A Circularly-Polarized Commercial-Style
FM Broadcast Antenna


FCC WARNING - This equipment, in some instances, is capable of exceeding Part 15 of the Rules for low-power broadcasting. Please be aware that it is possible to use this equipment in a manner that can place you in violation of federal laws. Before you construct any of these projects, you should familiarize yourself with FCC rules with regard to radio transmission devices, and be aware of the boundry between legal operation and illegal, unauthorized operation. It is up to you to assume legal responsibility for your actions as you use this equipment.


INTRODUCTION

There is considerable demand for antenna designs among FM broadcasters in the Free Radio world. The following circularly-polarized design is based on the Harris Dual Cycloid III, a C.P. antenna for FM up to 1KW. This antenna is not recommended for transmitters below 10 watts. However, because of its circularly-polarized pattern, this antenna will often outperform vertical antennas over hills and valleys. The gain with two bays is just about break-even, so more transmitter power is necessary to maintain the same intensity in your coverage area's strong spots. In my experience, this antenna produced a receivable signal in many isolated areas that were completely dead with a vertical J-pole that was in use before. The history of C.P. and some documentation I have on the subject indicates that doubling transmitter power is all that's necessary when switching to this polarization type. If you have adequate power to service your primary area, you'll find that your "dead zones" are greatly reduced or almost eliminated.

THE CHALLENGE

This design is not for the beginner. Because of the combination of curved and straight sections of radiating elements, standard formulae for calculating antenna wavelength do not provide accurate dimensions. The author discovered that much hand-tuning, with the aid of an impedance bridge, was necessary to find the resonant frequency. The curvature radius and the spacing between ring elements, as well as the length of the elements, proved critical.

CONSTRUCTION

The antenna is constructed entirely of copper. One-half inch type M copper pipe was used to form the transformer, along with two "L" fittings and an inch-long tube between the elbows. All fittings are sweat-soldered together. The radiating elements are " O.D. copper refrigerant tubing. I cut it to length, wrapped a little less than half of it around a Freon canister (about 10" in diameter) to get the curvature for the horizontal section, then used a 4" diameter cylinder as the form for the bend toward the vertical section.

The parallel tubes should be cut to 13.5" in length. For the element length, calculate two -wave elements (this will be too long, but we'll trim this during the tuning stage later on) and cut them to that length.

The elements are attached to the transformer parallel tubes by drilling " diameter holes in the tubes and forcing the rounded part of the elements into the holes just until they seat into both walls of the " pipe. Solder these joints, taking care to set up the piece so that all angles are kept aligned accurately.

Copper pipe hanger clamps are used to make the electrical connections. You may have already noticed the classic Delta Match used in this design. By sliding the clamps back and forth a few inches, this antenna's impedance may be adjusted. " copper clamps were bent and shaped as in the illustration. An extra hole was drilled where the flanges come together and a 6-32 stainless steel screw and nut with lockwasher is used to tighten the clamp. The end hole is where the coax feeder attaches.

I used RG-6-QS to make the interconnects between antenna bays, to keep the weight of the assembly down. At each connection, it is imperative that 4 turns of coax be looped into a coil, 3 or 4 inches in diameter. This serves as a balun, which decouples antenna currents from the outside of the feeder coax.


CP Antenna Top view
CP Antenna Front

TUNING

Some trimming and adjusting will be necessary before placing the antenna into service. I recommend use of the MFJ-259 SWR Analyzer, which is what is used in these adjustments. Text will describe the process accordingly.

After the copper is constructed and coax is attached, connect the analyzer to the antenna. Set up the antenna in the orientation in which it will be used, and preferably away from nearby objects. Position the clamps about 1" from the elbows. It is recommended that only slight movement of the clamps be used for fine-tuning the impedance. Moving them too far from the elbow will reduce radiational power, so try to avoid using them as the primary tuning adjustment.

For example, let's say you're frequency is going to be 99.9Mhz. Sweep the analyzer over the lower part of the band, starting at 70Mhz and slowly sweeping up. At some point the SWR will dip to 1.1:1 or better. This is the starting resonant point. Most likely, it will be below 88Mhz. You'll need to trim a couple of inches off the vertical portion of the antenna to raise the resonance. Once you are in the "ballpark", you can fine-tune the exact frequency by adjusting the spacing between rings slightly.

If you decide to use two or more bays, tune each one independently first, to get it into the vicinity of the operating frequency. Since these antennas have a typical impedance of 100 ohms or higher, connecting two of them in parallel makes a nice match to coax like RG-8 and RG-11. When connecting two units together, the mutual coupling affects the overall resonant frequency, lowering it by 300-400 Khz. The best way to compensate for this is to tune each element independently to be that much above your desired frequency. When you connect them together, the overall resonance should be at your desired frequency.

Once you're set with the adjustments, the antenna may be installed, taking care not to bend or shock the elements and throw the resonance off. All coax ends should have their exposed sections covered in silicone sealer such as Dow-Corning bathtub sealant to prevent moisture from working its way under the feeder jacket. Plug all open ends of tubing with silicone caulk to prevent water from getting inside.

Optionally, clear vinyl tubing of 3/8" I.D. may be slipped over the elements before tuning, and sealed with caulk to act as a radome and prevent water from detuning the elements during rainfall.

CP Antenna 3D View A 3-D view of the Circularly-polarized antenna. One bay shown here.

THE PATTERN

This antenna is somewhat omni-directional, with slightly stronger lobes coming off the front of the rings. Pointing the rings at the intended service area will result in excellent signal strength in that region. If two bays are employed, the pattern will be strongest along the horizon, with very little radiation up or down. Fall-off in signal occurs about 40 degrees off-axis in the vertical plane.

RELATIVE ANTENNA GAIN AND POWER
# of Bays Power db Gain Field
1 0.46 -3.3 0.67
2 0.99 0 0.99
3 1.56 +1.9 1.24
4 2.13 +3.2 1.46
5 2.71 +4.3 1.64
6 3.3 +5.1 1.81
Power gain is in each polarization.

CP Array Photo A photograph of the Circularly-polarized antenna array, as it is today.