Antenna Heights for
6m Sporadic-E
Steve Kavanagh, VE3SMA
CSVHFS 2004, Mississauga, ON
Introduction
•
What is optimum height for 6m antenna to be
used for Sporadic E contacts ?
–
•
Question triggered by looking at pictures of VHF contest Rovers,
mostly with fairly low antennas, and recalling my dissatisfaction with
6m results in my own June contest Rover experience
Methodology
–
1.
2.
3.
Approach is same as used by Jim Lawson, W2PV, for F2 propagation
at HF (Yagi Antenna Design, ARRL, 1986)
Determine elevation angle range of Sporadic E signals
Determine elevation patterns of 6m antennas at various heights
Compare – which antenna heights give best gain over the necessary
range of elevation angles ?
Sporadic-E Geometry
• h = approx. 110 km
(for sporadic E)
• Earth radius
Re = 6378 km
• Distance = paRe/90
• Can work out angle b
with trigonometry
• Elevation angle is
b - 90 degrees
Characteristics of Sporadic E
• Maximum distance is 2352 km
– single hop
– 0 degrees elevation angle using geometry on
previous slide
• Minimum distance typically about 1000 km
– limited by degree of ionization
Elevation Angle vs. Distance
12
Elevation Angle (deg.)
10
8
6
4
2
0
1000
1500
2000
Distance (km)
2500
Area Covered vs. Elev. Angle
• A is between 1000 and
1800 km distance (3 to 10
deg.elev.)
• B is between 1800 and
2352 km distance (0 to 3
deg.elev.)
• Area of A is about the
same as Area of B
• Potentially same number
of stations to be worked
in areas A and B
Elevation Angle Results
• Elev. angle range of interest is 0-10 deg.
• 0-3 degree and 3-10 degree ranges are of
about equal importance regarding number
of stations (and grids) available to be
worked
– disregarding variations in population density,
oceans, etc.
Elevation Patterns (from YA)
15
10
Dipole 8'
Dipole 20'
3 el 10'
3 el 20'
3 el 30'
3 el 40'
3 el 60'
3 el 20',5 deg
Gain (dBi)
5
0
-5
-10
-15
-20
-25
-30
0
2
4
6
8
Elevation Angle (degrees)
10
Observations
• Over flat level ground there is no antenna
height which gives optimum performance
over 0-10 degree range
• Best single height is probably about 50 ft.
– Higher antennas give more gain at low angles but
have nulls at the upper end of elevation angle range
• A choice of two antennas should work
well, with one at about 30 ft., the other
above 60 ft.
Observations, cont’d
• A rover with antenna at 10 ft will be 12 dB
down (2 S-units) compared to a fixed
station with the same antenna at 40 ft.
• A dipole at 20 ft. is as good as 3 elements
at 10 ft.
• A 20 ft. high antenna will be 10 dB louder
(at 2 degree elevation) over 5 degree
sloping ground than over level ground.
Limitations of Analysis
• Real ground may not be flat or level
– Could use terrain analysis software to evaluate performance at a
particular site
• Real ground may not be smooth
– Compared to HF, smaller obstacles will be significant at 6m
– It would be interesting to use software such as NEC to examine the
effect of power lines, buildings, cars, etc. on radiation patterns of 6m
antennas over ground
• Applies only to sporadic E
– Very high antennas will work well for tropo, troposcatter, line-of-sight
signals which propagate only at very low elevation angles
• Applies only to horizontal polarization
Conclusions
• A single antenna at about 50 ft over flat
level ground will work quite well for
Sporadic E
• But a choice of lower and higher antennas
should be better
• Most Rovers’ 6m antennas are too low to
be competitive – some extra height will
make them much louder on Sporadic E