Monday, 28 December 2015

Difficult to take a step back in this hobby

Some people do not have the option to keep moving forward in this hobby. Life changes, house moves, job commitments, lot of things, can cause many amateurs to take a step back from time to time and simplify what they are doing.

It is not a bad idea sometimes. We can often let things get out of hand and lose sight of the basic simplicity of amateur radio. Sometimes it is a good idea just to clear a lot of "stuff" out and go for the simple approach again.

Whatever the logic, I tried to change the 4m antenna a few months ago (see here). A smaller antenna seemed to be the only one available, given my limit of 1.5 on one side of the beam (or roughly 3m maximum boom).

My existing 4m antenna then,  a 6 element Sandpiper, was a great performer. However, it was basically an extended version of their 4 ele, and that was its main failing. The boom was not sturdy and featured heavy support braces. The elements did not stay stable in the winds we have here and I decided that it was not able to withstand the winds here.
This 6 element is not longer made by Sandpiper, and when I took it down I could see that it was quite a lot out of line. It should be fine as a 4 element, but 6 elements was too much for that boom.

The replacement was a 4 element Vine antenna. Nothing wrong with that. It was also the last antenna Vine sold as they stopped making them the day I bought mine.

There is nothing wrong with the Vine. However, it has a 2.6 metre boom, and the difference between that and the 3m boom of the Sandpiper was immediately apparent. Changing back from a 6 element to a 4 element was a frustrating experience. The two most useful beacons nearest to me (GB3CFG in Carrickfergus and GB3BUX in Buxton) both disappeared and I had to rely on GB3ANG (not a favourite of mine).

The thing about antennas is that you never know how badly they are doing. You know how WELL they are doing, but you cannot say what you might hear if you had a better antenna. In my case, I could remember how well the Sandpiper did, and somehow the Vine was not as "lively".

I cannot really define "lively" as a quality of an antenna. It is a sizzling noise, a feeling that you are beating the background noise and you might hear anything that crops up. I could not really put my finger on it. I knew that in theory reducing the boom length by about 15% might lose me half a dB or so, but it just felt that I was losing out on something but I did not know what that was.

In the end I decided to change the Vine for a 5 ele PowABeam. There was some confusion about that, as I thought it had a boom length of 3.02m, but it turned out that it was 3.20m. There was an error in the website. So after I ordered it, it turned out that it would not fit. After some discussion with the dealer, The DXShop, it transpired that, unlike the other antennas, the PowABeam does not have a central mounting but the clamp can fit anywhere. Subject to placing some side load on the rotator I could put the mounting point anywhere along the boom.

The PowABeam arrived with the boom in one piece - 3.2m in a cardboard tube. "A very long snooker cue" was the delivery man's observation. The DXShop said it would arrive two days after they sent it, but it came the next day - well done.

Once I fitted the elements and tried for the balance point, it was nicely placed, with 1.4 metres on the back side of the beam and 1.8 metres towards the front. As my overall limit is 1.5m on one side, I was able to get it to balance and fit perfectly.
The two antennas look very different beside each other. The 5 ele is 800mm longer than the 4 ele was.

Thus I have increased the boom length this time by 30%, in theory worth about 1 dB, hardly noticeable, you may think.

There was nothing wrong with the Vine, other than I lost confidence in it. It was, theoretically at least, a tiny bit less effective than the Sandpiper. It was bound to be, being shorter. But I missed an antenna that used all the space I had available. In practice, the PowABeam fitted perfectly.

The improvement is largely in my mind. As soon as I put it up I could hear GB3CFG and GB3BUX. The following day I could hear neither of them. Isn't tropospheric propagation interesting?

The Vine has gone into the garage. For anyone who wants a 70MHz antenna of about 2.6m boom it would be ideal. For me, who felt disappointed when I went for a shorter boom, it was too short.

Am I saying that every antenna any amateur has needs to be bigger than the last one?

I guess so.

73

Jim
GM4FVM

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