Thoughts on the SteppIR 4 Element Yagi

A Contester’s View

By Dan Levin, K6IF and Marty Levin, W6BDN – April 12, 2004

 

 

The Antenna

 

I have known for a while that a small company in the Pacific North West, Fluid Motion, was selling an antenna with adjustable length elements.  The elements are fixed in position on the boom of the antenna, but use stepper motors driving copper alloy tapes as elements.  A remote computer based control box can command the antenna to change the lengths of its elements, allowing the antenna to be tuned from the convenience of the shack.  The antenna includes, integral to its design, a high performance 2:1 balun – the designed feed point impedance is twenty two ohms.

 

When these antennas first came out, I was worried about their reliability.  In addition, Fluid Motion only sold small yagi’s (two element only at first, I think) – not big enough to interest me.  But this year marks their third in business, and several of their antennas have survived very difficult weather conditions (according to their owners – see the eham.net product reviews, for a start).  In addition, Fluid Motion recently released a four element yagi on a thirty two foot boom – big enough to be of interest as a main antenna at a serious contest station.  I discussed the matter with my father (W6BDN), and we decided to investigate in more detail.

 

To my dad’s amazement, when I asked Jim at Fluid Motion for a computer model of their new four element yagi, he said “sure, why not.”  Hours later, a zip file containing six YO files (one each for 20, 17, 15, 12, 10 and 6 meters) arrived in my email inbox.  We proceeded to model the antenna using NEC4WIN95 (a MININEC based program) and MultiNEC (using the public domain NEC2 engine).  We studied the antenna in detail, looking at gain, front to back and SWR for 10, 15 and 20 meters, as well as patterns in both free space and over real (modeled, of course) ground.  Below is a chart produced using MultiNEC data, showing the performance of the SteppIR yagi on 10 meters.  The antenna is modeled in free space.  On the mark (29.100 MHz), gain is 10.54 dbi, front to back is 8.3 db, and SWR is 1.2:1.  Given the design of this antenna, this level of performance can be achieved at any frequency in the band  by retuning the antenna.  The SteppIR controller actually uses this setting at 28.800 MHz, accepting slightly lower gain and higher SWR for better front to back.  The beauty of this antenna is that you can make such trade-offs to suit the operator’s needs!

Here are equivalent charts for 15 and 20 meters, for your reference.

 

At 21.225 kHz gain is 10.35 dbi, front to back is 19.68 db, and SWR is 1.5:1.   Again, SteppIR uses this model at 21.100 MHz, again accepting lower gain and higher SWR for better front to back.   Interestingly, we were unable to make the SteppIR do better than 1.5:1 SWR into a 22 ohm feedline (the design point).  We aren’t not sure why – both of our modeling programs show the same result.  We can only assume that this is the actual performance – but we have no real world data to back that assumption up.

 

At 14.325 kHz gain is 9.5 dbi, front to back is 21 db, and SWR is 1.2:1.  The actual controller uses this setting at 14.300 MHz.

 

One very interesting tidbit about this antenna becomes clear when you study the graphs above.  The 2 element SteppIR is a short boom, driven plus director, design.  Two element yagi’s with a driven element and a director, rather than the more traditional driven element and reflector, deliver higher gain but much narrower bandwidth.  L.B. Cebik produced a wonderful piece on this topic, which you can see here.  The executive summary is that if you are willing to live with very narrow bandwidth (for example, a driven director yagi with a 0.08 wavelength long boom on 10 meters might have a 2:1 SWR operating bandwidth of 250 kHz and gain within .5 db of its peak over less than 500 kHz), you can get quite a bit more gain from a two element array (peak gain of 7.29 dbi vs. peak gain of 6.25 dbi for “traditional” designs).  In the case of the SteppIR, since the antenna can be tuned at will (and even automatically), bandwidth is pretty much irrelevant.  That makes a driven-director 2 element configuration reasonable, and thus allows a very high gain antenna on a short boom.  Hence Fluid Motion’s claims that their two element yagi delivers performance similar to that of traditional three element antennae.

 

But for the four element yagi, this effect is much less important.  As a matter of fact, the four element antenna has a fairly broad operating bandwidth.  Not as wide as you would want for a traditional design, but plenty wide enough to live with just two or three settings across each band.  In fact on 20 meters, the SteppIR antenna looks pretty much like a “traditional” antenna!  This isn’t true in the case of their two element design – which is likely (I haven’t modeled it) so narrow banded that you really do need to retune it every 50-100 kHz.  The main benefits of the four element antenna derive not from the fact that you can make a narrow banded design work (although it certainly helps!), but rather from the fact that you use the entire boom on each band.

 

 

Comparing the SteppIR to long boom mono-banders

 

For the purposes of this comparison, we choose a ‘typical’ long boom mono-band yagi.  An example of such an antenna would be K6STI’s excellent design for 20 meters, five elements on a forty eight foot boom.  Detailed information on this design can be found at www.cebik.com.  This antenna is comparable to many other so called ‘long boom’ designs.  For 20 meters, these antennas are typically similar to the K6STI design, although some have six elements rather than five.  Analog’s on 10 and 15 meters would be on 24’ and 36’ booms, respectively.  These antennas represent the realistic 90th percentile of possible contest antennas, in our opinion.  Yes, we do see the occasional seven or even eight element yagi.  Some stations, K3LR leaps to mind, have forty eight foot boom antennas on 10 and 15 meters.  But for most of us, an antenna along the lines of the Hy-Gain Long John series (which would fall more or less into the same category as the K6STI design – yes, we know that the 205CA is on a shorter boom – but they are in the same league) is the realistic “dream antenna”. 

