Monday, January 23, 2012

Moxon Corrections and Antenna Software

Had another play with the Moxon over the weekend, I was fearing the worst that I would have to take it all to bits to get everything working correctly, but by simple detective work I managed to solve the problem and get things back up and running.

My initial problem was that although the antenna was working the FB was simply not there, I could rotate the antenna away 180 degrees from a signal and the fade was little if any. I was concerned that my calculations in originally building the antenna were totally out because I had used various different tubular widths (12, 10 and 8mm) and from correspondence with amateurs from QRZ you must be accurate for MoxGen, you cannot just input an average. Also the gap between the driver and reflector is critical and if out will certainly affect the FB.

So with this in mind I first rechecked the widths and decided that 10mm was the correct information to input in to MoxGen. Then I rechecked the gap widths and reset to the precise measurement given by the software, I found I was out by approximately 20mm, which although small could affect the FB. Then with a quick SWR check I found I was spot on with a 1:2:1 from 28.000 right up to 28.550 which was better than when I first set the antenna up.

After hoisting the antenna to its full height and rechecking the SWR, I then tuned in to a European beacon and started testing the FB. With the antenna pointed to the beacon I was getting a good 599 plus 10 and so (with fingers crossed) I rotated 180 degrees to see what the difference would be, I was amazed; the drop in signal was way down by at least 4 S points, finally it was working as it should.

So points to remember are;
If using different sizes of aluminium tubing either use antenna software such as 4NEC2 or EZNEC to work out the correct input in to MoxGen. Or work on an average width but test each sample to see which works the best.
Also take in to consideration what tubular width you have at each end of the driven and reflector elements as these will be the capacitance and need to be accurate. Each end of my moxon had the 10mm width so again another reason that I used that figure for calculating in MoxGen.
The gap between the driven and reflector has to be accurate, mine was out by only 20mm but made all the difference. Even with the gaps slightly out  you will get a good SWR, so check the individual lengths, once you find that sweet spot it does make all the difference.

On a slightly separate note I managed to downloaded 4NEC2 over the weekend and had some fun playing with the software. It's a bit tricky to start with, but do persevere, once you have mastered the basics you can easily set up some experimental antennas and it certainly helps me on the theory side, especially for the problems I had with the moxon! I also found some great tutorials on uTube which show you around the software and gives you the basics in getting started. Of course the trouble now is it has opened a whole new world for me in antenna design and I already have various ideas, especially a 10m Yagi to build!

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