As the prognosis for the higher bands looks quite sad now when the eleven year cycle is heading for a low, I decided last summer to try to build an antenna for the 80M band and eventually also 160M. These bands have not been that much activated in my home QTH yet and why not get more active? 🙂

As a general rule, I prefer to have dipole antennas, mainly to keep a balance in my antenna system but with a rather small garden and the fact that a dipole for 80M is rather long I had to leave my principles and look for alternative solutions, balanced or not.

I started my experiment in July with trying to fit a full length dipole, in some way, in the backyard. Since I knew from the beginning it wouldn’t be an easy task as the whole garden itself is far from enough space to put it up in a conventional way, I started to look at alternative ways to get it up. My final solution, far from the most effective one, was to elevate the feedpoint of the dipole and leaving one leg a about 3m above ground and the other straight vertical down to ground level, using the leg more as a counterpoise. A half dipole so to speak. 🙂 The result, well I did get it to resonate but the effectiveness is probably not that good and radiation diagram should be more or less high angles. I managed to work some European stations with the antenna in the SOP-activity and after that as well. The coverage seemed to be mainly Europe and no DX-stations either heard or worked so i guessed the antenna could use more elevation.

The only way to elevate the antenna about 1/4 wavelength at my QTH is to use the house to mount the center of a full length dipole in a Inverted-V configuration. The backside is that only one leg of the antenna would be able to go the full stretch without needing a “bend”. The conclusion was to look for other alternatives.

I wanted to reduce the length of the antenna itself to make it possible to fit, both in the garden and possibly mounted on the house. I stumbled upon an antenna construction consisting of a 80m dipole made shorter with inductances which made the total length only about 17m. I decided to give it try!

The antenna is made up of four segments of FK1,5mm2 wire and two 30uH coils, spooled with the same type of wire. The antenna is feed through a 1:1 balun, which I disregarded for the time being. The segment are two 3000mm and two 5600mm wires connected as shown below. The final length of the outer wires was not consistent with the drawing as it depends on where in the 80m band you will tune it to as well on other factors.

12

80m shorted dipole

The coils are made of the same wire as used for the antenna segments. For my part using 40mm diameter pipes, it took about 55 turns to achieve the correct values. The inductances was winded up on the PVC-pipes and, with the help of my Rigexpert AA30, carefully adjusted to about 30uH. I also had a small home brew center isolator with a fitted 50ohm cable which was used for the antenna. The build itself took about 8h, including the purchase of 40mm PVC-pipes on the local hardware store. To get the wires I needed I used parts of an old 3×1,5mm2 cable I have laying around looking for suitable project. After the build, it was ready to be tested.

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One of the antennas coils

I erected the antenna in a inverted-V position using my 5m fishing rod in the garden and measured the antenna. The end wires needed some extensions since the antenna was to short. A quick fix with some spare wire and the soldering iron solved the problem extending the antenna about 1m on each end. Since it is a shorted antenna, the bandwidth is no more than 20-30 kHz why it not suitable for use in the complete 80m band. I focused mainly on tuning the antenna to the lower segment of the band, aiming for 3576 kHz primarily (JT65/JT9). After some tuning in the rain and +2C (cold day) I was satisfied with the result and went in to the warm shack to do some on the air testing. After a few hours I decided to mount it more properly and moved the antenna to the back garden, removing the fishing rod. It is now mounted about 3m above ground as a conventional dipole with one end from the house and the other end in a tree, just at the edge of the garden.

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The 80m shorted dipole in the garden

When moving the antenna, the tuning made earlier of course changed why it need some more fine tuning, however the change was acceptable and I left it as it was. Further testing during the evening showed that, although mounted quite low, it still seemed to have a more favourable radiation diagram than my first experimental antenna. I actually got more heard stations outside Europe than ever before. Even a few VK-stations (VK6 and VK5) was decoded with acceptable levels. Since it’s now colder outside, I´ll leave the antenna as it is for more evaluation. Iplan to look for a mounting point on the roof of the house when the weather gets a bit warmer. 🙂

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The home brew center insulator and feeding point.

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The 80m short dipole viewed from the side with the coils to the left and right.

73 de SM7VRZ

 

 

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