Wednesday, August 30, 2017

20m Moxon Antenna Portable Build

I removed the last two sections of the crappie spreader poles.  I might put one back in, and use half of it, but it works now.


After having a great deal of success with a 20m moxon at my QTH, I decided to build a portable version, based on help from The Moxon Antenna Project by KD6WD.  My goal was to be able to work from a small mountain near my home, and do so at QRO power levels, with a beam antenna.  I also wanted to use up several scrap pieces of random PVC and pipe I had laying around.

I recognized the 100w QRO power part of the goal when I added a 10Ah Lifepo4 battery, with a 20Ah BMS draw.  With it, I can now operate for 3 to 4 hours at 100w typical hunt-and-peck SSB operation, along with some CQ'ing, before I have to drop wattage to 50w, or so.  With the Moxon beam, created by Les Moxon, I will realize my goal, and have a portable antenna capable of being constructed in 20 minutes, and fully guyed to as high as 8m on my Spiderbeam mast, in under 30 minutes.

I've been able to hit Europe with a 20m vertical dipole, and completed several 4.000-5,000 mile QSO's without much difficulty, from the mountain.  To run a pile-up into the EU, however, I know I have to increase my power.  There's no way for me to carry an amp (or to plug it in) on the mountain, so I was itching for a better antenna.  Adding a moxon beam gives me almost 5db of gain -- the same as running a 300w amplifier from the top of the mountain, with the added advantage of improved receive!  

Another useful feature -- 5db of antenna gain will make a 30w signal sound like 100w (or if QRP, 5w becomes as 15w).  That means I can run my portable rig on far less power, enjoy longer operation, and still complete the same QSO's as before... not to mention hearing (and contacting) others I was deaf to on a dipole.
Oops.. left some tape on the gap piece

The Moxon is a really great antenna design.  Its overall size is around 30% less than a full-sized beam.  Depending on your choice, you may prefer this reduction in size, when you're working with an antenna as large as 20m band antenna are.  A full-sized 20m 2-element beam is over 10m (32 feet) long, and  2.3m (7.54 feet) wide!  The sad reality is, once you get larger than 15m, building a portable beam becomes a bit of a challenge.  In any case -- please note that you may find constructing a basic 2-element 20m antenna even easier, depending on what's available to you.  Both ideas are great!

Another advantage of the Moxon is its wide bandwidth.  This means, if you build it right, you will be close enough to work both 15m and 17m bands with the same antenna, provided you have a tuner.  The SWR of those bands will vary on things such as wire size you use, but this antenna ends up 2.3 to 1 on 17m band, and about 3.4 to 1 on 15m.  Even though there are losses experienced (and especially on 15m) at these higher SWR numbers, K5LJ showed that there is gain to be experienced with the 20m Moxon, even when working off-frequency: See "Gain Antennas:  The 80% Solution" Part 1.  Part 2. I'm not using window line, so my gain will be down a bit, but I'll still enjoy a few db gain over a dipole, on both 15m and 17m.

The Moxon is recognized for about 10db higher front-to-back ratio, but is down about 1/2 to 1 db gain from a full-sized 20m beam (very small).  In truth, I find that the difference in front-to-back is maybe not as much as touted.  I have tried a few different 20m designs, and failed, because the parts I have available to me (or can find, I should say) are different than if I were at a Lowe's or Home Depot in the USA.  I have to hunt around the city to find what I need.  Also, I learned a lot about what wire size is going to work, as well as how much weight my cheap 5.2m (17 ft) crappie poles will take.  

I'm going to list the parts, and tell you a little about what I did.  Your part sizes may differ (metric vs. standard inches), but I have found that a little roll-around, or two, of electrical tape can fill in gaps between tubes.  Here's what I ended up using -- you may need to modify, based on parts available. My parts were in metric, so you'll have to check over your parts to see if they'll fit with the aluminum legs you slide in (you might need to adjust the design to do so):

1.  12m (21 foot) Spiderbeam HD mast.  One of the few semi-light masts stout enough.  A Sota pole or fiberglass flag mast will be too weak.
2.  One 3/4"  (20cm) PVC T
3.  One 3/4" (20cm) PVC Quad angle
4.  One 3/4" PVC pipe (cut into smaller 6cm or 2 1/2" long pieces, approximately)
5.  Two pieces of  36cm long, 20mm diameter aluminum element material.  Size must fit snugly, but freely inside of the 3/4" (20mm) PVC pipe.

