2 X 3 = 6

Two Three-Element Yagis for Six Meters

L. B. Cebik, W4RNL





Another project required me to model a couple of 3-element Yagis for 6 meters, so I thought I might pass along the designs. They are not especially new, but they might be useful to some one needing to home brew a small Yagi to take advantage of the occasional openings on that band.

The designs make a useful contrast with each other. Both are a bit over 6' in boom length. However, one presses for higher gain and good front-to- back ratio, while the other sacrifices a bit of gain and front-to-back for a wide operating bandwidth that covers all of 6 meters (50-54 MHz) with under 2:1 SWR with a direct 50-ohm feed.

Gain

A 3-element Yagi is capable of around 8 dBi free space gain with a front- to-back ratio of over 20 dB for something over an 3.2% frequency span (+/- 1.6% of the center frequency). In this span the gain will increase with frequency, while the maximum front-to-back ratio will peak in the 25-27 dB range somewhere in the middle of the band spread.

I have elsewhere shown a 20 meter model using single diameter materials with these properties, adapted from a stepped-diameter model developed by K6STI. The design covers all of 20 meters. When scaled to 6 meters, the design covers about 1.5 MHz with the desired characteristics. Here are the dimensions, using 0.5" aluminum tubing. (single-diameter elements are quite practical from 6-meters on up.)

Element        Length (ft)    Spacing from Driven Total Boom
                                Element (ft)       Length (ft)
Reflector        9.53              3.15
Driven Element   9.08              ----
Director         8.536             3.345            6.495

The performance characteristics are summarized in the following table:

Frequency      Gain      F-B Ratio      Feed Z         SWR (ref. =
  MHz           dBi        dB           R+/-jX           25 ohms)
50.0           7.92      16.55          26.9 - j20.2    2.14
50.5           8.07      22.59          26.4 - j11.6    1.57
51.0           8.24      25.86          24.9 - j 2.4    1.10
51.5           8.43      19.33          22.8 + j 7.8    1.40
52.0           8.64      14.66          20.3 + j19.2    2.34

Notice that the 2:1 SWR for this antenna, with a 51 MHz target design frequency, is well over 1 MHz, but assumes the use of a 2:1 feedpoint matching system. This can be a beta match, which will require shortening the driven element until it shows abut 25 ohms capacitive reactance, or a Tee match, which will require a bit of driven element lengthening to show inductive reactance.

The target center frequency can be adjusted up or down within 6 meters by adjusting all three element lengths by the percent of frequency change without concern for changes in element spacing of diameter. Likewise, for those more concerned with gain than front-to-back ratio, the performance at 52 MHz or a bit above that can be scaled to the target frequency, with the driven element adjusted for either resonance or a desired matching system. Changing the driven element length to create changes in its reactance by as much as 25-30 ohms has very little affect on the other performance figures.

Operating Bandwidth

A question often asked is how much gain and front-to-back ratio must be sacrificed to achieve a wide operating bandwidth, especially for a 4 MHz (8 %) bandwidth on 6 meters. The rough answer is about 1 dB of gain and 5 dB of front-to-back ratio.

From the same 0.5" diameter aluminum tubing, it is possible to build a 3- element Yagi with about 7 dBi free space gain and up to 21 dB front-to-back ratio. Such an antenna will show a 2:1 SWR bandwidth of over 4 MHz on 6 meters and have a direct 50-ohm feed as well. The design below is adapted from an old W6SAI design for 10 meters. The dimensions are these:

Element        Length (ft)    Spacing from Driven Total Boom
                                Element (ft)       Length (ft)
Reflector        9.734             3.391
Driven Element   9.008             ----
Director         8.01              2.73             6.121

The performance characteristics are summarized in the following table:

Frequency      Gain      F-B Ratio      Feed Z         SWR (ref. =
  MHz           dBi        dB           R+/-jX           25 ohms)
50             7.00      14.90          48.4 - j21.2    1.54
51             6.92      18.08          51.9 - j 9.9    1.22
52             6.96      20.31          51.9 + j 1.7    1.05
53             7.13      21.02          48.8 + j15.0    1.35
54             7.44      18.40          43.0 + j31.1    1.96

Notice that the gain curve is not the anticipated steady rise across the band, but shows a minor peak just below the band edge, with an insignificant dip and then a return to the normal 3-element rise. The front-to-back ratio does not peak on the target design frequency (52 MHz, for the wide-band model), but above that frequency. The SWR remains below 2:1 for a direct 50-ohm feed from below 49.5 MHz to just above 54 MHz.

The wide-band model is suited to wide-band interests, perhaps in a mechanical design that permits the user to flip it from horizontal to vertical polarization (where it, like every other Yagi, will show a broader beamwidth and up to 3 dB less gain over real ground). This would permit use with repeaters while allowing the operator to take advantage of SSB and CW openings.

Comparisons

Although the beams have comparable boom lengths and driven element lengths, the key differences lie in the parasitical elements. The higher gain model uses a shorter but more closely spaced reflector than the wide-band model. However, the tell-tale difference lies in the director. The higher gain model uses longer director at a considerably greater spacing from the driven element than the shorter, more closely spaced director of the wide- band model.

Figure 1 shows in rough scale the difference between the dimensions of the two antennas. Figure 2 contrasts the free space azimuth patterns.

Either beam is easily constructed from hardware store tubing and other supplies. Both will be very light and easily installed at moderate heights. You can take your pick--or you can use these notes as clues to the design considerations for adapting them to other bands, higher or lower.



Updated 7-25-97. L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.



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