HF Flashcards
Ionosphere?
The region extending 50km to over 500km high where some atoms and molecules are ionised due to radiation from the sun
Ionisation?
The process in which atoms become electrically charged, changing the density of ionosphere
*Occurs most during the day
Layers of the Ionosphere?
D: 50-90km
E: 90-140km
F1: 140-210km
F2: 210km+
Which layers of the ionosphere can refract HF?
E
F1
F2
Most important layer to HF?
F2
- It is present 24hrs a day
- Its high altitude allows the longest comms path with the least hops
- It usually refracts the highest freqs in the HF range
Which instrument measures the refraction of HF at different freqs?
Ionosonde
*The HF radar
Lower freqs are refracted ___? and higher freqs, ___?
Lower = Lower Higher = Higher
Reflection?
Occurs when an EM Wave bounces off a medium with an equal angle of incidence to reflection
Refraction?
Occurs when an EM Wave bends through a medium with different refractive index’s
Diffraction?
Occurs when an EM Wave bends around obstacles in its path
F1 & F2 layer merge at?
Night
*260-300km
5 Regular Variations in the Ionosphere?
- Diurnal
* Earths Rotation - Seasonal
- 27 Day Solar Cycle
* Rotational period of the sun - 11 Yr Solar Cycle
* Sunspot Variation - Latitude
* Proximity to equator
Fo (Critical Freq)?
Highest freq that will refract back to earth
MUF?
Maximum Usable Freq
*Max freq that comms is usable at over any given path
OWF?
Optimum Working Freq
- Best & ost reliable comms
- 85% of MUF
PUL?
Practical Upper Limit
Propogation Paths?
Sky Wave - Ground Wave
*Scattered wave
3 Types of Ground Wave?
1: Surface Wave
2: Direct Space Wave
3: Ground Reflected Space Wave
3 Irregular Variations?
1: SID
* Sudden Ionospheric Disturbances
2: Sporadic E
3: Ionospheric Storms
Free Space Loss?
FSL=20log(f.d)+32.4
LOS?
Line of Sight
*Typically less than 30km
BLOS using NVIS?
Beyond LOS via Near Incidence Vert Skywave
*out to 400km
WL of a 20MHz Wave =
15m
WL of a 10MHz Wave =
30m
WL of a 5MHz Wave =
60m
Speed of Light (C)?
300x10^6
