I have two excellent VHF antennas, a 5 element for 4m, and a 5 element for 6m. Both are PowAbeam designs, robust and very effective. They both have 3 metre booms, lots of gain, and work very well. Yet both have been taken down and are in my antenna-rich garage.
The cause of this action is that I have decided to make things a bit simpler. Less aluminium in the air, less strain on the rotators, one less linear amplifier and less yagi to annoy the neighbours. Simpler. An unusual concept round here, but that is what has happened.
Now shame on you who looked a my last post and did not mention that I had mounted the elements on the 4m/6m dual band beam in the wrong order. Tut tut. I am sure that the more observant noticed this but decided not to comment. Several of you commented by mail about the silent-key dilemma, and thank you for that. The silent key issue is a tricky one, and I am sure that you wanted to handle it delicately. So, you noticed that the elements were the wrong way round, but you kept quiet to spare my embarrassment.
I have now also put back up my 4m vertical, which fell over in a gale some time ago. Here are the two together, this time with the 4m and 6m driven elements on the yagi reversed. The vertical antenna on top of the yagi is a Diamond X-50 for 2m FM (well, actually used mostly for listening to Aberdeen Coastguard as there are no locals left on 2m FM).
|5/8th 4m ringbase vertical and 4m/6m dual band beam at GM4FVM|
I must offer something in defence of my stupidity. I took the old Vine dual band antenna out of the garage and gave it a spruce up. It must be ten years old, and last saw service temporarily as a 6m antenna in the past year or two. When I took it down I had carefully marked every element with labels including for this pair "4m BACK, 6m FRONT", not just on the mounting for the elements, but on the boom too. So when I put it together I made sure it was right. I did also notice that the coax connection to the driven element was getting a bit ragged.
As soon as it went up I had a high SWR. A quick examination revealed that water had got in and corroded the coax. So I took the driven elements off (they are mounted together) and took them into the house for surgery. This involved cutting back the coax, shortening the choke balun, soldering new ring tags and making good with lots of self-amalgamating tape. Then I put the two driven elements back, and, as I now know, I PUT THEM BACK IN REVERSE ORDER.
This was silly, but I was in a hurry to test the antenna. I still had the 5 element PowAbeam up so I was going to run a long series of tests to compare the two. Even with the elements in the wrong place, the results were plain - a 4 element with a 2 metre long boom is not as good as a 5 element on a 3 metre long boom. No news there then. But the tests showed that the reduction is only around the 1 dB expected and really quite manageable.
This dual band beam was in use at GM4FVM for some years before I decided that separate beams for each band were better. That is still true, but it comes with greater complication, two sets of coax, two rigs ... and just for now I would prefer something with less metalwork up in the air. So one old dual band beam has been tidied up and pressed into service in place of two larger ones. If it doesn't work I can always change back.
The trial I did between the two antennas involved pointing both at the GB3CFG 70MHz beacon near Carrickfergus in Northern Ireland (about 250km away) and monitoring the PI4 signal decodes simultaneously for 3 hours. The outcome showed all I needed it to show - the dual band antenna might be marginally worse than the single band one but not more than 1dB or so overall. I have decided that I can cope with 1dB on 4m (for now anyway).
As for putting the vertical back up, well it is now my second antenna. It works fine on 4m, I can receive on 6m, and with an IC-7100 attached (the one that covers for 70cms) I can switch it about to whatever band I want to hear while using the dual band beam for something else. Job done (well, once I put the elements on the right way).
What a job fixing the elements proved to be. I knew something was wrong, but I could not figure out what. I went out and straightened every element, measured the differences between the elements, tried to fathom why the 4m SWR was high, checked the coax, switched around all the leads in the shack .... I am sure it was only a matter of time before one of you sent me an email to explain that they were the wrong way round in the photo on the last blog ....
It dawned on me while watching a repeat of the 2017 Masterchef Australia on the Home channel. Yes, I watched every one of the 63 daily episodes in 2017, and I am now doing the same as they re-appear this year. And suddenly, while watching Kirsten Tibballs explain her chocolate mystery box challenge, it dawned on me - the elements must be back to front. And so it proved to be.
Spoiler alert. The mystery box was made of chocolate, so they could use the box in the cooking as well as the contents.
The moral? Stop thinking about your problem and you will do your best thinking. Distraction is generally my best way of solving problems.
So it was out at 08:00 next morning and, yes, they were the wrong way round. The SWR was now excellent. And then, as if by magic, the Es season started.
|VHF Es contacts at GM4FVM on 23 to 25 April 2019|
Since then, we have had three days with no Es. Such is life. I don't care, now that it has started it will be back.
Frankly, most amateurs using both 4m and 6m use a dual band beam, or a log periodic, or something similar. Separate beams might be better, but the final dB comes at a cost. It got harder to argue for separate beams when I was having such success with a 2m/70cms dual band yagi.
The dual band 4m/6m yagi antenna now works fine. I can listen to the other band with the vertical too.
First indications suggest that my simplification worked once I realised how to put the antenna together the right way. After fixing the TE Systems 6m linear I don't need it as the Gemini 4 covers both bands. The changes also clear the 4m single band beam away from the other mast to allow me to tinker with my 2m and 70cms antenna - but more on that later if I ever get round to it.
Simpler? Oh yes, but GM4FVM-style simpler which means still a bit mixed up.