We expect a ground mounted vertical to perform better with a set of radials. This page shows the results of adding radial pairs to a 30 Foot Ground Mounted Flagpole Vertical Antenna. In general, we expect two results as radials are added. These are:
As described here, the antenna was originally constructed with two 8 foot ground rods supporting the tuner and an Ufer ground in the concrete base. In the data below, this initial condition is identified as "0 Radials." Radial pairs were added as shown in the table below. The radials use 18 AWG Copperweld wire on the surface or barely below the surface which is covered with gravel. The radials have some gravel below and some gravel above.
| Date | Radial Numbers | Direction degrees clockwise from North | Length |
|---|---|---|---|
| 16 May 2023 | 1, 2 | 90, 270 | 25 feet (7.6 m) |
| 1 June 2023 | 3, 4 | 0, 180 | 19 feet (5.8 m) |
| 16 June 2023 | 5, 6 | 45, 225 | 25 feet (7.6 m) |
| 1 July 2023 | 7, 8 | 135, 315 | 29 feet (8.8 m) |
| 16 July 2023 | 9,10 | 22, 102 | 20 feet (6.1 m ) |
| 1 August 2023 | 11, 12 | 67, 247 | 25 feet (7.6 m) |
| 26 August 2023 | 13, 14 | 112, 292 | 25 feet (7.9 m) |
| 8 September 2023 | 15, 16 | 157, 337 | 22 feet (6.7 m), 38 feet (11.6 m) |
The graphs below show how the antenna impedance changes on each band as radial pairs are added.
| Band | Frequency (MHz) | Plot |
|---|---|---|
| 80 | 3.64341 | Plot 80 |
| 40 | 7.127905 | Plot 40 |
| 20 | 14.21705 | Plot 20 |
| 15 | 21.18604 | Plot 15 |
As discussed above, signal to noise measurements (SNR) were captured from the KO7SS Reverse Beacon Network receiver. That receiver is 14 miles (23 km) from W6IWI. It is far enough away to be out of the near field but close enough to receive only ground wave with no sky wave. Measurements were averaged over a period of about two weeks for each number of radials. The raw data is here.
| Radials | 80 m SNR dB | 40 m SNR dB | 20 m SNR dB | 15 m SNR dB |
|---|---|---|---|---|
| 0 | 15.0 | 38.7 | 39.1 | 42.0 |
| 2 | 16.81 | 39.50 | 39.94 | 43.63 |
| 4 | 15.6 | 38.64 | 39.27 | 42.91 |
| 6 | 19.55 | 36.18 | 40.18 | 44.55 |
| 8 | 17.5 | 36.0 | 39.75 | 42.56 |
| 10 | 18.44 | 38.0 | 40.25 | 42.25 |
| 12 | 12.54 | 39.80 | 41.53 | 42.07 |
| 14 | 17.60 | 37.93 | 40.67 | 42.33 |
| 16 | 18.36 | 34.64 | 40.07 | 41.79 |
As noted above, "0 Radials" indicates that two ground rods and an Ufer ground were used instead of radials. The radials are in addition to the ground rods and Ufer ground See a graph of SNR vs number of radials for each HF band.
All SNR values above that are the average of daily SNR measurements over a two week period.
According to the FCC M3 Map, the ground conductivity in Tucson is a relatively high 15 mS/m. Radials are slightly elevated above the earth, sitting on a couple inches of gravel. A very detailed look at radial systems, including elevated, is here.
It is interesting to note that the antenna impedance changes substantially as the first radials are added to the system with the two ground rods. See, for example, plot40 which shows the variation in impedance on 40 meters as radials are added (the antenna is about a quarter wavelength high on 40 meters). Resistance drops from 53 ohms to 28.5 ohms upon adding the first pair of radials. Adding additional radials slightly lowers the resistance to 23.8 ohms with 16 radials.
If the difference in resistance could be attributed to ground resistance in series with radiation resistance, would have decreased by 24.5 ohms. IF we assume the 16 radials result in a low ground resistance, the ground rods only situation would have had a ground resistance of 24.5 ohms and a radiation resistance of 28.5 ohms. Power would have been divided nearly equally between the radiation resistance and the ground resistance when ground rods alone were used. We would expect a 3 dB increase in the radiated field after the radials were added. However, looking at the table above (Relative Field Strength vs Number of Radials), there is no visibile increase in the radiated field. It goes up and down somewhat as radials are added, but there is no 3 dB increase, except on 80 meters where the SNR changed from 15 to 18 dB, and the driving point resistance changed from 24.5 ohms (ground rods only) to 9.6 ohms (ground rods plus 16 radials).
The relative radiated field measurement is quite imprecise. It is a measure of the signal to noise ratio at the KO7SS Reverse Beacon Network receivr, about 14 miles from W6IWI. This receiver was used since it is in the far field yet close enough to receive minimal skywave effects. The SNR values used in the table are from about two weeks of daily measurements with the specified number of radials. Besides being a measure of signal level, it is also a measure of noise level, and the noise level at the receiver probably varies, especially on the lower frequencies. Ideally the multiple measurements average out the noise level variations.
In addition to inaccuracies to due varying noise level at the receiver, the measurements are done at a single point in space. Variations in both the vertical and horizontal pattern of the antenna will cause the received signal level to vary. It is possible that the antenna efficiency increased while decreasing the signal at this particular point in space by redirecting the radiation vertically, horizontally, or both.
Many people make a considerable effort to make an antenna resonant (zero reactance) and/or make the resistance 50 ohms. To me, antenna length changes the vertical radiation pattern and driving point impedance, but does not make a significant change in efficiency (unless the antenna is very short with a very low radiation resistance). The simplicity of a vertical antenna without traps over the best ground system practical with an automatic antenna tuner at the base seems "good enough." These models show the antenna vertical radiation pattern, gain (dBi), and driving point impedance on each HF band.
