|Here is a description of my 80 m amateur antenna. It is a full size dipol requiering only a limited space. This is due to folding of the legs to an angle of 105 degree in the horisontal plane.
The legs are each app. 19,75 m, giving resonance at 3700 KHz. The feeder is 50 ohm URM43 (RG58), length is chosen as half a wavelength. The shortning facto for the cable is 0.66. Half a wavelength is: (300/3700)x0.66=26.76 m. By using a feeder of half a wavelength you can measure the impedance of the antenna at the end of the feeder, as there is no impedance conversion with half a wavelength cable.
I used 2 ferrite toroides to make a common mode filter at the feedpoint, There are 3 windings on each. Each give an impedance of 500 Ohm at 3700 Khz. In this way the antenna is balanced. To avoid the feeder to resonate with the antenna, I put ferrite tubes on the feeder at a distance of 2,5 m. The tubes give an impedance of 100 Ohm each on 3700 KHz.
The picture "Ferrits", shows the ferrites used. I laquered them with Linenoil, and baked them in an oven at 80 degree for 48 hour.
The hight of the anrenna is 8 m, that include the endpoints as well. In order to make the antenna is invisible as possible, I used a rather thin wire for the legs, 1,25 mm cobber enamel wire. It is wire for vinding transformers and motors. The enanel is very strong, preventing corrosion of the wire. The connection from cable to the legs are soldered and covered with silicone rubber.In order to lover man made noise and in order to lower the earth losses, I put a lot of wires on the ground internally connected, but not connected to earth or to the station. The wires vere laid where possible and convinient, on the lawn they were buried just below surface. Under bushes they are laying on the ground.
And it works - - - Noise on my receiver Drake R4a, is usually 1 to 2 S units, very low in spite, that I live in a residential area with many houses nearby. Signal reports with 100 Watts output are very good at short distances, and the antenna can also be used for connections up to 2000 Km with good results.
Measurement of impedance;
3500 KHz: 136 Ohm
3600 Khz: 78 Ohm
3700 Khz: 50 Ohm
3800 Khz: 80 Ohm
| Slideshow danish. (Choose "View Fullscreen" at the right button.)
| Slideshow english
| Here you see the ferrites for the feedpoint.
|Antenna filter||Here you see the antenna feedpoint erected in the garden.|