6.  One longer piece of the same aluminum, but make it about 2 1/2 feet (75cm) long.
7.  A roll of black electrical tape
8.  PVC Glue
9.  Some screws/nuts to hold the pipes and the PVC parts.
10.  About 22m of 16awg or 18awg wire (extra added, for safety scrap).  I was able to find some 16awg with a thin plastic shield.  I recommend this.  If not, you can get 18awg, but the thinner wire adds another .2 to .5db signal loss, and will lower bandwidth a smidge.
11.  The free MOXGEN antenna program
12.  A small/light 100w 1:1 balun (I got a cheap one -- emphasis on cheap -- from Aliexpress for $12), or just a feedpoint connector and make your own ugly choke on the coax, near the antenna top.
13.  A light plastic scrap long enough to measure and drill holes for the critical "end gap" where the elements nearly touch.
14.  Four crappie poles of about 4.8 to 5m length NOT INCLUDING the thinnest throw-away segment.

You'll note that the 4-way PVC has a hole in the bottom, where the mast fits in.  You can also replace the 3-way PVC "T" with another 4-way, and cut the top aluminum piece into two 36cm pieces.  This will allow you to slide your mast all of the way through the top (I would have done this, but was limited to parts on-hand).  

The "X" you create must be at a 40 degree / 140 degree angle.  That will give you the proper spread for the poles.  I haven't done so yet, but I plan to drill holes and add long screws to lock the "X" together.  I will also be able to take this apart, to save storage space in transport.  I haven't completed that yet, so I just have it taped 40/140 degrees with electrical tape, to keep it from moving.

The aluminum pieces are the right size to slide the open handle ends of the crappie poles onto. Again, you might have to improvise a bit, based on the size of your poles, the size of the aluminum piece you have, and how it fits into the PVC pipe X section you create.  My crappie poles are 5.2m in length, but I throw out the last piece to be able to extend the moxon.  In the photos above, I didn't use the final two pieces, but I will be putting one back in and using about 1/3 of it to hopefully straighten the sag.  Given that my parts mostly are in metric, and found in Korea, you might have to experiment to find what works for you.


I just use another wrap or two of electrical tape on each attached crappie pole, at this joint, to hold them to their respective PVC joint, when erecting.  I remove this tape and throw it away, when taking the antenna down.  Electrical tape is plenty strong to hold the pole to the PVC piece and over the piece of aluminum.

The wire sizing was done using the Moxgen.exe program, available free online.  If you use 16 AWG wire, with a thin PVC shield, you'll find the frequency ends up a 250 to 750 Hz too low.  You can fold over the ends of elements about 5cm (for starters), and get closer to your target 20m SSB frequency.  If your wire has a thicker plastic plastic shield on the outside, you may find your antenna frequency is even lower.  This is all due to something called, "the velocity factor of the wire."  Wire with PVC plastic on it will seem electrically longer than an equal length wire without it.  The wire doesn't have to make a perfect rectangle.  Just do your best.  It can even sag here and there, such as with this antenna, so long as it's all within reason.
These gap spacings are quite critical.

After calculating the wire sizes, and drilling the EXACT gaps in the pieces of light scrap plastic, you can measure and mark where the attachment points of the antenna wire will be to the ends of the poles.  I measured and drilled holes at the exact spacing given in the Moxgen program, and left myself a bit of slack wire, for adjustment.  I bent the wires into the holes, and taped over the small excess with electrical tape, to hold the wires in.  If you have a few inches/several cm of extra wire, just wrap it back and around the element wire, then tape it tight so that it looks like one wire.  The wire will appear electrically the length of the fold point (approximately).  It's safer than cutting!

After completing the center "X" with the added crappie poles, I begin taping the wire to the far ends of the poles, paying careful attention to where I marked, based on the Moxgen dimensions.  You might find a more elegant solution than electrical-taping the wire to the poles, but hey -- I find it works, and just a few small turns of electrical tape holds it really well.  The center feedpoint is pretty simple, here.  Some build more permanent moxons with a PVC pipe attachment added to anchor the feedpoint.  I find simply taping the coax to the pole, and angling it down to the feed, works fine enough.  I used a cheap Aliexpress 100w balun here -- it's really a weak balun, but works as a feedpoint, if nothing else.  You can do better.

Next, mount the Spiderbeam 12mHD mast to tree trunk, or fence post.  Lift the completed antenna over it, and and raise the correct-sized center pole fit inside (or through, if you built it with two PVC quad joints) the center "X" of the antenna.  You might come up with a more elegant solution, but to keep the antenna from spinning around by itself, I use 5 to 10 turns of electrical tape to hold it to the pole.

Spiderbeam sells some locking clamps for each section, but I find a stout twist and a few turns of electrical tape hold it well enough, in the case of light antennas.  This is also faster.  If you want to raise the antenna more than 5 or 6 meters, or if you have any wind, you will want to guy it.  In fact, I would highly suggest doing so.


I hope this project gives you some ideas.  There are better designs, for sure, but I'm glad that it breaks down small enough to be carried in a backpack.  The "X" can be broken down even further, if I want. Happy antenna building!

2 comments:

  1. Very nice design. Looks completely doable for any Ham wanting to make a light, efficient antenna. Thanks for the article!

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  2. Thanks! I just followed ideas on "The Moxon Antenna Project" and modified, based on parts available. Not pretty, but it works!

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