End-Fed antenna for 20m, 10 meter in length.
FT-140-43 ferrite toroid impedance transformer (UnUn).
To make it a multi band (10/20/40/80m) antenna when little space is available:
- Coil to extend to 40m. The coil must have a 3kΩ impedance on 14MHz.
After the coil approximately 2 meter of wire is needed to make the antenna resonant on 40m.
Effectively you end up with a quarter wave antenna.
- Coil to extend to 80m. The coil must have a 3kΩ impedance on 7MHz.
After the coil approximately 2 meter of wire is needed to make the antenna resonant on 80m.
Bandwidth is limited. On 80m the antenna has now two coils in the part of the antenna.
Effectively you end up with a eight(+) wave antenna where the maximum current (radiation) is in the wire between the two coils.
As desired you can skip one or both extensions.
The antenna will also work on 10m (with or without extension).
Impedance transformer (UnUn)
Ferrite Toroid FT-140-43. Should be enough for 100W PEP.
The impedance for a dipole antenna is high at both ends. Around 3 kΩ.
For a 50Ω coax feed line a 50x (2,5kΩ) to 70x (3,5kΩ) impedance transformer is needed.
See UnUn impedance transformer for End-Fed for more details.
Counterpoise (ground system)
The antenna is fed unbalanced. It needs something to push against, a counterpoise.
Because of the high impedance the counterpoise is not that critical. A mass connection with the mast at the feed point may be enough.
The length of the wires should be trimmed to get resonance on the disired frequency.
Coil impedance is about 3kΩ (34uH for 40m and 70uH for 80m extension). The 80m extension coil must have a larger value, about 110uH, in this dual coil configuration.
What about the current distribution.
- On 20m band it's a half wave antenna. Highest current (radiation) in the middle of the wire.
- On 40m it's a quarter wave antenna (-3dB).
- On 80m it's a compromise antenna. To short, limited bandwidth. Better than nothing.
I think (not sure) the highest current is between the coils.
An Inverted V Fan dipole antenna for 20m an 40m has been used as reference to compare different versions of the impedance transformer.
End-Fed 2x14 turns UnUn transformer (1:49)
End-Fed 3x24 turns UnUn transformer (1:64)
Be sure the extension coils (including there wires) are in place. They have some influence on the length of the main wire.
Start tuning 20m band by cutting (hopefully not extending) the main wire.
Thereafter tune 40m. And at last 80m.
This may be a recursive procedure. So be aware not to tune each band to precise (short) in the first round(s).
The 3x24 turns UnUn had an average 1 S-point better reception compared to the 2x14 UnUn version.
Beside the difference in primary turns, the transform ratio (1:49 versus 1:64) also differs.
So... still some things to figure out ...
To be continued ...
The U-bracket is mounted to the lid of the box. This way the transformer can easily be replaced by releasing the four lid screws.