Wednesday, 20 April 2011

We had a lovely weekend here in UK, as HF band conditions were reasonable (SFI=114, not great but OK). I decided to experiment with simple vertical dipoles. I have a 7m fibre-glass pole that I normally use for VHF but I used that and built three wire dipoles for 10m, 15m and 20m. These were centre fed with RG-58 coax attached with a simple home-made BNC connector unit.

The weight of the antenna and feeder bent the top of the pole slightly, as shown in the photo. This is not a bad thing as it separates the antenna from the pole, which does interact slightly with antennas. The top sections of the dipoles were nearly vertical. This is important as it achieves a low radiation angle. This is especially true for the 20m dipole, which would otherwise be inclined at a shallow angle, raising its radiation angle.

All dipoles were easy to tune using my MFJ-259 antenna tuner. I cut them slightly long then trimmed pieces off, 5cm at a time, until I got to resonance.

Would they work? I connected my Yaesu FT-817 and had a listen around on 15m. Lots of activity, especially from USA and this was just too tempting. I plugged in my key, called some contest stations and got replies! Not all stations I called could hear me, but I worked four American, one Russian and one Bulgarian in about 20 minutes on CW (Morse Code). I was on internal batteries so probably not running more than 3 Watts, so I think I can say the 15m antenna worked.

I changed over to the 10m dipole (six minutes) and again there were stations to be heard. But unfortunately the internal batteries in the FT-817 were about to give up the ghost so I did not work anything on this band.

This little 'garden portable' expedition shows that it is possible to work HF even with very simple antennas and low power. The vertical dipole has a very low radiation angle (17 degrees, according to my antenna modelling software) so puts your signals exactly where they need to be for working DX (long-distance contacts).

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