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Moxon for 11 meters
- Thread starter Bow
- Start date
Showing up late to the party but I have used the Moxon a lot and at low heights due to living in military housing. I had to hide it in my garage attic but even at that lower height I was still able to work DX at 12 feet off the ground under a roof. Hey you gotta use what you have. I had to run the measurements in Moxgen at 1 Mhz below my target frequency. So my center frequency was 27.025 mhz and I used 26.025 mhz in Moxgen. SWR was flat and impedance was on the nose at 50 ohms. Another thing I noticed was angle that your coax/feedline ties into the Moxon needs to be in the same plane but perpendicular to the element. The Moxon does very well at low heights and I do believe I could get better performance at a higher height. The back door is not completely closed off but there is a definite separation. Don't give up, its well worth the effort.
Hey 711. I've asked this question about how close Moxgen got to the frequency entered into the software before. I think I was told that Moxgen was pretty close to right on the money.
I can't question what you see, but your note that the software is off by 1 mhz. That is a lot of difference.
Did you use an analyzer on your antenna to figure that out?
I would like to consider to use somebody's real world specs and design ideas using Moxgen, and my using Eznec software to model the same. I would like to see how close to the real world results I can get with some of my model's results.
Thanks for all inputs.
This weekend I will assemble my moxon, as calculated :
Will be elevated on around 30 feet above the ground, with no SWR to tune, reuse aluminum tubing ex my Yagi 5 years ago and pvc boom.
Hahaha...
Let's see how it works.
Thank you very much gents, I'll let you know the result.
This weekend I will assemble my moxon, as calculated :
Will be elevated on around 30 feet above the ground, with no SWR to tune, reuse aluminum tubing ex my Yagi 5 years ago and pvc boom.
Hahaha...
Let's see how it works.
Thank you very much gents, I'll let you know the result.
Marconi,
I made the Moxon based on 12 AWG and Moxgen gave me the lengths. I put it up and used an MFJ and swept the antenna and seen similar measurements to Bows MFJ. I next went to the lowest SWR and reactance and found it to be centered at 28 mhz. So I lowered the center frequency by 1 mhz and everything was spot on. Not saying its wrong just from my own trials with the Moxon antenna.
711, Homer just told me that I was wrong, and that Moxgen also produces results that are not correct. He agrees with both of you guys...the frequency ends up well below what frequency was entered into the calculator.
Because I apparently recalled Homer's Moxgen generated real world Moxon gave near perfect resonance and match, I gave up on my Moxon models a long time ago, and that was because I too found the frequency was very low compared to the frequency I entered into the Moxgen calculator. I was trying to compare my model then to Homer's real world results...and the results didn't jive.
Sorry I led you guys on a wild chase for info.
As Marconi said, I have had to adjust the lengths to get the real world results spot on. It may be noted, however, that the wide banded nature of the antenna will produce great results for anyone using MoxGen without sweeping it with an analyzer.
I have built 4 of the Moxons - two wire and two tubing. In every case the MoxGen measurements produce a slightly long driven element that set resonance lower than I wanted. I simply shortened the driven element in each case to bring it up to where I operated. The value of Moxgen is the same as with all antenna calculators, it gets you started very closely to where you need to be. I have never built any antenna using any software that did not need to be fine tuned.
Not using the MoxGen tool can be a matter of too many guess work variables. There are numerous essential dimensions to be considered, not the least of them the end spacings between the two elements.
I never readjusted the length for the reflector. The antenna always worked well, and resonance followed the adjustments to the driven wire or tube lengths adjusting only the driven element.
One might do as has been suggesteed and put in a center frequency about 1 Mhz higher than desired and build it. Please note that the antenna parts will need to be adjustable in the event the antenna is still short of the mark. In other words, don't cut the wires or tubing in a permanent way before checking the antennas SWR and resonance.
I have built 4 of the Moxons - two wire and two tubing. In every case the MoxGen measurements produce a slightly long driven element that set resonance lower than I wanted. I simply shortened the driven element in each case to bring it up to where I operated. The value of Moxgen is the same as with all antenna calculators, it gets you started very closely to where you need to be. I have never built any antenna using any software that did not need to be fine tuned.
Not using the MoxGen tool can be a matter of too many guess work variables. There are numerous essential dimensions to be considered, not the least of them the end spacings between the two elements.
I never readjusted the length for the reflector. The antenna always worked well, and resonance followed the adjustments to the driven wire or tube lengths adjusting only the driven element.
One might do as has been suggesteed and put in a center frequency about 1 Mhz higher than desired and build it. Please note that the antenna parts will need to be adjustable in the event the antenna is still short of the mark. In other words, don't cut the wires or tubing in a permanent way before checking the antennas SWR and resonance.
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I have had to adjust my calculations as well. As stated in a previous post, I had to enter the center frequency 1 mhz lower then the center freq I wanted ie channel 6: 27.025mhz Moxgen: 26.025mhz. I also used insulated wire and staying cheap I cut up an old plastic gallon milk jug as the insulating pieces in the center. I also never used fiberglass or anything for the spreaders. I had it all suspended using nylon rope at each corner. Also stated earlier was relationship of coax as its hung from the wire. You want that perpendicular and in the same horizontal plane as the wire. Try it out, with the coax and moving it while watching your MFJ. The antenna works and its very efficient, plus its super cheap.
