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!

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.

Wednesday, January 11, 2012

Trials and Tribulations on Buying the MFJ 259b

The MJF 259b has finally arrived, it’s a fiddle though, you would have thought you’d get the power supply with it, but no, that has to be ordered separately, typical!

So now I’m busily checking around the local hardware stores for an AC DC adapter with the right connector, it never comes easy does it? On surfing the web I came across a UK Amateur dealer who wanted £68 for the MFJ 99x accessory pack which includes the dip meter coil, carrying case and you guessed it the power adapter! Having checked out the web I can get a power adapter for £6.50, makes you wonder doesn’t it?
The other pain is that I knew it took rechargeable batteries, but I was rather taken back when I realised it needed 10! Removing the front cover to install the batteries requires the removal of 8 screws, you'd think MFJ would have come up with a simpler design?

However I digress, last night I popped round to my next door neighbour and asked if by any chance he had a 12v power adapter, he did and blow me it fitted!  So I was able to do a quick test to make sure all was working as it should, interestingly enough all my antennas were slightly out on the SWR readings, most likely from the recent storms, but over the weekend as a simple experiment I will borrow my Brother in Laws MFJ that I originally used to set them up to see how good the SWR matches are on both units, that could prove very interesting!!

Monday, January 9, 2012

Antenna Analyzers and a 30m Switch Round

The Feature Tech AW06A Antenna analyzer I bought off Ebay has failed to show up. Originally purchased on December the 11th and having checked out the sellers reviews etc (as he was based in China), I thought I was getting a good deal. But to my cost it looks like I’ve been scammed, the feedback has suddenly come alive with other buyers complaining of not receiving their items. Hopefully I will get my money back, but it’s a lesson learnt which I wont forget in a hurry.

So last Friday I checked out all the UK dealers and bought myself an MFJ 259b, got a good price too, so all is not lost. To be honest I’m glad I got the MFJ, I’ve used one before and it does a lot more besides just giving you the SWR and it will certainly get well used in the shack.

Heard a ZL station for the first time on the 30m band this morning, checking my records I haven’t heard one on there since the last sunspot cycle. The explanation being that over the weekend I’ve moved the 30m inverted vee away from the cobweb mast and from facing north/south to hanging off the Moxon mast and facing west/east just as a little experiment and by all accounts it seems to have worked. Hopefully later this week when I get the chance I shall try and work both VK and ZL.

I also managed to take down the old Carolina Windom and replace it back with the W3DZZ which has now been serviced with the traps having a good clean up and should be back to being spot on their frequencies. After the past few storms we have had the W3DZZ certainly needed checking out as the traps had taken a right hammering in the high winds.

Wednesday, January 4, 2012

Storms over the Cotswolds

Another terrific winter storm this week and unfortunately the Moxon suffered somewhat even though the mast was placed in the tilt position. The aluminium is pretty flimsy stuff and any hard knocks will put it out of place, so with wind gusts of 60mph it will certainly affect the antenna in some way. On closer inspection the gap between the driven element and reflector has moved and it was noticeable when rotating the antenna that in some cases I was getting better reception with the antenna facing away from the target!

Once the storm had died down and I did some quick checks and quickly realised what the problem was and so will attempt to rectify this weekend. It’s most annoying though; my Ebay antenna analyzer has still not turned up, no doubt caught up with all the Christmas mail, so I cannot do any major alterations until it arrives. One other thought came to my attention which had completely passed me by was the fact that the boom was still longer than required, (see photo), but in all honesty I’m not sure that it would effect the antenna, however logic dictates that it shouldn’t be there so I will trim to the required length adjust the spacers and hopefully the forward gain will return.

At present all antennas except one are tied down purely for protection from the recent storms. The cobweb is fairly easy to drop as it is on a retractable mast and the only time consuming part is readjusting the guy ropes. The W3DZZ was replaced last week for maintenance as the balun needed some taping up and I wanted to check the traps were still aligned to their correct frequecies. So I had raised up the old Carolina Windom as a temporary stop gap and decided to leave it up during the storm so at least I had some form of communication available and all things considered it is working reasonably well.

I’m still having great thoughts of building a yagi for 10m in the spring, but sourcing the aluminium is a proving difficult as trade suppliers want vast amounts of money....Typical!!