General Classs Chapter 7

Transcription

1 General Classs Chapter 7 Radio Wave Propagation Bob KA9BHD Eric K9VIC

2 Learning Objectives Teach you enough to get all the propagation questions right during the VE Session Learn a few things from you about your experiences Have fun (it s a hobby, right?) Finish everything before noon

3 VE Session Three Exam Questions three groups: Sunspots and solar radiation; ionospheric disturbances; propagation forecasting and indices Maximum Usable Frequency; Lowest Usable Frequency Ionospheric layers; critical angle and frequency; HF Scatter; NVIS

4 Topic with no Place to Go Ground Wave ( meters) 50 miles maximumm Polarization dependent Vertical better Frequency dependent than horizontal Lower Frequency better than higher Due to refraction along the earth's surface Last time we re covering this today

5 Sunspot Cycle Sunspots First recorded observation in 364 B.C. by Chinese astronomer Gan De 11 Year max/min; 28 day life Strong magnetic activity inhibits convection and creates cooler layers on the sun s surface Magnetic activity on the sun is associated with high solar activity (UV and X-ray emission)

6 Sunspot Cycle Sunspots Sun spots can be activity Sunspot number calculated based on number of spots and number of groups Smoothed sunspot number used to determine cycle progression Only a relative assessment used as gauge of solar

7 Propagation Prediction More accurate indicators of conditions: Solar Flux Index (SFI) measured at 10.7 cm (2800 MHz). Corresponds to UV emissions from the sun. Higher numbers generally indicate better HF conditions. (You ll have to trust us on this for a few minutes)

8 Propagation Prediction K Index: Numeric value from 0 9. Indicator of the short term stability of the geomagnetic field Low values indicate more stability (better conditions)

9 The Ionosphere Why do we care about SFI, A Index, K Index, or sunspot numbers at all?

10 The Ionosphere

11 Caused by solar activity (UV and X-Rays)

12 The Ionosphere D Layer absorbs radio waves; lower frequencies more susceptible AM Broadcast ground wave by day, sky wave by night 40- through 160-meters (the low bands) primarily night-time DX bands Local contacts possible via ground wave or NVIS (more on NVIS in a bit)

13 The Ionosphere E Layer also absorbs radio waves, but not as dense as the D Layer, so less absorbtive; lower frequencies more susceptible E Layer also refracts radio waves (maximum skip 1,200 miles) E Layer refraction occurs on higher bands (into low VHF range)

14 The Ionosphere F2 Layer refracts radio waves F1 and F2 Combine into a single layer in the absence of UV/X-Rays (i.e., at night) Maximum skip 2,500 miles

15 The Ionosphere So, How does it all work

16 RF Propagation

17 RF Propagation Sky Wave ( meters) F2 Skip up to 2,500 miles E Skip up to 1,2000 Multiple refractions give longer distances Skip can also be shorter Short skip an indication that longer skip is possible on higher bands

18 RF Propagation Goes in two directions

19

20 RF Propagation Critical Angle: Maximum angle at which a radio wave will be refracted back down to the earth Above the critical angle, waves go out into space Depends on ionospheric conditions and frequency Low angles support longer communication distances as shown in the next slide

21 Critical Angle

22 RF Propagation Critical Frequency: The highest frequency on which a wave transmitted straight up will be returned to earth. Measured with radiosonde equipment Gives the height of the ionosphere s regions Provides day-to-day indication of ionosphere status and activity

23 Assessing Propagation Maximum Usable Frequency (MUF) Highest frequency where propagation exists between two points Above MUF, waves don t refract back to earth Depends on location of station, ionospheric conditions, take-off angle

24 Assessing Propagation Lowest Usable Frequency (LUF) Lowest frequency where propagation exists between two points Below LUF, waves are absorbed before being refracted Depends on ionospheric conditions For communications, frequency must be between the LUF and MUF

25 RF Propagation What can go wrong? Sunspots are associated with strong magnetic fields as are: Solar flares (large eruption of energy and solar matter) Coronal holes (weak coronal area allows steady stream of plasma to eject) Coronal Mass Ejections (CME) (large ejection of solar matter)

26 RF Propagation Sudden Ionospheric Disturbance (SID) UV and X-Rays from a flare rapidly increase ionization Lower frequencies impacted first due to D Layer absorbtion All sky wave communications may be blocked on sunlit side of the earth (radio black-out) Eight minutes after flare; lasts seconds to hours

27 RF Propagation Geomagnetic Disturbances Ejected solar material (charged particles) reaches earth in 20 to 40 hours May be trapped in and disturb the earth s magnetosphere near the poles Increased ionization in the E Layer Auroral displays and geomagnetic storms Paths over poles wiped out High bands (upper HF) go first Actually useful for 6- and 2-meter propagation

28 Northern Lights (Aurora Borealis)

29 RF Propagation Scatter Modes Scatter/Backscatte NVIS Signal bounces back into skip zone either from ionosphere or ground reflection Fluttery signalss Increasing take-off angle allows for short skip making local communications possible Good for 200 to 300 miles communications

30 Scatter Modes

31 NVIS

32 Questions? (Bob) (Eric)