80M Antenna experiments..

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.

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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

 

 

Antenna addition..

As I wrote in my last post I’ve started using a new exciting mode on HF, but that’s not the only thing that’s new. A friend of mine,  also a hamradio operator, was getting rid of one of his vertical antennas. As I’ve mode some experiments with a homebuilt quarterwave vertical before and was a bit curious of the construction of wideband verticals, I decided to buy it and make some tests.

The antenna is a Wimo GPM-1500 and is marketed as a wideband antenna covering 1.8 to 30MHz requiring a tuner for matching. Looks like a quite good antenna does it? Well, I as a bit doubtful that it would really be as wide banded as stated, partly from my own experience with building and measuring antennas and also from the theoretic point of view. I decided to make some measurements on the new antenna to see how wide banded it is observing the the SWR value.

For the SWR measurements I used my newly purchased antenna analyzer, the Rigexpert AA-30, measuring 0-30 MHz and a RG-58 cable for connecting to the antenna.

So I started setting up the antenna, mounting it on ground level about 50 cm of the ground on a steel pipe with no ground radials. The antenna is said to require ground radials to operate efficiently but can be operated without.DSC_0585

 

GPM-1500 @ groundlevel, no radials

The output from the AA30 showed the following values:

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First measurement, no radials, steelpipe mounting

As shown the SWR never dips down under 1,5. Although the SWR stays mainly about 4 and under most of the HF band. A built in tuner may be able to handle this and it seems like it may be usable on high frequencies over 30m band but on the lower bands it´s a nightmare.

I now tried to raise the antenna a few meters, because of the risk of touching the feedpoint and the antenna itself when mounted in the garden. I found a wooden pole with a length of about 2m. I mounted the antenna and it is now “free floating” without any ground connection.

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Second measurement, wooden pole 2m, no radials

Now the curve have become about the same as before but with a dip somewhere in the 20 m band area. Still not good in the lower bands and the higher is still around 4 with a dip in the 10 m band.  The next step is to reconnect the antenna to the ground by using a braided copper band and a 1 m copper piece driven to the ground.  This may not be a perfect ground but it is useful just to see how it affects the properties of the antenna.

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Antenna mounted on a wooden pole, connected to the ground with copper band.

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Third measurement with ground connection, no radials

This time the curve shows that the dip in the 20 m band is gone but is now widened with a value under 2 wich can be handled by a built in tuner. Lower frequencies are still bad and higher is still the same with a reduced dip in the 10 m band.

As I wanted to reduce the SWR value in the lower bands I now tried to connect a radial roughly in the 40 m band area, hoping that it may effect the curve. I also disconnected the grounding with the copper band as it had no effect on the lower bands.

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Copper braid disconnected and radial connected.

I decided to lay the radial on the ground as it is impractical with elevated radials with this length in my garden.

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Forth measurement, 40 m radial

The curves show no real improvement on the lower frequencies. More radials with the same length was connected later on but it showed no improvement as well.

I decided to stop my experiment at concluded that the antenna, as it is in my garden, is useful for 30 m band and up. Lower bands is to hard to match. I had a number of radials used for my old 20 m experimental vertical that i connected to the new antenna. They were mounted as elevated radials at roughly 45 degrees angle down to the ground. I also bought some steel tubes pushing the feedpoint of the antenna to a height of 3 m. The combination of 20 m radials and the ground connection of the steel tubing resulted in a deep dip in the 20m band with a SWR value of 1.1 and leaving the higher frequencies  between 4 and 2. The lower bands are still bad.

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The final result of steel tube mounting and radial system for 20 m band.

In it´s current configuration the antenna is useful as a listening antenna for the HF bands and a transmitting antenna for 20 m band.  Even though it is useful in transmitting from 30-10 m I´m choosing to use my more efficient Inverted V antenna for transmitting. I am planning to continue with grounding and radial experiments to try to make the antenna useful in the lower bands, at least down to 40 m.

My conclusion about this antenna is that, as expected, it is a typical wide band antenna that is a compromise to make it work at as many bands as possible. The compromise is in the matching, meaning that it is more or less good matched depending on the frequency but is tunable with a riggs internal tuner. As with other verticals, the risk of RF feedback is high when the matching is not good.

In my complete station configuration it´s a good compliment, despite it´s downsides. The use of a different type of antenna located at a number of wavelengths from the main antenna makes it a good reference antenna with somewhat antenna diversity feature for incoming signals.

73

//Anders