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[Articles Home]  [Add Article]  

40 Meter 'Free Beam' Antenna

Jon Greenwood (N8MUS) on December 28, 2010
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If you have a self supporting tower about 60 foot or taller with at least a small beam on it you might want to give this a try.

My current antenna for 40/80 meters is a Radio Works Super Loop. It works OK and has been up here for about 5 years. What I can tell is that for DX on 40M my super loop is way down compared with the 4 squares and the beams that are out there. It is the antenna I will be using for reference.

The challenge; how to get beam or at least 4 square type performance without the associated cost for what is commercially available. After weeks (more like months) of playing in eznec here is what I came up with.

I have a KLM KT34a at 60 feet it has a 16 foot boom. Using eznec I started trying to hang various types of antennas below the tribander. What seemed to be the best for performance verses least complicated was a Moxon antenna hanging vertically.

Unknowns. Would it "play"? Would it interact with my KLM and ruin the Tribander's performance? How would the proximity of the tower effect the Moxon?

For the last question I added my tower to the eznec model and found that as long as the Moxon was hanging 3 feet or more from the tower I would be ok. For the other questions the only way to find out was to try it.

I used the Moxon Generator available here: (and special thanks to AC6LA for this great tool.) http://www.ac6la.com/moxgen.html

NOTE: You can then upload the files this builds right into eznec! Saves a lot of time. Select vertical polarization and you basically have my antenna. I ended up using only about 1 inch of spacing between the ends of the folded elements at the top as well as the bottom. The Moxon design is forgiving. So if you only have a beam with a 14 foot boom don't worry about it. Your vertical section will just be a little longer. One obvious requirement here is an antenna height of at least 60 feet. (50 might work if you had a way to fold more at the top so as to have less hanging.)

November 2010: Purchased #14 insulated wire at Home Depot. Two 100 foot spools. cost less than $20 bucks, also purchased 3 fiberglass "driveway markers" each 6' long. Simply these are fiberglass rods about 1/4 inch diameter 6 foot long overlap them a foot tape together and you have your 16' bottom spreader support. They flex a lot but they do the job and are really tuff. Cost $2.87 each. That's it for parts. I did end up using a hygain BN86 1:1 balun not so much because I felt it needed it but because it was just laying in my junk box.

At first I was going to hang The Moxon a foot or two below the beam but I figured why not just go right across the elements first. That would be the strongest mechanically, and again easier! I used insulated wire and wrapped each element with a thick layer of electrical tape. I ran this parallel to the boom of the beam and about 4 foot out from the boom.

I had a concern that I did not want to stress the beam elements any more than needed was why I spaced this so close to the tower. I am actually to close. I didn't allow for the fact my tower is tapered. 6 foot out would be better. Next time I have the tower down I will fix that but I am good for this winters activity.

Findings? How did this effect the tri-bander? My SWR is slightly higher on 20 meters. On 15 and 10 no change at all. I still need no tuner on 20, 15 and most of 10. Which is what I like about the KLM antenna.

How about the Moxon? SWR is Good nearly as predicted in eznec. My swr at the high end of the band hits 2:1 for 7 to 7.25 MHZ I am below 1.5:1 my tuner is still in bypass mode for the frequencies I use. My swr is a little higher than predicted and testing this I found out it is the tower causing this. I am less than the 3 feet spacing I wanted. If I move the antenna away from the tower to the 3 foot spacing, then the swr matches the model and is flat for most of the band. That will get fixed in spring.

Last question. How does it play? Very well. For reference my QTH is in Michigan. First real test was a net that I check into early each morning. Stations are mostly east coast (USA) because of the 7 am eastern start time. There are however some Texas and Louisiana stations some in Florida. Net control is usually from Maryland. Pointing the Moxon South I had everyone as good or better than the Super Loop and I was better to them on the Moxon than the loop. If I pointed the Moxon in the opposite direction signals fell and the Loop might be better but what I found is the Moxon does not have a lot of front to back and often beats the loop even off the back side.

There is one station that checks into the net regularly from Maine that I can never hear. This morning was no exception. I could not hear him on either antenna. This was with the Moxon pointed south. I then started to swing the beam east and almost immediately I could hear him come up above the noise, as the beam came around on him he went up to a 5x6 to 5x7 signal! I switched to the Super Loop and nothing. Back to the Moxon and he is still 5x6 5x7. Wow I am impressed but this was all state side how about DX?

Over the next few days I worked a bunch of Europeans and I compared the Moxon to the loop. It was always better but still most of the time not by huge amounts either on receive or transmit. Usually a solid S unit and sometimes 2, but for the most part I could have used either antenna and still had a contact.