You are right 711, your Moxon working is all that matters for you, but I was just curious as to why the calculator produced such bad results.
Thanks for letting me know you too used insulated wire. Bow tells us that he also used insulated wire. I'm still not sure what Homer used, but I would bet he used insulated wire also.
To Conor,
There you go M0GVZ, they all used insulated wire.
So, what is the conclusion?
If I wanted the antenna work on 27.555, what will I have to put the number to moxgen? Shud I put the higher, 28.555 or the lower, 26.555?
Thank you.
Oops. It's already answered...
:wub:
So, what is the conclusion?
If I wanted the antenna work on 27.555, what will I have to put the number to moxgen? Shud I put the higher, 28.555 or the lower, 26.555?
Thank you.
If you are using bare wire, use the frequency you want. If you are using insulated wire you need to decrease the dimensions by roughly 4% as due to the velocity factor of insulated wire, it appears 4% longer to RF. Moxgen DOES NOT allow for insulated wire. If you want 27.555 as the target and you're using insulated wire, putting in 29MHz should get you close to where you want to be. The resonant frequency will be a little lower but this will be ideal for being able to access the soon to be legalised SSB frequencies on 11m
I've done quite some investigation into this and been discussing via email with Marconi. My own Moxon when built to the Moxgen dimensions was about 1.5MHz too low because I was using insulated wire. If I look at the dimensions I finally ended up with, they're roughly 4% smaller than the Moxgen ones.
Modelling in EZNEC confirms this.
I'm interested to know about the installations when it appears to generate lengths that are too long resulting in a build with a resonant frequency too low. In particular I'm interested in the height above ground, how long the coax is between the antenna socket and the analyser and how long you take the coax away from the feedpoint at right angles to the driven element and in what direction as these all have an effect on the resonant frequency. And also by resonant frequency are we talking about where X=0 or where the lowest SWR is? The lowest SWR is not the resonant frequency, the point where there's no reactance, i.e X=0, is and the SWR dip should occur several kHz away from this. If its occurring at the same point there's an issue.
The downside as I discussed with Marconi was that you can take 100 different Moxon calculators and they'll spit out 100 different set of dimensions even though they're all apparently following Les Moxon's formula or L.E Cebiks' revised formula. Fortunately modelling each of these shows that the variations are in forward gain and front to back rejection and that the differences aren't that massive. Some favour a bit more gain over less front to back and others favour more front to back at the expense of a little forward gain.
Here's the models I did for my own investigations. All are done for a centre frequency of 28.5MHz.
This is the first one. It is the one generated by Moxgen. The resonant frequency is what we expect, the performance is what we expect.
http://1drv.ms/1lErzMc
Now if we change that to insulated wire which is what most people will be using as they'll use speaker cable or mains cable, look what happens when we still use the Moxgen dimensions. Resonant frequency drops to 26.9MHz!
http://1drv.ms/UTHhrT
This is the model of mine which was shortened from the Moxgen model to get it to tune where I wanted it. If you do a SWR sweep up to 31MHz you'll see the dip is around 30.2MHz. The SWR at 28.5MHz is quite high and the front to back ratio is only a few dB. However in real world use it was resonant at just below 28.5MHz and had F/B of around -20dB.
http://1drv.ms/1lErd8y
This is the model of mine taking into account the insulation of the actual cable used. If you do a SWR sweep on that you'll now find that its at roughly 28.5MHz, a drop of 1.7MHz over the previous model which used bare wire. F/B ratio at 28.5MHz is around -20dB which is what we expect.
http://1drv.ms/1lEroQZ
Now you'll find that if you increase the size of my wires by 5% you get pretty damned close to the Moxgen generated lengths.
Its a good practical lesson into the real world effects of using insulated wire and how it affects RF.
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well I use aluminum tubing from 17, 12 and 9mm. How about that?
Using Moxgen figures for 28.5MHz generated using 2mm wire, changing to 9mm throughout results in a drop to 27.9MHz and using 17mm throughout results in a resonant frequency of around 27.6MHz so you do need to put in something larger than thin wire into Moxgen.
So if you use Moxgen to calculate sizes for the frequency you want using the midpoint of the pipe sizes, 13mm for the pipe, then you'll be about right. I did a model for 28.5MHz generated using 13mm pipe. Without changing the dimensions I then redid it using 9mm for all the wires and then 17mm for them all and the difference between using 9mm throughout and using 17mm throughout was 200kHz certainly not enough to be worrying about. RF flows on the surface of a conductor, not through the middle, so the fact you're using pipe is irrelevant.
So if you use Moxgen to calculate sizes for the frequency you want using the midpoint of the pipe sizes, 13mm for the pipe, then you'll be about right. I did a model for 28.5MHz generated using 13mm pipe. Without changing the dimensions I then redid it using 9mm for all the wires and then 17mm for them all and the difference between using 9mm throughout and using 17mm throughout was 200kHz certainly not enough to be worrying about. RF flows on the surface of a conductor, not through the middle, so the fact you're using pipe is irrelevant.
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