Monday, 27 January 2020

4m/6m dual band antenna

Well, that great television detective Vera Stanhope arrived in Burnmouth and my antennas were not visible in the background.

Burnmouth harbour had become a salmon fishery base, and a body was found floating in the tide. With murder brought so near to GM4FVM, I have been wondering about my security. I am a bit more visible than I was thanks to a new antenna for 4m and 6m. Well, actually it is the same size as my previous antenna up there, though it has more elements on a single boom.

I was not a big fan of dual band antennas until Antennas and Amplifiers started making carefully designed yagis with separate feeds. This was based on my experience with my Vine 4m/6m yagi, which had 4 and 3 elements respectively and a single feed. I just did not like a single feed.

I had a system whereby during the summer I would have separate antennas, and during the winter when the winds tend to be higher I put up the old Vine again. Even last year when the 6m antenna was just a simple 1/2 wave vertical I stuck to this system. If I needed any proof of the wind force the 1/2 wave bent when I left it up over the winter. An earlier 4m vertical there did the same thing and ended up crooked.
The old Vine (left with Diamond vertical), and the bent 6m half wave (right)
So I thought a compromise was needed. Could I get an antenna on a single boom which did not require a single feed, but which was also able to survive the gales? Ideally this would have similar gain figures to the two separate 5 element beams for 4m and 6m I use during the summer. Maybe not all of this would be perfect but it would be better than what I do now.

This question was then put to Goran, YU1CF, at Antennas and Amplifiers and he went away and poured over his beam modelling software. Actually, he spent ages on it. My specification was for a 3 metre boom maximum boom length and with any benefit of the doubt to be given to the 4m band. Eventually he came up with a new design called PA5070-9-3-2CB. This has 4 elements on 6m and 5 on 4m, and the gain figures are much the same as each of my previous single band 5 element antennas.

Goran has managed to get it all into the same space as my previous 6m yagi - i.e. 3m x 3m. This is good news for the neighbours who will have to look at it. I definitely did not want to put up anything that was bigger than before. Of course it does have 9 elements, but that is instead of 10 elements on two 5 element beams. The biggest gain for the neighbours is that there is only one of these antennas instead of two. So, all round skyline effect is smaller. The effective length of the 4m beam is 2.2m and the 6m one is 2.8m. The elements are interlaced together lengthwise on opposite sides of the boom, just like the 2m/70cms one.

This arrangement leaves the other mast clear of 4m and 6m, so it just has two antennas, the 2m.70cms dual band and the 23cm yagi. There is a reduction in wind pressure all round. Also reduced is the overall antenna count as the 6m vertical has been taken down before it fell down and the 2m/70cms vertical has gone there instead. Well, I call it a 2m/70cms vertical, but it is really only a marine band antenna due to the death of FM activity round here.

Right so, let's see it.
Antennas and Amplifiers dual band beam, "2m/70cms" vertical on right
It is no larger in any dimension than the 6m single band antenna, so I am very pleased with that. We shall have to see how it performs. Now I have a 6m beam all year round, or at least I hope so. Goran's meticulous construction looks likely to be long lived.

My dislike of single feeds is silly. After all, all the elements in a yagi are parasitic except for the driven element. Despite that, and for no good reason, I do not like the sleeve arrangement.

So we will see.