HOWEVER... There are those times where it is like what I experienced with the station in Maine, where the Moxon is like 6 or 7 s units better! This is not often but when it happens it is hard to describe. How can I hear a station 5x7 to 5x9 that I can't hear on the loop or can just barely tell is in there.

On November 22nd the band opened up LONG. I heard an east coast station working a ZL/VK station. I could not hear the DX on either antenna. I had the Moxon pointed West from my Qth toward ZL/VK Nothing. But then I heard the state side station mention he was running a 4 square and that they were working long path!

I rotated the Moxon 180 degrees and still could not really copy the ZL/VK however I could tell he was in there! I then started to tune up the band and Instantly heard JH1AJT calling CQ! I gave a quick call and got him! He was 5x9 he gave me a 5x7. I switched to the loop and he dropped down to just above the noise level. I could still hear him, but just barely. Not good enough to work. Flip back to the Moxon and he pops back up to 5x9. We finish our contact and The east coast station I was listening work the ZL/VK calls JH1AJT. What report would he get? Also a 5x7 Same as me! Now that is not a huge indicator, his report verses mine, but I could not have worked the Japan station on my loop. I did with the Moxon easily. I also was working him long path. He was on the frequency for some time working the pile up that formed on him. I was able to swing the Moxon around to the Short Path and he disappeared.

In short I feel that I ended up with nearly 4 square performance as far as transmitted signal gain and also received signal gain. I don't have the signal rejection off the back that I would have with a 4 square. I also didn't have any radials to bury. And this was EASY to put together. Total time including measuring and cutting the wire to testing was about 2 1/2 to 3 hours and most of that was messing with the tower tilting down and back up. I was able to final tune it buy adjusting the tails that I can easily reach standing on the ground. Although I don't think it would have needed any tuning if I had the 3 foot spacing from the tower as I had modeled.

Is this a beam? Yes! Is it equal to a 3 element or even a 2 element Yagi at 70 plus feet. No. However for me it works better than what I had before at least a little bit and sometimes amazingly better.

Drawbacks: Guyed towers. I don't see how this would work if your tower has guys. Part of antenna close to the ground creating a Danger zone. Note I am in a rural area and my home sits out in the woods over 500 feet off the road. I am not worried about someone walking into my antenna. However your situation may dictate a fence or something to keep kids off your antenna. I purchased my BX 64 used and It had damage in one of the top sections so it only about 58 feet and I have the beam mounted on the mast touching the top thrust bearing so I am not even gaining any mast height currently. This puts me closer to the ground than I would like. Only about waist high. If it was a full 64 feet I would be well above head height at the bottom. So if you have a 64 to 70 foot tower this would be a non factor.

If your set up is like mine and you want a 40 meter beam try the simple Vertical Moxon.

73

Jon N8MUS

Member Comments:
This article has expired. No more comments may be added.
 
40 Meter 'Free Beam' Antenna  
by WD8OQX on December 28, 2010 Mail this to a friend!
The "FREE BEAM" part of the title sounds like it would be something the FREE BANDER'S would use :-)

But seriously, being able to use the properties of "interaction" like that is interesting! - Years ago, I wondered just how one may be able to utilize that to it's fullest, as I'd already used it to make verticals [somewhat] directional. But I haven't played with the EZNEC enough to figure it out to it's fullest. So, for me, it was more the educated guess, & trial & error methods.

Happy Hamming...
 
RE: 40 Meter 'Free Beam' Antenna  
by N4JTE on December 28, 2010 Mail this to a friend!
Pardon me if my questions are off the mark but without a picture or two I am a little confused.
A vertical moxon on 40 is new to me. Not sure why, if it worked at all, would I aim what gain it has, straight up? The moxon was designed to shorten the footprint of a two ele beam and also maximize the front to back and results in a couple of DB gain in the pointed direction. Do not confuse received directionality with gain when comparing to another antenna.
Perhaps I am misreading your endeavor but as I read it, you have constructed a rotating, drastically shortened, 1/2 square.
One thing I do know from my moxon experiences they are not that forgiving, the spaces on the folded in elements are critical to obtaining the intended performance, at least when constructed in the typical horizontal application.
Best Regards,
Bob
 
40 Meter 'Free Beam' Antenna; INGENIOUS!  
by KC8FRJ on December 29, 2010 Mail this to a friend!
Ingenious, congratulations on coming up with that idea. I am sure you are going to have more than a few people giving it a go.

There is something about the way the article is put together that took me a while to comprehend; really do not know why. I have a feeling you are going to get a few posts from people who do not quite get what you are doing.

