This is a post about my experiences with the DL1ELU Folding Antenna.  This is not a thorough, scientific analysis of the antenna.  I’ll leave it to others to get into the precise details.  I’m posting this for other hams who may be interested in the planar hexagonal beam design or who may be thinking of purchasing the Folding Antenna kit and for whom my experiences may be of interest.

Introduction

I love contesting, but my property affords very little room for an antenna farm.  I’m limited in terms of both horizontal and vertical space.  I can’t put up real towers, and my antennas need to have a very low visual profile.  As such, I’m pretty much limited to relatively small and low wire antennas.

The venerable K4KIO hex beam on my roof (at about 35 ft) has been the mainstay of my contesting arsenal for the last couple of years.  But I operate SO2R, and I wanted another good gain antenna so I could work, say, 15m and 20m at the same time.  I home-brewed 15m and 10m moxons and found that I could just fit them in our tiny back yard mounted on a military surplus aluminum mast strapped to our kids’ wooden play set.  I was using this setup when I won the 2011 ARRL CW Sweepstakes in the single operator, low power, assisted class, so it worked pretty well.

Despite its effectiveness, this was a fairly inconvenient arrangement.  The agreement with my wife is that I’ll only raise additional antennas for the major contests, and assembling, raising, lowering and disassembling these two antennas was a bit of a pain.  On top of that, I wanted another 20m antenna that I could mount in the clear — further from the noise and degenerative effects inflicted by our house on the roof-mounted hex beam.  I really didn’t want to add another moxon — a big 20m one — on top of the others, so I concluded that another hex beam was called for.

I ordered another hex beam from Leo Shoemaker (K4KIO) with the intention of modifying it to make it lighter and easier to quickly assemble and disassemble.  After ordering, I asked Leo a couple of questions about my plan and he pointed me to DL1ELU’s Folding Antenna suggesting that it might be a better solution for me.  Well, I really liked what I saw on on the website so I contacted Christian (DL1ELU) and ended up purchasing one of his antennas in short order.

The Folding Antenna arrives

The FA (Folding Antenna) arrived in under two weeks which is pretty darned fast given it was shipped from Germany to Seattle.  The box was a bit battered, having been jostled around the globe, but Christian had liberally packed the contents with crumpled newspaper, and everything survived the journey intact.

The first thing I noticed about the FA kit was that the parts were of very high quality and ingenious design.  It turns out that Christian is an expert in plastics and molding, and fabricated the specialized parts himself.  Some of these parts — like the detachable wire clamps — are super useful for general antenna building and can be purchased separately.

Building

I’ve built a lot of homebrew antennas, so I figured I could whip the FA together in a couple of hours.  I was wrong.  It’s a substantial project, and though I didn’t measure the time it took to build it, I think one should expect to spend at least 8 hours from the time you take it out of the box to the time you can start using it for QSOs.  The instructions are very good and include a clear, step-by-step guide covering every detail of construction.  But there’s a lot to do, and you really can’t rush through it.

If the only antenna kit you’ve ever built is Leo Shoemaker’s K4KIO hex beam, then you’ve been spoiled.  In Leo’s kit, all the hard work is done for you.  All you need to do is connect a few things together and you’re on the air.  The FA kit is more like the spiderbeam kit in that you’re really building just about everything yourself.  Even though it’s a lot of meticulous work, I found it to be a generally positive experience because the directions are clear and accurate, the design is cool, and the parts fit together very well.

I spent a lot of time measuring, cutting and assembling the elements.  Having had a lot of experience building moxons, I knew how crucial it was to get the elements right — including the spacers between reflectors and driven elements — so I took great care in this process.

With the elements assembled, the center post constructed, the support strings cut, and all the spreaders put together, I was ready for the final assembly phase.  We have a heavy wooden umbrella table on our patio that is perfect for this kind of work.  Just insert the center post in the hole in the middle of the table and string it all together.  My 6 year old son gave me a hand, and we had the whole thing wired together in about two hours.  It took longer than expected because I spent extra time trying to get the (very) flexible spreaders to lie straight.  They naturally tend to warp one way or another and straightening them requires many adjustments to the 5 elements and 20 strings.

Raising and adjusting

After getting it all put together and getting the spreaders as straight as I could, I raised the FA up (push-up style) using my trusty aluminum mast sections.  I raised it to a height of about 18 ft to conduct my testing.  The moment of truth had arrived.

I connected my RigExpert AA-230 to a 25 ft feedline attached to the FA and…    The 20m resonant frequency was up around 14.4 MHz.  Given that I’m a CW guy and a contester, I want the resonant frequency down closer to 14.03 MHz, and I want the lowest SWR between 14.00 and 14.05 MHz.  The numbers I was seeing on the analyzer were WAYYYY too high.  Grasping at straws, I raised the antenna up to about 25 ft and tried again.  Same result.

