Thursday, June 23, 2011

Fun with Antennas - Part 1

Having been licensed for a number years I like many have gone down the commercial route in both rigs and antennas, buying various base units from Kenwood’s, Icom’s to Yaesu’s and using different commercially made antenna’s having little or no patience to build my own.

A while back I took a break for a year or two and came back in to the hobby with a slightly different attitude, especially with regard to antenna’s and understanding how they work. Having sold all of my equipment I had to start all over again and in my search came across a local radio amateur who was selling a Yaesu Ft990 belonging to the late Frank Watts G5BM.

The legendary frank watts G5BM
Frank was a bit of a legend round these parts, a confirmed CW enthusiast I actually visited the legendary Frank years before when starting out on CW. I watched him do a few qso's and was amazed at his speed and light touch, he was a master of all types of CW keys. He was also kind enough to show me all his homebrew equipment including a fair few antennas he had made over the years. I always remember him saying, “You may own a nice rig but remember it is the antenna that does most of the work”.

Remembering our conversation all those years ago got me thinking that I should really start making my own antennas, partly because of cost and more importantly because I could learn another part of Amateur Radio. I figured the best way for me to learn was to start tinkering and having a go at building a simple antenna, that way I get the satisfaction of  knowing that when I hit the jackpot of a DX station I’d  know that I’d done a lot of the work to make it all happen!

Since having the break I had moved house and with a nice new garden to play in I could start afresh. The whole plot was nearly half an acre and the amount of the garden I had negotiated with the members of my family for radio work was approximately 90’x 50’ which gave me plenty of room for some reasonably sized antennas. At one end of the garden is a silver birch tree about 40’ high which could give me and ideal spot to hoist a dipole and at the other end I had managed to raise an old aluminium scaffold pole with an extension to give me a similar height of about 40’. If necessary I could also use the roof apex of the house which again was around 35’.

Whilst searching around in the shack I found I still had an old Carolina Windom 80, it was very long but with a bit of planning it would just squeeze it in the garden, however it would definitely look odd and out of place. So I did some research on the web and came up with the idea of reducing the size to make a Carolina Windom 40; 66’ in total, which would fit snugly down the length of the garden. After much measuring and re measuring I did the deed and cut the antenna to its new length 25’ short and 41’ long and reducing the size of the vertical element from 20’ to 10’. Borrowing an MFJ 259 I did some quick checks and trimmed each side to get a reasonable SWR reading of 2:1 for the whole of the 40m band, further checks showed acceptable SWR readings for 20m, 15m, and 10m and using my tuner I could happily use 30m 17m and 12m with no major problems. 80m could be tuned up with patience but since I rarely used that particular band I thought I could live with this problem. All in all the Carolina Windom 40 was a good antenna and produced great results but to be honest it still felt like I had cheated and was still using a commercially made antenna.

I have to be very careful when it comes to antennas, I happened to live in an area of outstanding natural beauty (the Cotswolds) so there is no way local residents are going to enjoy seeing a very high antenna or tower! What I needed was to build something that would happily fit in the garden be fairly discreet and be a true home made multi band system.
I trawled over the internet looking at various designs of multi band antennas, seeing what would be relatively easy to build and more importantly put out a good signal.

There’s no question that before the yagis and beams the old boys had some brilliant and intuitive ideas when it came to antenna design. I came across two easy designs; the classic random length dipoles fed with open wire feed line and the W3DZZ a simple a multi band trap antenna. The W3DZZ was cheap to build and a reasonable size to fit neatly in the garden. More importantly I could learn about traps, something in my early radio days I didn’t really understand and yet are actually quite simple to build, especially coaxial traps.

Taking the sensible route, I decided to start with the basics and chose the random length dipole with an open wire feeder which was even easier to build. I had some spare 14 gauge wire and trimmed to a half wavelength for 40m. Another rummage in the shack and found some 300 ohm ribbon cable. Within a short time I had completed the dipole and connected the feeder, now to connect to the rig; but there was a problem; my ATU would not accept the open wire feeder. Back to the web I searched frantically for an answer, how to transfer a 300 ohm feeder in to a 50 ohm cable for a connection? I quickly started learning about baluns; this is what is so great about the internet, by the end of the day and after some experimentation I had built my first 4:1 balun. I connected up my newly made balun and plugged the coax in to the back of the tuner and switched the rig on, hey presto I was up and running and within minutes was talking to a Frenchman who was giving me a good 5/9 report.

Pleased as I was, this antenna was definitely fine for local European traffic but was quite weak when it came to DX. Within days of realising this I immediately set about building the W3DZZ hoping that I might have some better luck.  Following instructions from various websites the coax traps were fairly easy to build although getting the coax to right length sometimes proved a little tricky. I used an MJF 259 to check that all was well they tuned up nicely, 1:1 right on the cw frequencies I required. For my first attempt the traps worked well on 7 and 3 MHz and I was getting some great contacts on these bands but disappointingly although titled a multi bander it was very quiet on the other bands compared to the Carolina Windom and the random dipole.

I was still certain that I could build an antenna which was better than the Windom and so once again I did some research on the internet and quite by chance came across the Mystery antenna. This very simple to build antenna used a mixture of coax and 14/16 gauge wire, it only took me a day to construct and after a quick tune and trim I soon had it hanging up in the Silver birch as an inverted vee.
First results were fairly good and unsurprisingly matched against the W3DZZ it was down on 3 and 7 MHz  by a couple of Db. But it worked extremely well on 14 MHz and as a multi bander it easily matched the Carolina Windom, at last I had built something that was near to matching a commercially made antenna.

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