HF Antenna Radiation Angle

Transcription

1 HF Antenna Radiation Angle

2 Introduction HF antennas for working DX Focus on the effect of the height above ground Touch on antennas for 80 and 160 m 2

3 Scope Antennas dipoles verticals small beams The concepts are applicable to other antennas 3

4 Scope Height above Ground radiation patterns take off angle / angle of radiation Ground Slope 4

5 Scope Low Bands (160 m and 80 m) Are not my field Different propagation modes applicable Show why vertical have advantages at LF 40 m and 30 m Transition Zone Principals still apply 5

6 Scope DX Bands 20m 15m 10m WARC Bands 17m 12m 6

7 Scope Antenna Height The old adage: The higher the better Why that is true (for DXing) Why it is not always true 7

8 Angle of Arrival Propagation Angle of Arrival Angle relative to the ground that signals arrive/leave an antenna The ionosphere determines Angle of Arrival Solar Cycle, time of the year, time of day You have no control (other than to pick your frequency, time of day and the path) 8

9 Antenna Radiation Angle The radiation angle of an antenna is determined fundamentally by: Height above ground Polarization What your antenna does has to fit in with what the Ionosphere demands 9

10 Requirements ARRL Antenna Book Accompanying CD Data base of Propagation Angle of Arrival ZS to South America, USA, Asia, Oceania HFTA (HF Terrain Assessment) software 10

11 Angle of Arrival 20 Distribution of the Angle of Arrival on 80m Occurance (Percent) Elevation Angle (deg) 11

12 Angle of Arrival 8 Distribution of the Angle of Arrival on 20m 7 6 Occurance (Percent) Elevation Angle (deg) 12

13 Angle of Arrival 15 Distribution of the Angle of Arrival on 10m Occurance (Percent) Elevation Angle (deg) 13

14 Angle of Arrival Obtain the statistics of the data for all destination on each band Cover 80% of all cases Lowest angle - 10% of the time Highest angle - 90% of the time Median Angle 50% above & 50% below 14

15 Angle of Arrival Band 10% Median 90% 80 m m m m m m m m

16 Angle of Arrival For ZS-land the statistics for DX bands (20m 10m) 10% of the time < 2 90% of the time < 14.4 Median

17 Up to this point we have established what the required radiation angles are The next step is how to achieve them Or how close we can get This is where antenna height comes in 17

18 Antenna Height What does antenna height do for radiation angle? Vertical Antennas Independent of height (almost) Ground properties are the major factor Horizontal Antennas Ground properties are (almost) irrelevant for DX 18

19 Antenna Height Verticals Ground properties determine the Brewster angle Below the Brewster Angle, the gain falls off very rapidly. At HF the Brewster Angle is only really low over sea water This limits the usefulness of verticals 160, 80 & 40 m are another matter 19

20 Antenna Height Horizontal Antennas Reflection from the ground are out of phase Reflection cancel or reinforce the direct signal maximum gain (1 st lobe) 180 from ground reflection 180 from path length Lobes at odd multiples of

21 Dipole at H = 12 m Pattern Lobes 10 m band 13, 40 & m band 17 & m band 26 & db75 0 db = e-005 dbi db db

22 Optimum Height What is the height required for the lowest lobe to match the median angle Band Height 80 m 171 m 40 m 68 m 30 m 53 m 20 m 38 m 17 m 34 m 15 m 29 m 12 m 24 m 10 m 19 m 22

23 Practical Height Achieving optimum height is too expensive for most of us What loss can you expect with practical antenna heights? m are common m less often 23

24 Loss at 10 m Band 10% Median 90% 80 m m m m m m m m Loss (db) 6 db = 1 S-point 24

25 Loss at Median Angle Band 6m 12m 20m 80 m m m m m m m m Loss (db) 6 db = 1 S-point 25

26 Antenna Height The old adage: The higher the better Have shown why that is true (for DXing) The ideal is higher than what most can afford Have not yet seen why it is not always true For short ranges, up to about 300 km, the required angle is high 60 to 90 NVIS For this a lower horizontal antenna is more suitable 26

27 Verticals Radio ZS November-Dec 2009 Compared vertical to horizontal dipoles DXing - 20 m and up Conclusion: A dipole at modest height will outperform a vertical, except over sea water What about LF? 27

28 Verticals Gain at 8 of a Dipole up at 12 m compared to a Vertical Band Dipole at 12 m Vertical 10 m 6.4 dbi 0.9 dbi 20 m 1.5 dbi -0.8 dbi 80 m -10 dbi -3.4 dbi Verticals and Horizontal Dipole at 12 m Gain (db) m Vertical 20m Vertical m Vertical 80m Dipole 20m Dipole 10m Dipole Elevation (deg)

29 Ground Slope Ground Slope at New Zealand (blue) Antartica (red) Europe (green) Done with HFTA (14 MHz) 29