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A 2 Element Direct Feed Yagi
by WB2VUO, Keith 

  

Probably the easiest rotatable beam is a 2 Element Yagi, both in the mechanical and the electrical sense. 
While the gain won't be "up there" with the big tri-banders & the like, a useful amount of gain can be realized without breaking the bank. Below is a "sketch" of a 2 Element Yagi. 

2 Element Yagi Direct Feed
Gain is about 5dBd.


The Reflector and the Driven Element are slightly different lengths, in actuality, the Reflector is 5% longer than the Driven Element. 

The formulas for the element lengths are listed below:

DE (ft) = 470/F(MHz) [HF]  :  DE (in) = 5600/F(MHZ) [VHF]

REF (ft) = 494/F(MHz) [HF] :  REF (in) = 5880/F(MHz) [VHF]


The Spacing can vary from 0.15-Wave to 0.25-Wave,  with little change in the array gain.  According to the charts in the "ARRL Antenna Book", 14th Edition, the variation in gain is less than 0.5dB.  What does change is the feed impedance. 


Depending on how you want to feed the array will determine the spacing.  The closer the spacing, the lower the feedpoint impedance. 
At 0.20-Wave, the impedance is around 40-Ohms, resulting in a 1.25:1 SWR, and at 0.25-Wave, the feedpoint impedance is around 60-Ohms, resulting in a 1.17:1 SWR.
  This is with direct feed with 50-Ohm cable.  The feed can be direct, or through a 1:1 balun, and will show very little variation either way.
(Editor's note....by extrapolation, you should get very near 50 ohm impedance if the spacing is about .225 wavelength between reflector and driven element. Swr should be near 1:1)

Construction of the 2 Element Yagi

Various construction methods can be used for a 2 Element Yagi.  The most common is to take a length of rigid tubing, cut to the desired boomlength, and mount the elements with U-bolts, pipe clamps, muffler clamps or what-have-you.  The boom can also be wood or a lattice-like structure, or even PVC pipe.  The main consideration is mechanical strength.  The elemenst can be any conductive material that will support its own weight, or a suitable length of wire or braid that is supported by a rigid non-conductor.  One of the really inexpensive antennas I saw in a publication used lengths a #14 house wire, taped to bamboo poles.  Of course, the author lived in the South, and just went out in his backyard and cut his own canes!  Some people have all the luck!

                                                            
Below, I have dimensions for some of the HF and VHF bands, and some construction "tips":

        Freq.                REF.            DE.             Spacing
        14.1 MHz         35.04'           33.33'          13.95' - 17.45'
        18.1 MHz         27.29'           25.97'          10.87' - 13.59'
        21.2 MHz         23.30'           22.17'           9.28' -  11.60'
        24.9 MHz         19.84'           18.88'           7.90' -   9.88'

       
28.4 MHz         17.39'           16.55'           6.93' -   8.66'
        29.3 MHz         16.86'           16.04'           6.72' -   8.40'
        50.4 MHz       116.90"        111.33"        46.96" - 58.69"
        52.5 MHz       112.00"        106.66"        44.99" - 56.23"
       146.0 MHz        40.27"          38.36"        16.18" - 20.22"
       223.5 MHz        26.31"          25.06"        10.57"  -13.21"
Editors note: You should be able to "split the difference" with the spacing numbers above for very close to 50 ohms impedance which should give an excellent match to coax.


Let's say, for example, that you need a 10 Meter beam.  Looking at the chart above, (numbers in red), the longest element is just under 18', and the boomlength would be about 7' to 8.66'.  The elements could be DIY aluminum tubing (expensive), or it could be EMT (conduit), which is heavy, but inexpensive.
The boom could be a length of TV mast, Chain-link fence rail or a 2 x 4.
The least expensive would be the heaviest (2 x 4 boom & EMT elements), but the total cost would be $LESS. 

With about 5dBd gain, and a 25 watt rig (Uniden or Radio Shack, etc), this would be like going with a 75 watt amp, at $0.33/watt, and if you were running 100 watts, you would have effectivly 300 watts for $0.08/watt, plus the cost of the rig, of course.
Any way you look at it, it's a big bang for the Buck! (NOTE, prices are for comparision only)

If you wanted to make the Yagi for, say, 20 Meters, the "boom"could be a ladder, (No, I'm not kidding!), or a lattice-construct made with 2 x 2's or 2 x 4's.  It would be HEAVY, ... but the price of 20 Meter beams are HEAVY, also.  It might be feasable to do the ladder boom, and beef up the mast instead of depleteing your budget.


For the VHF beams, the boom could be PVC pipe, 2 x 2's, TV mast or whatever.  One suggestion that was in "73" Magazine awhile back was to use threaded elements, drill holes in the PVC pipe and "bolt" the elements through the pipe wall.  This would provide the mechanical support and the insulation for the feedpoint all at the same time.  For vertical polarization, the boom could be extended back beyond the Reflector, and the beam then end-mounted, putting the mast out of the field of the antenna. This would allow the antenna to even be side-mounted on an exsisting mast.

The beamwidth of a 2 Element Yagi is about 110-Degrees, so aiming is not critical, however, the front-to-side ratio and front-to-back ratio is 10 - 20 dB, providing a high degree of rejection to unwanted signals.
To put this in perspective, say you lived in Brockport, NY or somewhere in the Western edge of Monroe County.  The 2 Meter version of this antenna could be mounted up in the clear, and "sited" on the center of Rochester.  You would have gain from the North Greece area, all the way around to Rush, and would reduce the VE3 repeaters by a couple-or-3 "S"-units.  Not bad for a few chunks of wire & a piece of PVC.


The possibilities are limitless.  Consider a 2 Element Yagi this Antenna season.

WB2VUO
73, Keith WB2VUO

Editor's note: Many years ago, the 10 meter version of this 2 element Yagi was built by myself using much the same construction techniques as mentioned in the article using small aluminum tubing. It was installed on a short mast about 15 to 17 feet high with a rotor on the ground. This turned out to be a fantactic performing antenna and I made contacts worldwide on it. I still have the "parts and pieces" of it resting comfortably in my "junk" pile as it is called by the XYL!
I hope to revive it from it's "vacation" someday! Thanks to Keith, WB2VUO for sharing his article with us! Contact Keith via his QRZ email address.....N4UJW



 

 

 


 


  


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