 

So what can we expect from a forty eight foot boom monster – or even its twenty four foot boom 10 meter cousin?

 

We can expect gain on the order of 10.5 dbi in free space, with a maximum gain excusion (that is to say, the difference between the gain at the point in the band that delivers maximum gain and the point in the band with minimum gain) of less than half a db.  We can expect a front to back ratio of around 40 db at the peak, and 20 db or more across the band.  And we can expect an SWR of less than 2:1 across the band, if we use a sensible matching system like a beta match.  You can see graphs of the performance of the K6STI design at L.B.’s website.

 

By comparison, the SteppIR does quite well on 10 and 15 meters.  It delivers about the same gain (10.3 and 10.5 dbi respectfully) – and of course it does it with zero gain excursion.  10.5 dbi is actually better than most of the large mono-band designs that we have looked at, which tend to fall between 10.0 and 10.5 dbi at their gain peak.  The SteppIR doesn’t hold up on front to back however.  With peak F/B of around 20 db on 15 meters and 8-10 db on 10 meters – you are giving up quite a bit compared to the single band designs.

 

On 20 meters, the SteppIR falls quite a bit short – delivering 9.63 dbi, or around .9 db less.  That is entirely due to its shorter boom – you just can’t make up for the missing 16 feet no matter what you do.  Front to back is also down a bit – 21 db from the SteppIR compared to something well over 20 peak from the mono-banders.

 

Comparing the SteppIR to “big” Tri-banders

 

For this comparison, we choose the Force 12 C-31XR.  It happens to be the antenna that we have at our own station, but it is also the antenna of choice in this category amongst contesters.  For those not familiar with the C-31, it is a fourteen element antenna covering 10, 15 and 20 meters on a thirty one foot boom.  It uses twenty five feet of boom with three elements on 20 meters, twenty six feet of boom with four elements on 15 meters, and twenty five and a half feet of boom with seven elements on 10 meters.

 

We did model the C-31XR, but to give the manufacturer the benefit of the doubt we will use their claimed performance numbers rather than our own modeled numbers.  The comparison is to our modeled performance numbers for the SteppIR, over flat average ground at a height of 74’ (the conditions specified by Force 12 for their claimed data).  You can see the comparison between the two antennas below:

 

 

C-31XR Gain

SteppIR Gain

C-31XR F/B

SteppIR F/B

10 Meters

7.4

8.14

21

8

15 Meters

6.3

7.98

22

20

20 Meters

6.0

6.88

20

21

 

We modeled the C-31XR using the same tools that we used for the SteppIR, just to validate the comparison.  Our modeled results for the C-31XR match very closely with Force 12’s claimed numbers for gain (they claim 14.0 dbi gain on 20 meters, our model showed 13.98 dbi of peak gain.  They claim 15.6 dbi gain on 10 meters, our model showed 15.67 dbi of peak gain).  We did see quite a bit more gain excursion than we would have liked to in our C-31 model.  On 10 meters, gain peaked at 15.67 dbi, but at 28.600 it was down to 15.28 dbi and falling fast.  Worse, on 20 meters, peak gain was at the bottom of the band.  At the top of the band (14.300) gain was down from its 13.98 dbi peak to 12.76 dbi, for a delta of 1.22 db.  We don’t trust our modeling skills enough to be sure, but our guess is that the gain curves for the C-31 show a bit more excursion than the mono-band designs mentioned above do – especially on 20 meters.

 

In summary, the SteppIR uses more boom than the C-31 on each band, despite the fact that their total boom lengths are the same.  That longer boom delivers more gain, as you might expect.  Interestingly, on 10 and 20 meters, the SteppIR’s advantage is only about .7-.9 db.  On 15 meters, for reasons that aren’t completely clear to us, the SteppIR delivers a healthy 1.7 db more gain.  The C-31 delivers about the same gain on 15 and 20 (.3 db difference).  The SteppIR delivers about the same gain on 15 and 10 (.15 db difference).

 

As one might expect, the SteppIR falls far short of the C-31XR on 10 meter front to back ratio – but it is close enough for ‘government work’ on 15 and 20 meters.

 

Conclusions

 

Our goal in this exercise was to inform our own decision about what antennas to install at our single tower contest station.  We have limited space, and hope for the best possible performance given our constraints.  For us, stacks of long boom mono-banders simply aren’t possible.  We compared the SteppIR to them mostly for our own education – and also to understand how much performance we would be giving up to our competitors.  We currently use Force 12 yagi’s, including a C-31XR as our main high band antenna.  We were very interested to know how this new fangled antenna would compare to our tried and true C-31XR.

 

Our conclusion is that, mechanical issues aside, the SteppIR 4 element yagi is a serious contender.  It offers gain about equal to that of a five to six element long boom mono-bander on 10 and 15 meters, and does quite well on 20 too.  Front to back, especially on 10 meters, is quite a bit less than any of the antennas that we compared it to.  The addition of a dedicated 10 meter element might fix that – but for West Coast stations we have always felt that front to back ratio is irrelevant anyway.  For domestic contests, there isn’t anyone behind us (just the big, empty Pacific Ocean).  For DX contests, when beaming Japan, there is more of an issue – but with South America off the back of the beam and likely calling at the same time, we’re happy to be able to hear off the back.

 

There are serious questions in our mind about the mechanical reliability of an antenna with motors.  These antennas are also quite expensive, a fair bit more than their direct competitors. But those issues aside, the SteppIR is clearly worth a hard look.