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 Approach is same as used by Jim Lawson, W2PV, for F 2 at HF (Yagi Antenna Design, ARRL, 1986) propagation 1.

2.

3.

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 R e = 6378 km Distance = pa R e /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 10 8 6 4 2 0 1000 1500

Distance (km)

2000 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)

-10 -15 -20 -25 -30 15 10 5 0 -5 0 2 4 6 8

Elevation Angle (degrees)

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

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