Short 40m dipole antenna, part 1

With my vertical antenna I can listen to 40 and 80 meters, but we all know that is not the best setup. I could transmit with it by using an antennatuner. Using a tuner near the transceiver with a coax fed too short antenna is not the solution. Losses are building up in the tuner and coax.

The solution : build an antenna for 40 and 80. The roof on the building is not that big that I could install a full size dipole or inverted V. I have to shorten it. Let’s try this first with a 40m inverted V dipole.

I found a site in Portuguese with the desired plans. I tried to wind the coil to my best knowledge of that language. Measured value was about 14,1 µH (microHenry). Not the claimed 26 µH. CT2JKO Rui helped me to translate the plans from Portuguese to English so now I know what I did wrong. I didn’t used enameled wire and close wound it. The original plan used windings seperated by about 1 mm.

With the help of the K7MEM site I learned that the dipole would be about 12 meter long.

My goal was 8 meter. On the site I found out that 8,5m would be better. The design choosen after all required parameters had this values :

  • B = 2,55 m
  • C = 1,7 m
  • inductance = 30,251 µH

The letter B stands for the length between the feedpoint and the coil, C is the length from the coil to the end insulator (see also on the site of K7MEM).

2 coils for the short 40m dipole antennaThe K7MEM site has also a calculator where you can design the needed coil. About 43,859 turns would be needed on a 50mm pvc pipe and 1,5 mm² stiff copper wire.
I started winding the coil (by hand) and stopped at 50 turns instead of 43. I was exhausted after winding the coils. You have to keep the tension of the wire with one hand while winding with the other.

Measured value this time was 44 µH for both coils.
Now I’ll have to redesign the antenna elements itself. Back to the site.

With the new coil value this is the outcome :

  • B = 2,85 m
  • C = 1,15 m
  • L = 44 µH

Total dipole length will be 8 meter (coil length not included). Perfect.

The used sites :

Stay tuned for the next article on this subject.