MFJ Moxon

[HomerBB]

Let me straighten this out.

1. when I built Moxon antennas I used the MoxGen application, a center frequency of 27.385, and I had to change the center freq from 27.385 to something higher to get an antenna that operated on center where i wanted it. MoxGen made both wire and tubing antennas lower down on the band than I wished.

2. when I posted the above two diagrams it was from an online calculator. I used a center frequency of 27.2 for the 11 meters dimensions and 28.4 for the 10 meters antenna. I know nothing of its accuracy.

3. I used 1/2" tubing for the metal antennas, and #14 wire for the wire antennas. I left the insulation on the wire antennas.

I believe the tubing antenna is more durable.

Here is the exact measurements of my Moxon I have up now. according to MFJ-259 it has a center frequency of 27.3.

 

[222DBFL]

Just out of curiousitiy, what length coax jumper did you use to tune the antenna in the pics above?? How long exactly was it?? Just curious? Thanks for any replies.

 

[222DBFL]

Or was a 50ohm matching system used??

 

[Marconi]

Let me straighten this out.

1. when I built Moxon antennas I used the MoxGen application, a center frequency of 27.385, and I had to change the center freq from 27.385 to something higher to get an antenna that operated on center where i wanted it. MoxGen made both wire and tubing antennas lower down on the band than I wished.

You can't beat that for good information, and I will fix your model accordingly and let you know. Hopefully you will work with me a little if I have questions in the process.

With this design I'm lucky, because the Moxon can produce an almost perfect direct fed match, no matching required, so I don't have a mismatch to try and explain which can be confusing with the results.

2. when I posted the above two diagrams it was from an online calculator. I used a center frequency of 27.2 for the 11 meters dimensions and 28.4 for the 10 meters antenna. I know nothing of its accuracy.

I get it Homer, nobody would likely know at the start of our using such a calculator if it made errors for us to even consider...that is normal. We generally only know about such things if we are unsuccessful in our build and construction, and/or unless somebody advises us from their experience.

3. I used 1/2" tubing for the metal antennas, and #14 wire for the wire antennas. I left the insulation on the wire antennas.

I believe the tubing antenna is more durable.

Homer depending on the thickness and the dielectric constant of the insulation, I find there can be as much as a 1/2 to 1 mhz difference in the frequency results with insulation vs. bare wire sometimes used on a Moxon, and that is a big difference.

If I can remember, I will try and address such results when I get back to doing the Moxon.

Thanks for your opinion and the heads up.

 

[tba02]

Attached are the figures and calculations I used to set my measurements for 27.385.
I am using an MFJ-1890 - the actual tubing dimensions are here, but I used .625 inches - http://www.mfjenterprises.com/Downloads/index.php?productid=MFJ-1890&filename=MFJ-1890 manual REV 1A.pdf&company=mfj
I fudged/nudged the numbers slightly in a couple cases to better meet my tape measure increments.

I originally used 27.385 for my calculations but they did not meet my needs so I change to 27.245. I didn't own an analyzer at the time so my numbers were all based on SWR measurements at 20 feet. I'll try and connect the analyzer here soon and show those results as time allows.

 

[Bearcat]

I didn't know you needed a matching system on these antennas?? I thought center wire/shield direct fed? Anyone?

 

[Marconi]

Attached are the figures and calculations I used to set my measurements for 27.385.

tba02, thanks for the response.

It is OK that you used 27.385 if that is where you wanted to work your radio. It's what one should expect early on in the use of these calculators. Plus it is normal to expect every location is different...so location might also have an effect. I commend you for realizing you didn't hit the frequency you wanted, and for sure since you were so close.

I am using an MFJ-1890 - the actual tubing dimensions are here, but I used .625 inches

I took a chance you used tubing and set your wire diameter at .50" inches. It showed very good gain, match, and rejection, and I did not consider tinkering with your model. Good work and without an analyzer no less.

http://www.mfjenterprises.com/Downloads/index.php?productid=MFJ-1890&filename=MFJ-1890 manual REV 1A.pdf&company=mfj

This link does not open for me. Is it an Eznec wire definitions out-put generated from you calculator?

I fudged/nudged the numbers slightly in a couple cases to better meet my tape measure increments.

Rounding and re-tuning is fine.

I originally used 27.385 for my calculations but they did not meet my needs so I change to 27.245.

Homer talks about doing something similar on his Moxon antenna I think.

I didn't own an analyzer at the time so my numbers were all based on SWR measurements at 20 feet. I'll try and connect the analyzer here soon and show those results as time allows.

Please follow up with your analyzer info and I will send you the model I made based on your earlier dimensions. If I need to change any dimensions let me know.

 

[Marconi]

I didn't know you needed a matching system on these antennas?? I thought center wire/shield direct fed? Anyone?

The Moxon does not need a matching device...it is direct fed and is capable of showing a great match...if you get it setup correctly and in the clear.

 

[222DBFL]

I didn't realize this. Thus my reason for asking. Thanks for all the info gents!!

 

[Marconi]

tba02, here is the model you posted dimensions for. Now that Homer set me straight the models below for you both are very similar at 27.385 mhz.

 

[HomerBB]

I used an electrical 1/2 wave RG58 jumper of 11' 10" length as my memory serves me. I would have to go measure it to be absolutely sure.
CG net's online wavelength calculator puts a 1/2 wavelength at 27,385 MHz at 17.966'
multiplied by the Velocity Factor of my RG58 coax cable I get a length of 11.85756 or about 11'10-1/4"
Wavelength x VL = e1/2ƛ
17.966 x .66 = 11.85756

12" x .85756 = 10.2929072"

Bearcat is correct. This antenna is a direct fed 50 Ohm antenna at center frequency and close to it well beyond center. No matching network is needed.

 

[222DBFL]

Thanks Homer. Makes sense.

 

[vkrules]

The calculator will only get you so far.
The moxon's a good two el beam .When set up correctly it has a good match to 50 ohm, wide bandwidth and a high F to b ratio.Even when set up less than perfect you can get good performance.
But like they say the devils in the details.
Its easy enough to get a match but can take some tuning to get the high F to b ratio and bandwidth. In fact most dont even bother .
What I like to do is tune the reflector first for F to B . Then tune the driven for resonance.( hence the idea of the tails).
Couple of observations .
If you see the bandwidth go up while tuning ,it corresponds to the high F to B point
Very little change to Forward gain no matter how you tune it( unless of course it is well out of band i.e 8 or 10%)
Definitely needs a choke, you can get some strange results if you have a CMC problem
Guess that is why some report that the position of the coax is so important.
Finally it's a 2 el beam forward gain is about the same as any other 2 el. what makes it special is the coupling of the bent elements providing that bandwidth , match and F to B. I think the future of yagi antennas will be with critical coupled (bent) elements.
This is some of my observations based on making dozens of Yagi, quads, moxons, skyppers and quagis .
Would love to hear your real world experiences and results.

 

[HomerBB]

What I like to do is tune the reflector first for F to B . Then tune the driven for resonance.( hence the idea of the tails).

Can you share the details of your process for tuning F/B for everyone?
Do you use the corner tails, or change lengths or spacing?
Thanks.

 

[HomerBB]

@tba02 the calculator you used is the MoxGen app I used to build my antennas originally. I made 6 Moxons with it, and all of them were lower on the band than I shot for unless I used a higher frequency to land more or less where I wanted to operate.

@Marconi When you get the time, and are feeling well enough, if you could take the posted dimensions in my graphic I provided and see what a model reveals I would really appreciate it. As you can see in the photo, I use a coaxial choke about 24" back along the coax feedline from the feed point.