kHz
AM is a standard for short waves broadcast.
Single-sideband Modulation (SSB)
One method of producing an SSB signal is to remove one of the sidebands via
filtering, leaving only either the upper sideband (USB) or less commonly the lower sideband (LSB). Most often, the carrier is reduced (suppressed) or removed entirely.
With other words (10000 + 3)kHz is left.
Assuming both sidebands are symmetric, no information is lost in the process. Since the final RF amplification is now concentrated in a single sideband, the effective power output is greater than in normal AM (the carrier and redundant sideband account for well over half of the power output of an AM transmitter). Though SSB uses substantially less bandwidth and power, it cannot be demodulated by a simple envelope detector like standard AM.
SSB is the modulation - most often used for short waves radio telephony.
Most modern transcievers work on SSB.
Frequency Modulation (FM)
In FM, frequency of the carrier wave is varied by the amplitude of the modulating signal. FM is not used for short waves. Wherever used for VHF the depth of the modulation usually varies from 5 to 100kHz.
On FM, AM or SSB it is possible to encode digital signals - which is an interesting alternative of radio telephony.
3. Antennas
As a rule - the antennas for 10-16 used by non-government enthusiasts are home-made. Yaggies are possible choice for long distance work. Nadenenko dipole has wide spectrum (O,78l < length of the wave < 2l) (l = dipole length) but is quite easily spotted (your neighbours are going to "dislike" such weird thing on the roof :) ).
Half-wave or 1/4 wave dipols are often used.
Practically the length of the antenna could be less than 1/4 wave, for adjustment of the resistance in this case - a coil is placed in the base of the antenna - (2-4 cm in diameter - 10-50 winds). (The number of the winds is adjusted experimentally).
Anyway, do not expect very much from such antenna...
HF antennas are generally horizontal because horizontal dipols have directed emission - very useful for ionosphere reflection.
HF communication is very difficult to be untracable. Size of the antennas, powers used for reliable work (no less than 50-100W) and the short spectrum are majour drawbacks. Anyway, if very directed antanna is used - which does not emit towards the ground - it will be impossible (in theory) to catch without airborn assistance...