No. 37: 6- and 7-Element OWA Yagis

L. B. Cebik, W4RNL

Last time, we looked at some Yagi designs for fairly short-boom OWAs with direct 50-Ohm feedpoint impedances across the 28 to 29 MHz span. The 4-and 5-element designs provided about 8 dBi free-space gain and 9.2 dBi gain, respectively, which is quite good for monoband Yagis with their boom lengths.

This time, we want to look at some longer versions of the OWA with more gain, but still keep the 50-Ohm feedpoint impedance. We shall look at 6- and 7-element Yagis on 24 and 36 foot booms.

Fig. 1 reviews the element structure that we are using in this set of designs, a combination of 3/4, 5/8, and 1/2 inch diameter aluminum tubing. One reason that we shall review the 6-element OWA, when we already did an entire column on that antenna, is the new, larger tubing sizes. As a result of changing both the tubing sizes and the taper schedule (the places along the element length where we change tubing size), the element lengths will also change. However, the end result will be very similar performance between the two versions of the antenna.

A 6-Element OWA

The original 6-element OWA is a very good antenna. So too is the new version. The only difference is in the tubing, which may be a matter of what an individual builder likes to use or has access to in his or her local area. Let's see how the new version turns out.

Fig. 2 gives us the outline of the 6-element OWA, as well as the inter-element spacing. The following table provides the structural details.

Element            Space from          Tip Length          Total
                   Reflector           Length              Length
Reflector          0                   5.46'               17.92'
Driver              3.66'              5.13'               17.26'
Director 1          5.77'              4.66'               16.32'
Director 2         10.94'              4.49'               15.98'
Director 3         16.06'              4.51'               16.02'
Director 4         23.50'              4.13'               15.26'

The free-space gain of this 6-element OWA varies from 10.1 to 10.2 dBi across the band, with a front-to-back ratio that never falls below 21 dB. Only at the upper limit of the frequency spread does the SWR get above 1.25:1 with a direct 50-Ohm feed system (including the recommended 1:1 choke/balun to suppress common mode currents).

A 7-Element OWA

Adding one more element increases the boom length from 24' to 36' or so. Whether the added length is worth the effort depends on how important another dB of gain is to the operator.

Fig. 3 gives us the outline of the 7-element OWA, as well as the inter-element spacing. The following table provides the structural details.

Element            Space from          Tip Length          Total
                   Reflector           Length              Length
Reflector          0                   5.47'               17.94'
Driver              3.66'              5.17'               17.34'
Director 1          5.76'              4.64'               16.28'
Director 2         10.89'              4.50'               16.00'
Director 3         17.50'              4.51'               16.02'
Director 4         26.40'              4.47'               15.94'
Director 5         36.00'              4.11'               15.22'

The gain ranges from 11.2 to 11.6 dB across the first MHz of 10 meters, with a peak near mid-band. The front-to-back ratio is a solid 20 dB. Although the SWR curve reaches 1.6:1 at 29 MHz, it is very tame across the remainder of the band. In fact, Fig. 4 gives us a sampling of both antenna SWR curves.

Remember that these designs are specific to the tubing sizes and transition points from one size to the next. Any deviations from the prescribed tubing schedule would require refiguring the designs.

As with all OWA designs, we must have 1 extra element for the given boom length and gain in order to secure the relatively smooth 50-Ohm SWR curve across the band. The element marked as Director 1 in all of the designs does not so much contribute to the antenna gain as it functions with the reflector to establish the antenna bandwidth and feedpoint impedance.

How big do you want to go? I have seen an 8-element OWA design for 10 meters with something over 12 dB gain. But it was 48' long. The design can be found at the NW3Z web site via your search engine (search for "NW3Z"). As well, these designs might be tweaked for even better performance with an optimizing program. The versions shown here were hand-developed. If you prefer a direct 50-Ohm feedpoint impedance in a good Yagi antenna, then the OWA designs are the way to go.

Updated 10-10-2002. © 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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