Last year, during an Castle operation for DCI award, I had problem with my usual antenna: it worked than suddenly stop running etc. I have finished my activation with many difficulties. Went home I found the problem: a screw not well screwed.
This event pushed me to make a “spare” antenna to be used in case of breaking of the main antenna.
This antenna being a “spare” one, has to have the following characteristics:
short, light, efficient, working on different bands.
I have used a magnetic base, born for V/UHF, as support.
The antenna is about 2,5 m. long, the coil is placed high (at 2 m.) to get a better efficiency.
The antenna is made: one aluminium bar diameter 8 mm., long one meter threaded to screw into the base while on the opposite end is bored to accept a second aluminium bar 6 mm. diam., one meter long.
The coil is supported by a plastic retain that accept also the rod.
The coil: I have tried to get it as light as possible; the wire is enamel copper 0.7 mm., wrapped on a plastic tube diam. 33 mm.; it has 64 turns and it is long 48 mm.
The coil is located at two meters from the base; it can be joint to the mast and to the rod with crocodile pliers.
To work different bands I have changed the size of the rod to maintain the same coil, infact:
for 40 m. the rod has an effective size of 60 cm.;
for 12 m. and 17 m. it is 20 cm. long; for 10 m. I take out the coil and use a rod 52 cm. long put directly on the mast to get ¼ wave (2 m. + 0,52 m.); all rods have the diameter of 2 mm.
In 40 m. the antenna has a loss of 1 S-meter point in comparison with a vertical 10 m. high, radiation resistance is 4,70 Ohms.
I believe that the photos of details and mounted antenna let understand how to build it.
If you have any doubt please contact me.
73 de Gianni I1UWF