Variable length Dipole antenna
Hello!
I am SV3AUW, and my name is Peter. I would like to thank you for the warm welcome at the club. In exchange I would like to introduce to you an antenna I made many years ago.
A variable length dipole antenna.
The frequency range depends on the element length. As elements, I use two collapsible radio antennas. If you can find them around 17cm when closed and 1m when extended, you'll have an antenna, which resonates from 70Mhz to 450Mhz.
You can use then a small piece of conduit pipe 12-15mm in diameter and around 10cm long and make a small hole at the center to pass the coaxial cable.
Separate some 5cm shield from the core of the coaxial and connect them at the end of two elements each side of the conduit tube. Or even better solder them.
Then use some epoxy glue to glue them inside the ends of the conduit pipe with an overlap of 3-4cm.
Attach a connector to the other side of the coaxial cable (5m is fair enough) and that's it. Your antenna is ready. It needs only tuning at the frequency of your interest.
Extend each of the elements according the formula 72/f and with the help of a SWR meter do a fine adjustment.
Even better, you can make a very small hole (like 1mm) opposite the coax hole and pass a strong piece of string or fishing line. Draw the string parallel to one of the elements and do a knot where it ends. Next time you'll have to extent the elements up to the knot for that certain frequency and you are on air!
That string will help you to hang your antenna from a branch or the ceiling on a Field day or when you can't have an outside antenna.
If you tight it horizontal atop of a pole and you rotate it you'll have a slight directivity.
The same concept you can use for HF dipole if you have a pair of discarded portable CB radios. Especially those with the center loaded whips. You can use then a plastic junction round box and three pieces of 20mm conduit.
With the help of an ATU you can work from 21MHz and up!
I am terribly sorry but I can't provide you with any drawings. I don't think though you need them. It's just a variable length dipole antenna!
Many 73's
de M0/SV3AUW
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