Wednesday, August 2, 2017

Why a 2-Element Yagi?

2-element 20m Moxon homebrew costing $70 to make, including mast.

Like many new hams, I spent a lot of time reading about antennas, trying to decide what would work best for me.  I came across designs such as monoband dipoles, 1/4 wave verticals, fan dipoles, off-center fed dipoles, end-feds and of course, the G5RV.  I probably spent hours reading about antenna makers extolling the virtues of their product, and studied Youtube videos intently as they operated from back yards and beaches. I asked for opinions in ham radio forums, and got a variety of mostly good answers. I loved it all.

Eventually, I settled on an MFJ G5RV with ladder line.  I chopped-off the coax connector, trimmed the legs to 33' each (a better fit for my roof) and extended my own ladder line all of the way to the tuner.  I had read about how ladder line is extremely low loss, and that I could tune most bands with a simple tuner with a balanced input. It proved to be an excellent choice, and I barely lose 1 watt, while even expensive LMR 400 coax can lose far more.  If you don't mind working with ladder line, and can keep it at least 12 to 16 inches away from metal, you're good.  You can cross metal (such as between metal bars) so long as you are about 3 to 4 inches away... just don't run along metal and be closer than 12 to 16 inches, or your SWR will go wacky due to decoupling.  You also need to be careful you aren't at a resonant length.  Basically, I don't worry about it -- I found myself at a resonant length, once (SWR went bad on a band) so I just lengthened the ladder line by soldering a 4 foot piece on, to extend the length, and I was good.  

Later, I experimented with vertical dipoles, and became so impressed that I wanted more.  The vertical dipole taught me about the importance of pattern and take-off angle.  I was impressed by how much better the 20m vertical dipole was at transmitting, compared to my cut-down G5RV  The vertical dipole was great, although the receive was a lot noisier in my city locale, due to its vertical polarization.  I found myself transmitting on the vertical dipole, then switching to the doublet for receive.

But I wanted more.  I wanted a yagi.

The idea of owning a yagi was a distant dream.  I soon learned that the prices of towers, rotators, not to mention the antenna, could be quite high.  No problem, I thought.  I would just build my own.  Immediately, I made a mistake that I think many hams make -- I went straight to digging-up 3-Element yagi designs, and planning my own build.  It was too long before I became overwhelmed by concepts such as gamma matching, what diameters of tubing I would need, and how I would be attaching these things to the boom.  The more I looked, the more complicated it became, and there seemed to be a precious few finished designs available online, which were not pretty massive.  Pricing parts soon taught me that it would be cheaper to just buy a proven design, than build one which may not work.

More frustration.

Now, let's back up a bit. I mentioned that I made a mistake in going right for a 3-element yagi design.  How could that be a mistake?  In truth, it's not so much of a mistake.  It's more like trying to jog before I learned how to walk. Why?  Because in going for the 3-element yagi, you skip right over the design the ARRL calls, "the best bang-for-the-buck antenna" around:  The 2-element yagi.
The 2-element yagi is, in the eyes of many hams, completely underrated.  Like Rodney Dangerfield, it gets no respect.  Many amateurs have no idea just how good a 2-element yagi is.  In fact, it's a lot like a college sports team in a really difficult conference, where they come close to winning most of their games, but lose nearly all of them by just a few baskets, in the final seconds.  Their losing record is nowhere near an indication of how good they are.  That's the 2-element yagi.  It's actually close -- very close -- in performance to a 3-element trapped yagi, and only about 25% less powerful than a full-sized 3-element.  And here's the kicker:  even the novice antenna builder can make one.  If you can build a dipole, and know how to cut and glue some PVC, you can build your own 2-Element yagi. And provided you don't have picky neighbors to worry about, you can put one up on a mast for less than $200, including the mast!

My 2-element yagis have been cheaper.  They are made of wire, and fiberglass crappie poles. They cost less to make than the cost of a quality balun. They aren't built to last, but I haven't cared, because I keep upgrading every year.  Even still, I've had loads of fun with them.  I've run pileups from Korea, to Europe, when I couldn't even receive the signals on my doublet or dipoles.  I've picked-out difficult remote DX when friends with dipoles and noisy verticals couldn't find it.  I've had DX ask me, out of much curiosity, "Hey, what are you using for an antenna?"  People have assumed it's a 3-element yagi, sometimes, but it's not.

I do have a point of reference for how much of an improvement a 2-element yagi is.  Watch the above video to see one example.  Of course, tests like this are very subjective, depending on signal, the band, and other factors.  It give you a basic idea of how much better a 2-element yagi can be, compared to a dipole.

One of the first things you learn, when building yagis, is that boom length is what determines gain -- it's not the number of elements.  I highly encourage you to download the free (and wonderful) program called MMANA-GAL Basic.  It's a Yagi design program that has some basic templates already installed.  I have had oodles of fun with their 2-element 20m yagi template.  You can change the frequency parameters for other bands, and have it optimize and recalculate lengths to match.  You can tell the program to figure for different criteria, such as better SWR, or more gain, or better front-to-back.  It's a lot of fun.  If you get confused, I recommend asking about your problem in the QRZ "Antennas, Feedlines, Towers & Rotors" forum.  

In a future posting, I'll add a simple 2-element wire yagi design, with approximate dimensions.  I'll be sure to add a link to that post, here, once it's finished.  In the meantime, never discount building your own 2-element yagi (or buy a 2-element monobander, such as a moxon -- they're not so expensive).  You'll have a blast.

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