A Performance (Yet Simple) Vertical Antenna

Matt Roberts - matt-at-kk5jy-dot-net

Published: 2017-07-01

Updated: 2017-07-06


While discussing simple and easy Field Day antennas with a local ham, I happened to remember a design that I had seen in the ARRL Antenna Book many years ago.  The design had interested me at the time, and I even modeled it in EZNEC+, but I never built it, because I had other antenna designs for 20m that were already working well for me.

My friend, however, didn't have an effective antenna for 20m, as most of his work is on 40m during the evenings.  Since he wanted a dedicated 20m antenna for Field Day and other weekend events, he decided to build the design from my model.  As it turns out, the antnena is every bit as effective as the model suggests, and can be built for a few dollars in just about any yard.

The design was originally suggested in a September 1984 QST article, by K8CH.  The idea is simple enough — hang a wire vertical from a support, using two elevated but sloped radials to form a basic "ground plane" antenna.  Since the radials are elevated, the ground losses are minimized, even though there are only two of them.  The antenna wire frame and its elevation pattern are shown in Figure 1:

The Model

I rarely build any HF antenna unless I model it first.  Neither will I recommend one to others without being able to show the theoretical performance in a computer model.  So here is the model for the K8CH antenna, modified and scaled for 20m:

EZ-NEC+ Model EZ-NEC+ Model EZ-NEC+ Model EZ-NEC+ Model
Figure 1: EZ-NEC+ Model and Data
Peak at 25'

The model above assumes the peak of the antenna is at a very reasonable 25' above the ground, which is an elevation that anyone should be able to achieve.  This results in radials that are roughly 6' above the ground at the tips, and a feedpoint at about 8.5'.

By raising the antenna to 30', a modest improvement in pattern, elevation angle, and 50-ohm match can be realized, bringing all three close to optimal values, as shown in Figure 2:

EZ-NEC+ Model EZ-NEC+ Model EZ-NEC+ Model EZ-NEC+ Model
Figure 2: EZ-NEC+ Model and Data
Peak at 30'

Either antenna is a good match to 50-ohm feedline, which should be directly attached.  As with all such antennas, a choke at the base is recommended, but not strictly mandatory.  Note that the eccentricity (the front/side ratio) is about 3/4 of a dB for each antenna, increasing slightly within creasing height above ground.

The antenna design can work with any band, provided that a support of sufficient height is available.  The original design by K8CH was for 30m, and was hung from a tree.  Each wire in the vertical is λ/4 long, adjusted for insulation, if appropriate.  The angle between each wire is roughly 120°, although this can be adjusted to improve the feedline match, as needed.

When Chris installed the antenna at his residence, he split the difference between the two models above, installing the antenna peak at just over 27', as shown in Figure 3:

KF5VRD Diagram   AA-30 SWR
Figure 3: KF5VRD 20m Vertical As Installed - Peak at 27'4"  Figure 4: AA-30 SWR Sweep of the Antenna


Field Day 2017
20m Contacts
Trial Run

He used PSK31 as his first test of the antenna, running QRP from his KX3.  One of the first 20m contacts he made was R6AV on the northeast coast of the Black Sea.  Given the conditions at the time (SFI = 73; SSN = 11), the antenna immediately demonstrated its DX prowess and efficiency.

The real test of the antenna was ARRL Field Day 2017. During that event, the antenna netted 101 20m CW contacts, using 5W or less.  Among those contacts were KH6J and KH6RS in Hawaii.  A map of the 20m FD 2017 contacts from KF5VRD is shown at right.  Even running less than 5W, this antenna is an effective communicator.

Installation Considerations

The antenna is a good match for 50-ohm feedline, as the sloped radials raise the impedance from the 36-ohm theoretical value.  The slope can be adjusted as needed to obtain the best match to the cable.  As a full-size antenna, the design has a very flat SWR over a wide bandwidth, as shown in Figure 4.

The feedpoint can be assembled in any number of ways.  Chris likes outdoor enclosures for his ham gear, so he built a junction box to connect the wires to the coax.  The box had three 1/4" stainless fasteners for the wires, and a waterproof N connector for the coaxial cable.  Figure 5 shows the antenna pulled into a tree next to a treehouse, and a closeup of the junction box.

Antenna Antenna Junction Box
Figure 5: Installed Antenna

The coil of coaxial cable underneath the box is a common-mode choke, which is a good addition to just about any HF antenna.  Even though the antenna is a good match for the 50-ohm cable, a choke virtually eliminates any common-mode current that might try to flow due to environmental interactions or cable lengths that happen to be a multiple of 1/4 λ in length.

Care should be taken when locating the radial anchors.  If the anchors are attached to the ground, they should be located such that they do not present a tripping hazard to people or animals.  If more than QRP is used, the ends of the radial wires should be insulated, to prevent people or animals from coming in contact with an energized conductor.  While the voltages present at the end of a quarter-wavelength radial are not excessive, even at QRO, they are high enough to cause RF burns if touched.  The rope used to attache the radials to the ground should have good insulating properties.  Alternatively, ceramic insulators should be used between each radial and its support rope.  The ends of the radial conductors should not be allowed to touch the ground (or anything else).

Like all vertical antennas, installation height matters.  This is particularly true of elevated-radial antennas.  Placing the radials too close to the ground increases ground losses, and lowers the efficiency of the antenna.  Raising the antenna too high causes the elevation pattern to become distorted, and raises the peak elevation angle, which reduces the antnena's DX capabilities.  The antenna dimensions shown in Figure 3 are close to optimal, and can be scaled for use on other bands.  Many hams have been told that "higher is better" but this is not true of vertical antennas.  One nice feature of this design is that the optimal height happens to be close to the lowest reasonable height that will allow the ends of the antenna's radials to be kept above the ground.  Since most of the radial current is concentrated near the feedpoint, the ground losses are minimized, even though the ends are fairly close to the ground.

Advantages

The elevated radials greatly reduce ground losses when compared to a ground-mounted vertical antenna.  The antenna is resonant and a good match for 50-ohm cable; it can be easily adjusted for the best match to eliminate the need for extra tuning devices, like shunt inductors.

The antenna is lightweight, and can be quickly deployed for portable operation, when adequate supports are available.  A mature tree with stable branches near 40' could support an antenna built for 40m or shorter wavelengths.  Temporary supports, such as fiberglass masts, are ideal for this kind of antenna, since most of the height of the antenna does not generate additional wind load.  Supports do need to be nonconductive, to prevent coupling to the vertical element.  The antenna's "flat" profile allows it to be easily located in areas where other antennas might not fit, e.g., behind a stand of trees along the back or side fence line of a property.  If proper colors are chosen when selecting materials, this type of antenna can be made very stealthy, even when not hidden by trees or other natural visual barriers.  There aren't many "stealth" antennas that are also efficient, but this is one of them.

Conclusions

The performance of the antenna is without question.  Even running QRP, the antenna is efficient enough, and with a low angle of radiation, that it works DX with ease.  As an unloaded wire antenna, it should be able to withstand any legal-limit power level.  The only power constraint would be in the feedline chosen.

The K8CH article continues to be published in the ARRL Antenna Book, as well as in another book, Simple and Fun Antennas for Hams.

Article and Models: Copyright (C) 2017 by Matt Roberts, KK5JY.
Pictures and Diagrams: Copyright (C) 2017 by Chris Willis, KF5VRD.
All Rights Reserved.