“Okay,” I thought, trying to stay calm, “The instructions said to leave a 2 inches of extra wire on the ends of all elements in case adjustment is required…”  I checked 17m and found the same result: the resonant frequency was high by ~350 MHz.  In fact, I found the same thing on all the bands. I pulled the antenna down, lengthened all the wires by 2 inches, and raised it back up.  The resonant frequency was still way up above the CW segment and the SWR in the CW segment was unusable.  Ouch.

I quickly sent an email to Christian letting him know about my problem.  I wondered if he had heard of any similar problems from other builders.  He responded very quickly apologizing for the trouble.  He hadn’t heard of similar trouble from others but he said that he had recently changed wire suppliers.  I’d like to think that the wire he got from his new supplier was the problem — that the dielectric was different enough — but I admit there’s a good chance that I somehow messed up when cutting the elements.

Christian also immediately changed the instruction manual once I reported the problem, increasing the recommended extra element length from 2 inches to 4 inches just in case more length was required. I should also note that he has been very responsive and helpful all the way through this process.

Modeling and building (again)

What to do next?  I had invested a lot of time in building the FA, so it was a bummer to realize I had to partially disassemble it and build new elements.  And of course I wasn’t about to just start randomly cutting new wires in the hopes that I’d end up with good performance, so I knew I had to model the antenna before proceding.  Luckily, I have a lot of experience with 4nec2 so I figured the modeling wouldn’t be too tough.

I developed an initial model of the FA in which I could vary the vertices, DE length and reflector length of each element for each band.  It worked okay, but it turned out to be too awkward to use for optimization.  What I really wanted to do was vary DE length, reflector length and length of the gap between them for each band.  This turned out to be a bit trickier, and I even had to call on my math skills in order to brew up the right formulas.  Once I figured the math out, it was easy to build the model in 4nec2.

Since I only plan to use the FA for contesting in the near term, I decided to model and build a tri-bander (20-15-10) version.  I call this the FA3.  It’s lighter than the FA5 (the original 5 band version) and, in theory, has less inter-band interference.  I used TheWireman 532 insulated, stranded copper wire in the model since I happened to have it on hand.  You can download the FA3 and FA5 nec files here.  [NOTE: The FA5 model uses the original dimensions from the kit and, just as I witnessed when I built it, it doesn’t tune up well on the bands.  You will have to tune the model in order to get dimensions for the lower segments of the bands.]  I recommend using 4nec2 to play with these models, and you’ll have to increase the default “Max-nr of SYmbols” (under Settings–>Memory usage) in order to load it.  I set mine at 1024 with no degradation in performance.

It took me a few days to get the model built and tweaked enough that I felt good about it.  I’m really not an expert at antenna optimization and I have not had the time or patience to sit down and really work on optimizing the FA3 as much as it deserves.  The FA3 as modeled is almost 1.0 dB down in gain relative to the K4KIO.  I’ll bet someone could get it up another 0.5 dB purely by optimizing the model.  I would love it if someone would do that. If you do, please let me know!  Also, let me know if you find any errors in the model so I can correct them!

For my purposes, gain is the priority.  I’m not concerned with the front-to-back ratio, and I generally ignore it.  [The reason for this is simple.  I focus on domestic contests which means my antennas point ESE from Seattle and there really aren’t many strong signals coming from the WNW that I need to worry about.]  Once I got decent looking gain figures with resonance and low SWR in the CW segments of the bands, I declared the modeling process complete.

The task of removing the 5 old elements, building my 3 new elements, and attaching them to the antenna took a couple of hours and then I was ready for moment of truth #2.  I raised the new FA3 up to 18 ft, connected my analyzer and…  The results were very encouraging.  SWR below 1.5:1 across all three CW segments with resonances pretty close to those shown in the model.  I decided to do a bit of tuning to get the SWR even lower.  This consisted mostly of lengthening the gap between DEs and reflectors.  I was still using Christian’s wire clamps so making these adjustments was easy and quick.  The FA3 was finally ready for action!

