20 Meter 3-element Reversible Wire Yagi

Recently I was asked to design a reversible 3-element wire yagi. The constraints were a maximum height of 30' (9 meters) and inverted vee elements. Since many hams do not have towers or tall supports this antenna can provide good performance in two directions on what is the workhorse DX band. For this reason I thought to make a few changes and write it up for the blog.

Although the antenna is simple and easy to construct it requires attention to detail. Unlike single element antennas such as dipoles, verticals and delta loops a yagi relies on coupling and phasing between elements to develop its directive pattern. Care is required when measuring the wires, getting the angles right and building the coils and switching system. Mistakes will incur a cost.

Another consideration is antenna placement. Because it is fairly low there is the risk of coupling to house wiring and plumbing, metal fences, power lines and so on. A minimum spacing of 10 meters (λ/2) of these obstacles is highly recommended, especially to the front and back. More height, if you can manage it makes this easier.

Design

The design is similar to that of a 3-element inverted vee wire yagi for 40 meters that I described several years ago. Refer to that article for aspects of design and construction that are not repeated in this one. The element spacing is 3.5 meters, for a total length of 7 meters.

The interior angle of the inverted vee elements is 90°. A larger angle will improve performance but don't try it without a redesign of the antenna since the resonant frequency of an inverted vee rises with larger interior angles. Elevation angle of the main lobe is lowered by bending a dipole into a vee since the average current height is lower. That, too, depends on the interior angle.

The elements are made from 2.5 mm (AWG 12) copper wire with 0.7 mm insulation. Thinner wire can be used since the tension doesn't need to be very high, however ohmic loss will be higher. The wire loss for the antenna as designed is approximately -0.4 db. There is ground loss as well despite the antenna apex being up almost λ/2 since the ends are much lower. Bare copper wire elements will need to be longer because the velocity factor of the insulated wire is ~2%.

Each half element (inverted vee leg) is the same length: 5.06 meters. To feed the antenna with a beta (hairpin) match the driven element legs are lengthened to 5.11 meters. Although you'd expect the driven element with a beta match to be shorter consider that the reflector element is in fact electrically longer. The beta match uses a nominal 150 Ω shorted transmission line stub at the feed point. The antenna radiation resistance is quite low at ~15 Ω mid-band. An L-network can be used instead but a beta match is simpler and lighter.

A 0.7 μH coil is placed at the centre of each parasitic element. Relays short the coil to convert the element from a reflector to a director. To allow use of the antenna without powering relays choose a preferred direction and place a SPST-NO relay at the reflector and a SPST-NC relay at the director. To reverse the yagi power both relay coils.

A larger value coil can be selected to broaden the 200 kHz 2:1 SWR bandwidth somewhat, but at the expense of gain. In my judgment the trade off is not justified. A switchable L-network at the feed point can switch the best match between the low and high ends of the 20 meter band. A tuner (ATU) can be used in the shack at the cost of additional transmission line loss. Unfortunately an antenna of this type is difficult to engineer to achieve a good match across the full band without significant performance reduction.

Performance

At the modelled height of 9 meters the elevation angle of the main lobe is 30° which is not ideal for DXing, if that is your interest. More height is needed to lower that angle. Like all yagis with 3 or more elements its performance remains stable at greater heights without the need to adjust the match or element lengths. However it will worsen at lower heights since the element ends are already only λ/4 above ground.

The free space gain of the antenna peaks at near 8 dbi high in the band, and is greater due to ground reflection as is true with any horizontally polarized antenna. Gain is quite flat across the band, increasing only 0.35 db from 14.0 MHz to 14.35 MHz. Although a 2-element yagi has peak gain only a little more than 1 db worse the gain bandwidth is quite narrow.

The F/B, though not exceptional, is quite good across the band and, again, far superior to a 2-element yagi. It peaks at close to 30 db mid-band. As is typical of yagis the frequencies of maximum gain and F/B do not coincide, and can be far apart. In this case over 150 kHz apart.

Parting thoughts

It is the rare ham who does not want more antenna performance. That is difficult to do in the urban and suburban settings where so many live. This antenna is a simple and inexpensive way to achieve performance on the ever-popular 20 meter band. It can be almost invisible to neighbours if there are trees. Trees can also serve as convenient supports.

The particular physical design described here was chosen by the intended builder. Your circumstances and needs may be different. The antenna design can be modified. However, guessing at the impact of changes to dimensions and construction materials is not recommended. Yagis require more care than that if performance is the objective. Modelling those differences is wise. A high quality antenna analyzer, and the knowledge to use it, is so valuable it can verge on being mandatory.

Spring, and therefore antenna season, is around the corner.