|VK6AAL 20m 4 Element Monoband Yagi|
Boom Length 27 FT or 8.23 M
REF: 10.660 M
DE: 9.894 M
D1: 9.54 M
D2: 9.34 M
REF to DE: 4 M
DE to D1: 1.6 M
D1 to D2: 2.63 M
F/B 21db Gain 12.68 dbi
Elevation 22.8 Degrees to 15.8 Degrees depending on direction due to sloping ground 4-6m below tower base for 180 degrees.
Adjust DE for resonance at 14.230
Add Reflector at calculated length.
Add D1 and D2 at calculated lengths.
Check for Impedance at resonant Freq
Add Hairpin match coil to suit
Re-Adjust DE for resonance.
Check overall bandwidth and impedance matching using a noise bridge at 10m and 15m from the ground.
Initially I used an Air Wound Balanced to Unbalanced Balun with a split Driven element insulated with large Nylon bushes.
Jasta 2007 saw my airwound Balun finally burn out and I replaced it with a DX Engineering DXE-BAL050-H11-C this is a very rugged industrial balun rated at 10KW continuous so it will never fail.
With the addition of the Ferrite Balun it required the addition of a Coiled Hairpin to match the DE impedance of 30 ohms to the feed line at 50 ohms.
The coil dimensions where 5 turns of 12 Gauge enamel coated copper wire 1.5 inches in diameter and spaced over 2 inches.
By soldering lugs onto the ends of the coil it can be fitted onto the insulators of the balun using the same fixtures for the DE connection.
For extra element stability split the REF, D1 + D2 elements and insert a wooden stiffener pole 3 m in length into the centre of the element (neat fit) and add a thin wall sleeve over the outside of the centre for additional mounting strength 200 mm length.
The Driven element has a 6 mm wall thickness centre tube 150 mm in length that the bushes are inserted into and the elements are pressed into this section the tube is slit along one side to apply a clamping pressure as the bracket is tightened.
The mast to boom clamp is made from a piece of 6 mm thick aluminium checker plate with a 2 slots cut 75 mm into the ends and the wings bent under and over the square boom.
The large U bolts that pass around the boom and through the holes drilled in the plate also fit into a T section of aluminium 30 mm x 300mm in length this also distributes the support and clamping pressure at the mast attachment point.
A guy cable is passed from one clamp at each end over a pulley on the mast 500 mm above the Beam a 150 mm turnbuckle is used to pretension the horizontal lift and then wired so it can not vibrate undone. This cable has earthing straps in the clamps attached to the Boom to element U bolts to prevent any poor connections and RF rectification.
The boom is a 50 mm x 3 mm wall thickness SQ tube section this came from a special batch of high grade structural aluminium run in 1977 by a local ham.
Useful products for Antenna Builders
Industrial Grade High Power Continuous Rating Balun Suppliers
Ferrites to wind your own available from CWS ByteMark
Good Balun reference books Understanding, Building, and Using Baluns and Ununs also from CWS ByteMark
Tape for the RF/Electrical industry.
Scotch 23 self amalgamating tape is designed to keep
water out of cables, use Scotch 23 at the feedpoint of antenna's, simply use a small
strip, stretch it and wind it on. The tape will bond together like glue and make it
impossible for water to access your coax cable.
To remove it - simply use a very sharp knife.
There is a USA designed product that is a putty that
you can apply and remove at ease any time !
A cheaper alternative to Scotch 23
Available in Australia through...
Australian Radio Ind. Pty. Ltd.