Performance

I conducted an initial set of tests of the FA3 versus my K4KIO hex with both antennas at about 35 feet.  It’s not really apples-to-apples, though, because the K4KIO is about 10 feet above our roof whereas the FA3 is in the clear.  In addition, there are 3 antennas nearby  — a 40m wire moxon, a 40m doublet, and a 160m inv-L — and I’m sure that coupling and interference from those is felt more by the K4KIO given it is basically surrounded by them and each of them is within 40 feet.  At any rate, my testing “protocol” goes something like this:

1. point both antennas the same direction (usually straight east for this test)
2. send a test message (e.g. “vvv test kk7s”) about 5 or 6 times, with a few seconds pause between them
3. note the snr numbers on the Reverse Beacon Network
4. move up a few KHz and switch over to the other antenna
5. repeat steps 2-4 a few more times and then compare the results

Okay.  It’s not the most scientific approach, but it works well enough for me.  The results I got for the FA3 were quite good.  The average results for the FA3 were on par or better than the K4KIO on all bands.  Keep in mind that in my setup the K4KIO has all the handicaps I’ve already mentioned, so I’m not saying the the FA3 outperforms the K4KIO.  However, the FA3 works very well, and I’m very encouraged by this, not only because it solves the problem I set out to solve, but also because it opens up another avenue of exploration for us hex beam fans.

I knew that the real test of the FA3 would come in the August 2012 NAQP CW contest.  Maybe band conditions were better this year (though many folks reported otherwise), so take this for what it is worth.  My score this August was 50K higher (200+ more QSOs) than last August and using the FA3 on my run radio garnered sustained run rates that were above normal for my station.

Folding and unfolding

Once I got my FA3 up and running, I still had a couple of weeks before needing it for the NAQP, so I decided to take it down and fold it up until then — partly to please my wife and partly to see how well the process would work.  The folding process seems pretty simple, but it requires more forethought than you might expect in order to ensure you will later be able to unfold the antenna easily.   Christian made a valiant effort to describe how to do this in the instructions, but I honestly don’t think it can be described with words, so I won’t even try.  You just have to figure it out on your own.  With a little thought and after a few times through the folding/unfolding process, it’ll be no problem.

Mechanically, folding is as simple as loosening a bunch of wing nuts, pivoting the fiberglass spreaders together, and wrapping the whole thing in big velcro straps.  This shouldn’t take any longer than about 10 minutes, not counting the time it might take you to figure out what to do with all the wires.

Unfolding is similarly simple from a mechanical point of view.  After all, you’re just reversing the process — removing the velcro straps, pivoting the spreaders down, and then tightening the wing nuts.  But the devil is in the wires…

Keep in mind that with the FA, you’re dealing with several long wire elements and 14 support lines.  These are all pretty long and they all get smooshed together as part of the folding and strapping process.  When you first start to unfold your FA, it might feel like you’re about to sort through one of your old junk wire boxes — like there’s no way you’re going to untangle this big mess of wire and strings.  Well, that’s certainly what it felt like to me.  But it really wasn’t that bad.  I just set the thing down on my umbrella table, unfolded the spreaders, and then worked my way around the antenna ensuring that the wires weren’t tangled and that wires and lines were on the proper side of the spreader.  (Hint: none of the wires or lines should be passing under a spreader.)  Frankly, I thought it was going to take me hours to unfold the FA3 the first time, but it ended up taking about 20 minutes to get it unfolded and another 10 minutes to get it back up in the air.  And it didn’t require any tools.  Not bad at all!

I should also mention that I bought the FA bag.  It’s made of durable synthetic material and has useful carrying straps.  When folded, the FA fits neatly into the bag and there’s room left over for extra feedline and other accessories.  It’s a really nice solution and if you’re going to buy an FA for portable use then I highly recommend the bag as well.

Conclusion

I believe the FA is a noteworthy development for the hexagonal beam and for hams that want a lightweight, low profile directional antenna with decent gain.  It’s an excellent design for portable use.  In fact, I have seen nothing comparable.  Christian is to be commended for the design, fabrication and promotion of it.  If you pair an FA up with, say, a KX3, and hike up a summit somewhere (or fly to a remote location), you’re guaranteed to have a heap of fun.

I am personally left believing that there is something amiss with the element dimensions in the kit (the current version is 1.04) as borne out both in building and modeling the antenna myself.  But if that’s a general problem, rather than just an anomaly experienced by me, then I’m sure it will be fixed soon enough.  Keep in mind that a lot of other hams have built this kit and as far as I know they haven’t had the problem that I had.  One thing is certain: any determined ham can get this antenna up and running and will be happy once they have.

I’m sure many hams will want to build the FA or FA-inspired antennas.  If the FA looks like it is just the antenna you’ve been looking for then I recommend that you get Christian’s kit.  The components in this kit are of the highest quality, and even if you have the same experience I did and you have to determine the element dimensions yourself, you’ll find every other aspect of this kit is simply great.  I’m confident that most of us really couldn’t homebrew an FA and end up with something as light, strong, durable, and portable as the original DL1ELU Folding Antenna.