Zero Five 30 Foot Flagpole Antenna

The Zero Five 30 Foot Flagpole Antenna is a very sturdy vertical antenna that doubles as a flagpole.

Installation

The antenna installation is not yet complete. The coaxial cable will be in 2 inch conduit buried 24 inches. It is currently on the ground. The "radial ring" and copper strap is not yet soldered. There are no radials installed yet.

The image at the right shows the base of the antenna as currently installed (not complete!). The concrete base is two feet in diameter (with a two foot by two foot square top) that extends 40 inches below the surface and six inches above. There are five #5 vertical rebars, each 36 inches long. They are tied to three 18 inch diameter "stirrups" made with #3 rebar.

There is an Ufer ground consisting of 20 feet of 8 AWG bare copper wire in the concrete with the excess coiled at the bottom of the hole. The ground wire is connected to the tilt base of the antenna by wrapping it around one of the J bolts and held down with the associated flat washer and nut.

In addition, 2 inch wide copper strap is routed through the tilt base and held down with two pieces of angle aluminum, each held in place by the mounting J bolts. The copper strap connects to the radial "ring" construced out of 1/2 inch copper pipe.

The MFJ 998RT antenna tuner is mounted on two eight foot ground rods. Copper strap also goes from the antenna tuner to the radial ring.

There is a 3 position J plug mounted on insulators on the antenna mast. The bottom connection is connected to ground through the antenna base using 1/4 inch copper tubing. The middle connection is connected to the mast using 1/4 inch copper tubing. The top connection is connected to the antenna tuner output using 1/4 inch copper tubing. For normal operation, the J plug is between the middle and top connection, connecting the antenna tuner to the mast. During storms, the J plug is moved to the bottom connection grounding the antenna. In addition, an ammeter J plug was modified by removing the ammeter and installing an SO-239 connector. This can be placed between the top two connections to make antenna impedance measurements.

Antenna Model

Richard Fry provided this analysis on the antenna.

Below is some number-crunching for those systems, starting with the use of NEC4.2.

Ground Rods vs Ground Rods with Buried Radials with 5 mS/m Ground Conductivity

*Using the "classical" method, NOT the same as shown for these systems using the methodology of NEC-based software.



/Richard Fry, October 2022

Ground Rods with 15 mS/m Ground Conductivity

The feedpoint impedance with ground rods (76.9 -j50.2) is quite a bit different than the measured impedance, below, at 7.2 MHz (39 +j11). The assumptions in the above model include that the ground conductivity is 5 mS/m. According to the FCC M3 map, the ground conductivity of Tucson AZ is 15 mS/m. Richard ran the model again with the higher ground conductivity.

In addition to ground conductivity, the models use the ground dielectric constant. Typical dielectric constants are shown below.








Increasing the ground conductivity from 5 to 15 mS/m decreased the driving point impedance from 76.9 -j50.2 ohms to 68.7 -j38.1 ohms, but the resistance is still considerably above the measured 39 ohms, and the reactance substantially different (predicted -38.1 ohms, measured 11 ohms). A possible explanation for the difference is the coaxial feed line from the antenna tuner to the transceiver acting like a single ground radial. The line extends about 30.75 feet from the antenna tuner, then has excess coiled, forming a common mode choke. The line is on gravel about 3 inches thick, making this a slightly elevated radial.

To test whether the coax is acting as a ground radial, the cable was disconnected from the tuner and another impedance sweep run. As shown below, the connection or disconnection of the coax has a minimal effect.

Ground Rods with Ufer Ground with 15 mS/m Ground Conductivity

It's interesting that the measured driving point impedance is lower than what is shown by the model. It seems like the most likely reason is that the ground resistance is less than predicted. Above is the consideration that the feedline is acting as a radial. As shown above, disconnecting the coax had a minimal effect on the driving point impedance. The antenna also has an Ufer ground. It is 20 feet of 8 AWG wire passing through the 46 inch concrete base with the excess coiled at the bottom. Richard Fry added the Ufer ground to the model and compared it with the antenna with only the ground rods.



The driving point impedance dropped from 68.7 -j38.1 ohms to 59.3 -j40.1 ohms. Subtracting out the radiation resistance of 27.5 ohms, the ground resistance dropped from 41.2 ohms to 31.8 ohms. As noted below, the measured driving point impedance at 7.2 MHz is 39.13 +j10. The gain increased from -2.8 dBi to -2.2 dBi.

Consider Antenna Diameter

The vertical antenna is 2.5 inches in diameter at the base and 1.5 inches at the top. The model was modified to show the average diameter of the radiator (2.25 inches). Previous models used a diameter of 20 mm (0.394 inches) To determine the radiation resistance, a perfect ground was used. The radiation resistance increased from 27.5 ohms to 29.5 ohms. The measured driving point impedance at 7.2 MHz, listed below, was 39.13 +j10 ohms, leaving about 10 ohms for ground resistance, which still seems low compared to the 31.8 ohms above that considered the ground rods and Ufer ground.

Antenna Impedance Measurements

Antenna impedance measurements are being done as radials are added.

Relative Radiation Measurements

Signal to Noise measurements are being made at the Reverse Beacon Network station KO7SS, which is about 41 miles away. Ideally this is close enough to reduce variation in signal strength due to skywave. The graph shows there is still some variation (though some of these transmissions were made at 500 watts instead of the usual 100 watts), it is hoped that with a large number of measurements, averages will show the change in radiation from the antenna as radials are added. Measurements are being made about noon MST each day.

Additional Resources