Monday, January 16, 2012

Re Analyzing the Moxon

Been suffering with the typical cold virus that’s been doing the rounds, so my amateur activities have been somewhat limited this week. However, over the weekend I managed to have a play particularly on 10 and 12 meters just to see how the moxon was.

Now that I have the MFJ analyzer I did some checks and the moxon was out on the SWR giving me a reading of 1:6:1 which although not disastrous was a fair change from what it had previously been at 1:2:1. possibly brought on by the recent storms we’ve had. The readings were showing that the driven element was short by a few cm. Since I have a separate piece of aluminium tubing on each end of the driven element I can easily adjust or replace, but it has been proving a little tricky probably because the whole antenna was only a few feet off the ground while I was working on it.

Suffice to say after adjusting and checking and then raising the tilted mast back up the SWR reading although better was still out at 1:4:1 but acceptable. So when I get the time I am going to completely restart and get everything properly sorted. The other problem that is proving difficult is that the tubuler aluminium is of differing sizes, 8, 10 and 12mm which causes and issue in selecting the right figure for inputting in to Moxgen, so I've been using 10mm as an approximation.

Two questions I have for any budding moxon enthusiasts, firstly is it right that the SWR readings are just coming from the driven element, as the reflector is not connected presumably it does not play any part? I can have the reflector a foot away from the director or 2” but there doesn’t seem to be any difference in SWR. From what I’ve read though, the gap between director and reflector is critical so presumably this is for FB, naturally as guided I use the distance per the moxgen software for my set 10mm, but it still leaves me slightly confused. Obviously if I alter the length of the director in anyway then the SWR is compromised.

Also how do I measure FB? Certainly if I rotate the antenna there is a difference in received signal, but it seems you have to take in QSB and other factors. On occasions I tried to test by listening to a station that is transmitting and on rotation the signal does fade but sometimes comes back up when I am facing 180 away, is this normal? Any help would be much appreciated.

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