Best Regards

Chris
 
RE: 40 Meter 'Free Beam' Antenna; INGENIOUS!  
by K3AN on December 29, 2010 Mail this to a friend!
I'm sure that by "Vertical Moxon" he means the two elements (radiator and reflector) are vertical, and separated horizontally by the proper distance. This article has given me inspiration to hang such an antenna between two of the towering pine trees in my back yard. Yes, I wouldn't be able to rotate it, but maybe I can aim one northeast and another toward the west.
 
RE: 40 Meter 'Free Beam' Antenna; INGENIOUS!  
by N4JTE on December 29, 2010 Mail this to a friend!
Thought I was missing something, now I am curious on how you maintained the seperation on the bottom element and supported the feed line connection. Appreciate the clarification but I still reaffirm that Moxon dimentions are more critical than other parasitic beams.
Good luck and tnx for an innovative idea.
Bob
 
40 Meter 'Free Beam' Antenna  
by N8MUS on December 29, 2010 Mail this to a friend!
Well to reply to a few of the questions.

Picture a moxon antenna basically a rectangle shape. For 40 meters the long side of the rectangle is about 49 feet and the short side about 18 feet. Now take the rectangle from horizontal to vertical. Now the dimension needed for "boom" length is still 18 feet but the long dimension is now vertical instead of horizontal. This horizontal length is just to much for my tower and even if I could put it up that is a lot of aluminum to buy and a lot of effort to build. I wanted simple and cheap.

I shortened the spacing on the short side "tails" to only about 2 inches and added the extra length to the long side of the rectangle to get only 16 feet on the short side of the rectangle and fit what I had available on my beam.

I played with the dimensions in eznec and yes this does have an effect on front to back and gain but according to eznec it is very little.

As to radiation pattern per eznec it is the opposite of what the thought was per prior post here. It has a very low angle of radiation. Peak is between 15 and 20 degrees. The link I provided for the moxon calculator works great and if you just down load the free demo of eznec you will get the basic idea. On the right side of the calculator just choose vertical and it builds the eznec file for what I did here. So obviously I was not the first person to think of the idea. In fact this was how I came up with this.

What I was wanting to share here was how easy this could be set up if you had a tower and tri bander similar to mine. yes this would also hang in a tree or between 2 trees 20 foot or so apart. The cool thing about hanging it from the beam is I can turn it.

I uploaded pictures of the set up but I don't see them posted here. I am a newby to this so if some one could advise how to add those I will.

As to the question about the bottom spreader. that is where I used the 1/4 inch fiberglass driveway markers. each is 6 foot long I overlapped one to the next by 1 foot and taped them together. In essence building a 16 foot fiberglass pole. It sags or has a belly in the middle but it keeps the rectangle "shape" basically there. I have no idea how I would model that curve in eznec. It did not seem to change the swr for what that is worth.

For the coax feed I ran coax about midway up the tower and then ran about 12 to 14 foot extra out from the tower to the BN86 so picture a piece of coax between the tower and the driven element with a "belly" in it. The coax wraps around the tower as the beam is turned so extra is needed to allow for that. I brought the coax out from the tower on the north side as my rotor has stops at the south. So the coax only has to wrap at the most half way around the tower if I am pointed South and of course less for any other direction.

73

Jon N8MUS
 
40 Meter 'Free Beam' Antenna  
by N8MUS on December 29, 2010 Mail this to a friend!
Note to: Bob, N4JTE

I just wanted to comment and note that your article on your 2 element raised vertical system was really what started my research and where I came up with this "Vertical Moxon" When I read your article I wondered with the raised radial system why couldn't I hang that down from my beam and have a 2 element rotatable array using the beam as the ground plane. Where I could have the performance that you achieved as well as be able to rotate it AKA 4 square performance. From all the modeling I think this comes close. I also of course have the eznec file of my final design which I can email to anyone that wants it. Thanks Bob for your article as it was the inspiration for this crazy idea!

Jon
 
RE: 40 Meter 'Free Beam' Antenna  
by NR9R on December 30, 2010 Mail this to a friend!
Thanks for sharing your experience. This statement really concerns me though:

"I purchased my BX 64 used and It had damage in one of the top sections so it only about 58 feet"

1) The BX64 is designed for TV antennas and has a wind load capacity of 6 sq ft (the tallest medium duty HBX tower is 56' and the tallest heavy duty HDBX tower is 48 ft so I assume this is the light duty BX type)

2) No BX tower, even "the heavy-duty" HDBX series has the torsional resistance to support an antenna with a boom length longer than 10 ft.

