There has been quite an upsurge in interest in the 4m 70MHz band of late. Antennas are always a good fun project.
The old 5 element 6m antenna from F9FT ( Tonna ) 20505 was a reasonable performer but it was never going to be a DX buster. Its a little short for a six metre antenna but on 4m the extra (electrical) boom length makes this antenna better.
It's probable that there are a fair few of these lying about unused as people have moved on to a more interesting band or a bigger antenna for six.
Recycling as well- an environmentally sound project!
Having processed the antenna dimensions using a simple computer optimising programme we are left with a design that has 7.5dbd gain and a f/b ratio of 28db.
This is an NBS yagi, you can improve on it but you can over optimise with other software - this is a safe, forgiving design.
Here is the EZNEC PLOT
of the antenna
Here is the EZNEC FILE If you want to play.
This is the PA5DD way of converting this old antenna using a different optimisation, DK7ZB antennas also work very well.
All that's left for you to do is spend an hour with a pipe cutter or hacksaw and it's possible to turn one of these antennas into a solid performer on 4m. The antenna elements simply need shortening as the spacing along the boom remains the same. Take care to follow the dimensions below and do not be tempted to change them.
Each of the elements comprises of an aluminium tube that fits into a centre plastic mounting.
NOTE - remove the element from the housing to cut and shorten the element so that the plastic end cap remains intact.
You will need to make sure that you cut the element SLIGHTLY short to achieve the correct length as the tube doesn't push all of the way to the centre of the plastic housing. Don't worry - All you need is a tape measure!
All dimensions are given first in inches then (mm):
The first figure is the length of the element from the plastic end cap to the centre of the boom, so when it says 41" you will end up with an antenna element 82" wide. The figure after the @ is the distance from the reflector, the rear most element. This is given for guidance only as you don't need to change it.
Reflector 41.5" (1054mm) @ 0" (0mm)
Driven element 40" (1016mm) @ 21" (533mm)
Dir one 37.75" (959mm) @ 44" (1118mm)
Dir two 35" (889mm) @ 82"(2083mm)
Dir three 33.25" (845mm) @ 136" (3454mm)
When you have done this you will find that the antenna length requires no further tuning to make it work on 70MHz BUT the antenna will require a matching network to make it 50+/-J0R
Here is a photograph of the matching setup Which is simple to make with hand tools and will handle plenty of power
The antenna "as is" is capacitive and requires an inductance across the feedpoint to make it look like a nice resistive load. You need a 1 x 1/8 inch strip (25mm x 3mm) of aluminium cut to 21 inches (533mm) long. First twist the ends about 2 inches ( 50mm ) from each end through 90 degrees Bend into a U shape that ends up 9 1/2 inches long and 6 inches wide ( approximate )
Drill 1/4 inch (6mm) holes in the end of the strip, close to the end. Use the original clamps that originally held the 4mm wire matching inductor to attach it to the driven element.
This is the Return loss of the antenna which is more than adequate.
Here it is on the mast five element Its not the antenna that I normally use for 4m but its a simple solid performer with very little wind load and weight. This would be an ideal way to make a cheap antenna for either home or portable use.