Other Antennas
20m,15m,30m Minibeam
This antenna started off trying to make a minibeam antenna from an old Band 2 VHF antenna. However the rods were not long enough so new elements had to be bought. These I purchased from a metal tube supplier called Metals4U. The driven elements are each 1.5meters long. The director is made from a single length 3metres long.
The two elements are mounted onto a central pole so that they are 2m apart. This is a compromise distance for all 3 frequencies, none of which is ideal.
The load coils at the end of the elements are made from 22mm waste pipe. The coils are wound with 20swg enammeled copper wire.
Winding lengths are as follows
Inner coil 10m 25turns
Middle coil 15m 45turns
Outer Coil 20m 85turns
The feed point is made from plastic chopping board and the remains of the cable entry from the VHF antenna. One of the only pieces to survive from the original project. Across the feed point but not shown is a balun made from 10 turns of low loss caxial cable wound on a 100mm diameter ferrite ring which you can buy from rallies.
Between and beyond each coil is a series of capacity hats. These were made from 3mm diameter aluminium rod which can be bought from some local DIY stores. Each capacity hat is made from 2 rods each 400mm long formed into a cross shape. There is a capacity hat after each coil so three at the end of each element. To increase the length of the reflector an additional 500mm of aluminium rod was attached to outer point of the non driven element.
Tuning needs to be carried out with the antenna as high as possible before it it paced into its final resting place on top of the mast. At a low height the antenna will go off low so this should be bourne in mind. Tuning is carried out by clipping the length of the capacity hats until a frequency about 50 to 100KHz is achieved. When mounted up at about 30ft (10m) or more the antenna should tune about 100KHz higher
Thses pictures show my minibeam mounted on my antenna mast on top of my house, in the picture you can see the balan close to the feed point and the capacity hats at the end of each coil section.