Take a look at this link:
http://www.dxham.net/ham.html

The KT34XA at 68 lbs, with a boom length of 32 ft and a wind area of 9 sq ft is grossly overloading that tower.

-Anthony
 
40 Meter 'Free Beam' Antenna  
by N8MUS on December 30, 2010 Mail this to a friend!
Anthony,

You are correct on the torsion rating for BX 64 If you check out my QRZ.com page I explain how I overcame that problem. Short answer is don't apply any torsion in the the top section that can't take it. I have my rotor mounted only 24 foot off the ground, so all the torque is handled by a section of tower that can more than handle it. Its been up here for a couple of years and survived some major winds as well as Ice loads.

Jon
 
40 Meter 'Free Beam' Antenna  
by N8MUS on January 2, 2011 Mail this to a friend!
Guys, still have not figured out how to add any pics here. I did add some pics to my QRZ.com page just scroll to the bottom. Because this is a wire antenna it is tuff to see in the photos, but if you look close I think you will pick it out. My email address is n8mus at yahoo if you would like a copy of the eznec file I would be happy to forward that to you for your review.
73
Jon
 
RE: 40 Meter 'Free Beam' Antenna  
by KE7VUX on January 3, 2011 Mail this to a friend!
Nice idea.

Build a vertical moxon, hang it off the side of the existing beam and let it float in the breeze off the side of the tower and rotate it around.

Any issues with heavy wind smacking the antenna around? I would expect some variation in the interaction with the tower as the wind blows - especially if it hits the tower. :)

Hang the moxon out far enough and you can still shunt feed the tower, too.

You are adding some weight loading to the elements of the regular beam that they were not designed for - maybe an issue, maybe not - depends on how far out you go.

(A shame you can't hang two moxons.. one on either side and phase them.. I think you'd exceed the available real estate)

I have a BX series tower as well, but only a 40' HDBX. I looked around for another bottom section to make it 48' but got a lot of "8' won't matter" responses and no sections came up.

I bought the tower and beam (Sommer XP708) as a package, and my first thought was "too much antenna for the tower" but later realized that the SK that had set this all up had a pretty fair idea of how to deal with the BX tower's shortcomings.

It was up for 10 years before I took it down and moved it, and as you've already struck on, one of the keys to living that long in the wind and ice of Montana was to mount the rotator at the ground, with 48' or so of mast running from the ground up the center of the tower and out the top.

All of the torsional load of the big Sommer beam (20' boom) is taken up by the slab where the rotator is mounted.

Too bad, at only 40', there isn't enough extra altitude to hang a 40 meter Moxon.
 
40 Meter 'Free Beam' Antenna  
by N5RMS on January 4, 2011 Mail this to a friend!
How do you splice two 6 ft. rods together and get one 16 ft rod?? I went to public school and I was taught 6 plus 6 is 12. :)
 
RE: 40 Meter 'Free Beam' Antenna  
by KH7DX on January 4, 2011 Mail this to a friend!
Aloha Jon
I get it ! this would play just fine with my three element steppIR at 65ft. on my Sky needle tower.
If you can send me the eznec file to kh7dx@arrl.net
That would be great.
I have virtually no wind here so I have been using fishing weights at the bottom of some of my vertical dipoles with good success.
Aloha, 73, Stuart KH7Dx
 
40 Meter 'Free Beam' Antenna  
by N8MUS on January 4, 2011 Mail this to a friend!
re: bottom spreader. I must have missed typed it is three 6 foot rods total 18 foot but I overlapped one to the next a foot so net was 16 foot. They sag too so not a full 16 foot is obtained with these. I just used them because they were cheap, easy to get, and strong.

RE: does swr change in strong wind. Yes. If the bottom swings in close to the tower it does change swr. I have not had trouble with that but I am sure some locations could.

Sending eznec file to email request.

73

Jon
 
RE: 40 Meter 'Free Beam' Antenna  
by K6SGH on January 8, 2011 Mail this to a friend!
Vertical moxons work great, and hanging them under existing antennas have been explored by several moxon makers over the years. There's additional info at the Moxon Antenna Project...dozens of different designs.

www.moxonantennaproject.com

73, k6sgh
 
RE: 40 Meter 'Free Beam' Antenna  
by W4JKL on January 25, 2011 Mail this to a friend!
Hi Jon,
Please send the eznec file to me at w4jkl@arrl.net
Thanks.
 
RE: 40 Meter 'Free Beam' Antenna  
by K6RF on January 28, 2011 Mail this to a friend!
I am not surprised Bob, N4JTE inspired you in this project. When I saw it listed on the home page of eHam I was sure it was another of his articles !
 
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