40 meters band Vertical Dipole

20/03/2015 15:10

I have already a last high pine tree (Canadian Douglas) growing on my plot. Its high is almost 30 meters. I use the one like a mast for my low band antennas. That is the mast of 30 meters band dipole and 80 and 160 meters band long wire antenna as well. My vertical dipole for 40 meters has also the “Douglas” like its mast. Now I would like to briefly describe this antenna and used software and measurement equipment for optimization of antenna parameters. If somebody is interested in details please contact me via e-mail or Skype.

I used software (for 32 bits PC) EZNEC (demo) v.5 for modeling the antenna.

 

The picture above shows the vertical antenna (green color).  The half of the  dipole was 10,27 meters long (cooper wire diameter about 1 mm). The horizontal part (3.65 meters) in the low part of the antenna must have been used for the dipole resonance and it also had the role of some coupling with the ground (about 2.5 meters over ground - the antenna was without any radials, no place for them). This antenna should have been relatively good for DX communication and its gain was only cca 3dB under the horizontal dipole's gain in its optimal high over optimal ground. It must have been good enough for me. I did not have another reasonable possibility for installing the 40 meters band antenna on my plot. The vertical radiation angle was 15 degrees  (almost optimal for DXing).

The modeled elevation (vertical) diagram of radiation follows:

I would like to remark that I used common ground quality for modeling. The dipole had a voltage balun in its center and about 10 meters of 50 Ohm coaxial cable to a respective point for feeding located on the ground. The modeled SWR was about 3. It was of course too much. Because of that fact I measured the real feeding parameters of the real antenna in the respective ground point with AA-200 antenna analyzer. The real measured resistance was 30 Ohms and the real reactance was 40 Ohms. I saw that I would have used a simple LC network for matching the real antenna to the feeding 50 Ohm coaxial cable. Therefore I used SMITH V3.10 (demo) software. It was normalized for 50 Ohm system.  The impedance point of the antenna (including about 10 meters of the feeding coaxial cable) is seen in the Smith diagram below:

The needed LC network (one L and one C) was designed with SMITH V3.10 (demo)  software again:

 

The result of designing the matching network:

L  =  0,9 uH

C =  580 pF

The scheme of the LC network is in box : “Schematic” in the picture above. Final SWR of the real antenna was 1,1 (7,0 MHz) to 1,4 (7,2 MHz). The real matching network was put into a small plastic box. I have used the vertical antenna for 40 meters band successfully (CW,SSB) also for DX operation for a few months.

References: 

-          Program EZNEC v.5 (demo) and user’s manual

-          EZNEC Modeling Tutorial by W8WWW

-          Program SMITH V3.10 (demo) and user’s manual

-          AA-200 Antenna Analyzer user’s manual