The Zero Five 30 Foot Flagpole Antenna is a very sturdy vertical antenna that doubles as a flagpole.
|The Zero Five 30 Foot Flagpole Antenna as seem from the street. The MFJ 998RT antenna tuner is the black box at the bottom.|
The image at the right shows the base of the antenna as currently installed. 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.
On the right, 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.
On the left is a plastic junction box on top of two inch plastic conduit buried 2 feet. The conduit carries the RG-8/U transmission line. The end of the coax in the junction box has a PL-259 connector that connects to an SO-239 bulkhead mount barrel connector in the bottom of the box. A short piece of RG-8/U goes from that SO-239 to the input of the MFJ 998RT antenna tuner.
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 bottom two connections to make antenna impedance measurements.
Finally, the base is surrounded by 1/2 inch copper pipe to serve as a "radial ring." At this point, radials are not yet installed. They will be installed in pairs and then the impedance swept and signal strength checked at the WO7SS RBN receiver to see the effect of the additional radials.
|The image at the right shows a base impedance sweep being made. A J-plug designed for inserting an ammeter was modified to add an SO-239 instead of the ammeter. The SARK-110 antenna analyzer is plugged into the SO-239. As discussed above, the three terminal J-plug socket has the top terminal going to the MFJ 998RT antenna tuner, the middle to the antenna mast, and the bottom to ground. The antenna analyzer is measuring the impedance between the bottom two terminals, or between the mast and ground.|
|The image at the right shows where the buried conduit comes up at the house. The RG-8/U from the coduit passes through a hole in the wall to another junction box in the house. In addition, a short piece of RG-58/U passes through the hole. The RG-58/U has a PL-259 on the end in the outside junction box. The PL-259 is connected to a bulkhead mount SO-239 barrel. On the outside of the box, another piece of RG-8/U goes to the vertical VHF/UFH antenna. Finally, a short piece of 6 AWG bare copper wire passes through the hole in the wall and connects to an 8 foot ground rods near the junction box.|
|The image at right shows the junction box inside the house. The RG-8/U from the buried conduit is connected to the left SO-239. The RG-58/U os cpmmected tp the right SO-239. The 6 AWG wire connects to the ground screw in the junction box. The left cable goes to the directional wattmeter after the Dentron Clipperton L amplifier. The right cable goes to the Wouxun VHF/UFH transceiver. Two inch copper strap is clamped between the box cover and the box and provides an RF ground to the SEA245 transceiver and the Dentron Clipperton L amplifier. The wide copper strap has lower RF resistance and reactance than wire with the same crossectional area. The short 6 AWG ground wire between the inside junction box and the ground rod should contribute minimal impedance. Similarly, the very short piece of RG-58/U should contribute a minimal amount of loss to VHF/UHF.||.|
Richard Fry provided this analysis on the antenna.
Below is some number-crunching for those systems, starting with the use of NEC4.2. In addition, I used EZNEC PRO 7.0 to model the antenna in the middle of each of the HF amateur bands. Those results are here.
In addition to ground conductivity, the models use the ground dielectric constant. Typical dielectric constants are shown below.
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.
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.
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
Antenna impedance measurements are being done as radials are added.
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.
As discussed above the antenna originally had two ground rods and an Ufer ground. Over time, radial pairs are being added. The results are described here.