I just wrote a long posting explaining everything about why I have two masts, why the 6m antenna cannot go on one of them and why the 23cm should go on the other. It explained why I need to have at least one antenna covering two bands (even though I dislike that whole idea) or none of it works, and why I do not like sleeve fed single coax dual band antennas. Boring. I deleted it.
So you will just have to take it as read that I need to deploy them the way I do. Yes, I would love one taller free-standing mast with a stack of antennas on top, but it is never going to happen, so I have to do this.
This is where I am now. It is very similar to the last settled situation a while back. It might be settled now after a whirlwind of changes which went off the rails when I could not complete the programme, I was in the middle of swapping things around when my knee went and it has taken about 18 months to get the new knee fitted and for me to be able to work with them again now.
In the past I had the Dual 2m/70cm yagi with two feeders on my main "CUG" mast. The boom length of this antenna was 3m and I did very well with it over a period of five years. On 70cm, as with all designs of this type of antenna, the whole boom is not utilised and my single band SHF/Wimo 70cm yagi with 3m boom would have given better performance.
After a lot of head scratching, and some chin stroking, I decided that my 3m boom limit was getting in the way of results on those bands. However, stepping up to about 4m boom length put me into territory where no suitable commercial antenna was available. Most manufacturers stepped straight from 3m to 6m. I can hardly blame them as going to around 4m was only going to produce 1dB or so of gain, but that was not the point. I could free the 70cm antenna to be the full 3m boom if I switched to separate antennas, and that was what I did at first. I changed over to a I0JXX 8 element 2m single band yagi with a 4.2m boom, and put the SHF 3m boom back up for 70cm.
I have to say that the I0JXX is a beautiful antenna. Not only does it look wonderful, it performed brilliantly. It is light and strong, so what more could I want? The 70cm SHF also did very well, but therein lay the problem.
The only place to mount the single band 70cm antenna was on the Tennamast and that caused RFI problems. Also, and for the same reason, it broke (the UK regulator for amateur radio) Ofcom's rules setting limits for field strength outside the boundaries of my QTH. The Tennamast was too close to the boundary for the higher RF levels once I got my 70cm linear working again (that might be a later post). The solution would be to move 70cm back to where it was, on the same boom as 2m, as the CUG mast is far enough away from the boundary to comply. BUT, I had already moved it away from there as the dual band antenna lacked gain. I cannot put three antennas on that mast, and the 6m one will not fit anywhere else.
There did not seem to be a sensible answer so I took my standard approach and left it on the side while I worried about something else. When I eventually returned to thinking about this I decided to check out all the figures. It was then that I discovered that Dual now make an antenna with the same boom length as the I0JXX, with similar gain figures for 2m, but which also covers 70cm. Although the 70cm side is shorter than the 2m one, it is as long at the SHF yagi and the claimed gain figures are almost the same. Result!
Here is a photo under threatening skies.
 |
| The new Dual 2m/70cm yagi above the 6m PowAbeam at GM4FVM |
2m and 70cm back on the same boom with broadly similar performance, and complying with the Ofcom rules? Just what I needed, but at a price. I had to buy a new Dual "PA144-432-25-4.5-2CAP" which has 9 elements on 2m and 16 on 70cm. This set me back about £250 but it should resolve RFI issues created by the SHF yagi being mounted too close to the site boundary.
Compared to the previous settled arrangement with the earlier Dual antenna, Goran claims +1.3dB on 2m and +1.8dB on 70cm. I am sure that this will not quite make up the difference to the single band antennas, but it means I can operate at full power which is +3dB. With the single band set-up I could not run my full power on 70cm. That did not matter until the linear came back. I might want to do some more moon bounce now.
To complete my antenna array, I am currently using my two 5 element PowAbeams for 4m and 6m. Once again this gives maximum performance for those two bands. If I need to in the future I could still revert to the single Dual 4m/6m antenna, but there would be some losses involved in that and no benefit which I can see at the moment. Later I might think of a reason for doing it (13cm dish Jim? That Mini-beam in the garage?).
The 4m and 23cm antennas are on the Tennamast. In that position and at lower power levels they easily comply with the Ofcom rules.
So that is it. Settled.
Until I change it again.
73 Jim
GM4FVM
No comments:
Post a Comment