HF Vertical Antenna
by DJ9CB (collected by Achraf, 3V4-002, MDXC#235)

On the higher bands, the signals often are extremly weak, so you need an optimised
antenna. A vertical dipole, made of wire and the length cut for 15m would do a good job.
Later you can optimize it by adding in parallel dipoles for 10m and 20m. Take any wire you can get!
Dipole length is calculated half wavelength, multiplied by 0,95. Feeding into the center
of the dipole by coax cable. TV-coax does a very good job! Connect the center wire of
the coax to the upper part of the dipole and connect the outer conductor of the coax to the lower part of the dipole (The outer part is grounded on the receiver chassis and must be connected with the outer side of the coax. The inner one connect to the inner part of the cable). Both of the ends of the dipole must be free and should be insulated. If you let the dipole hang down from the top of a tree, let hang the coax a bit to the side to avoid the lower end of the dipole to be too near of it. A distance of 10 cm or more is good.Don´t hang up the dipole extra hight. The center of the dipole 2m above the ground is ok. In this case you must let hang a part of the lower end of the dipole horizontally.

Only the vertical part of the antenna receives vertically polarisated waves. Specially the center part of the antenna (round about 0,25 wavelength) needs to be vertically. The vertical antenna has got the same sensitivity to all horizontal directions. At flat angles (dx!) there is best sensitivity, and perpendicular to the ground (that is the
direction of the antenna wire) there is a zero of sensitivity. A horizontal antenna
(horizontal dipole) has two zeros of sensitivity: both towards the wires direction. When
the horizontal dipole hangs lower than 0,25 wavelength, then there is poor sensitivity for flat signals. On the upper bands you always receive signals with very low elevation angles (5 to 15 degrees).

For the HF you don´t need to connect any ground ( only for the safety, you need of
course on the station). You could make an RF-choke instead of a truly balun. Simply
form a coil of the coax just before the connection to the dipole. 5 to 10 turns, diameter 10 cm, air wound.

What is the role of the RF-choke?
Imagine the dipole hangs a bit apart from the house and receives solely the nice weak signals from dx you want to hear by the help of your receiver.
The HF walks along the center conductor and along the inner side of the outer conductor of the coax (the screen or the shielding) into the receivers antenna input.
All right so. When there is electro -smog made within the house (from switching light
on and off, electro-motors, TV-Receiver etc) or electro-smog coming into the house
along the power line, then it cannot come into the inner side of the coax because of the shielding.
However, all the noise walks along the outside of the coax, walks into the dipole and
then it is fed into the inner side of the coax, the same way like the very weak signal from dx. You of course hear very strong man made noise, but it is almost impossible to hear the weak signals. If you make a choke of the coax near the antenna connection -it is a high inductive resistance- then the man made noise cannot pass into the dipole. On the other hand the HF comming from the antenna throught the inner side of the coax "sees" only a coax but does not "see" the windings of the choke. (The choke does not do any transformation. It only hinders man made electrical noise to come into the receivers input.)

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