4.
Ladder-line-fed HF horizontal wire antenna

The photo here shows the feeder of an antenna system for low band applicaitons. In low band operation, it is not practical to install a horizontal antenna with high elevation suitable for DX contacts. So I made the one which is considered appropriate just for local contacts.

This is a 7 MHz antenna and is fed by a ladder line so that multi-band operation is achieved. As its feed point is only 3 m above the eaves, the antenna is regarded to be narrowly above the ground in terms of the wavelengths.

The feeder consits of 1-mm diameter stainless steel stranded wires separated by pieces of acrylic-resin plates keepig 30-mm parallel distance.

The characteristics impedance of parallel lines is given by the figure of 276 times the logarithm of twice the line distance divided by the line diameter. Thus, in this case, it is 276 x log (2x30/1). So I assume around 500 ohms is not a bad guess.


As this is to be tuned through a float balun and a manual tuner, the feeder length can be a convenient value. In this case, however, I chose the feeder length which tuned to 7-MHz CW band when connected to the antenna. The feeder length resulted in about 18 m.

As intended, the system smoothly tuned to the 7 MHz, 3.5 MHz, 1.9 MHz bands.

The low elevation horizontal antenna is not suitable for DX contacts because of its inherent high angle of radiation. However if we just switch to the contacts within the areas around several hundreds km apart, its characteristics can be conversely and effectively utilized. The so-called NVIS corresponds to this.

Actual operations have indicated that the antenna is far superior certainly for JA stations compared to the existing antenna of 18 MHz vertical dipole + AH-4.

It is usual that the horizontal antennas for low bands which radio amateurs can make end up as low height ones. Because of this, the majority cases will be that stable contacts with local stations are materialized incidentally, without a particular consideration for the NVIS.

To my delight, this antenna enabled me to work on 1.9 MHz band. That had been only in my radio license because of the antenna restriction. Thank for the ladder line!

The tuned feeding is an old and matured technology, but higly recommended.