Commercial-Style CP Dipole FM

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A Circularly-Polarized Commercial-StyleFM Broadcast Antenna

FCC WARNING - This equipment, in some instances, is capable of exceeding Part 15 of the Rules for low-powerbroadcasting. Please be aware that it is possible to use this equipment in a manner that can place you in violation offederal laws. Before you construct any of these projects, you should familiarize yourself with FCC rules with regard toradio 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. Thefollowing 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-polarizedpattern, this antenna will often outperform vertical antennas over hills and valleys. The gain with two bays isjust about break-even, so more transmitter power is necessary to maintain the same intensity in your coveragearea's strong spots. In my experience, this antenna produced a receivable signal in many isolated areas that werecompletely dead with a vertical J-pole that was in use before. The history of C.P. and some documentation Ihave on the subject indicates that doubling transmitter power is all that's necessary when switching to thispolarization 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 radiatingelements, standard formulae for calculating antenna wavelength do not provide accurate dimensions. The authordiscovered that much hand-tuning, with the aid of an impedance bridge, was necessary to find the resonantfrequency. 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 thetransformer, along with two "L" fittings and an inch-long tube between the elbows. All fittings aresweat-soldered together. The radiating elements are ¼" 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 thehorizontal section, then used a 4" diameter cylinder as the form for the bend toward the vertical section.

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The parallel tubes should be cut to 13.5" in length. For the element length, calculate two ¼-wave elements (thiswill 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 ¼" diameter holes in the tubes andforcing the rounded part of the elements into the holes just until they seat into both walls of the ½" pipe. Solderthese 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 theclassic Delta Match used in this design. By sliding the clamps back and forth a few inches, this antenna'simpedance may be adjusted. ¾" copper clamps were bent and shaped as in the illustration. An extra hole wasdrilled where the flanges come together and a 6-32 stainless steel screw and nut with lockwasher is used totighten 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. Ateach connection, it is imperative that 4 turns of coax be looped into a coil, 3 or 4 inches in diameter. This servesas a balun, which decouples antenna currents from the outside of the feeder coax.


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TUNING

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

After the copper is constructed and coax is attached, connect the analyzer to the antenna. Set up the antenna inthe 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 theimpedance. Moving them too far from the elbow will reduce radiational power, so try to avoid using them as theprimary tuning adjustment.

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


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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 theoperating frequency. Since these antennas have a typical impedance of 100 ohms or higher, connecting two ofthem in parallel makes a nice match to coax like RG-8 and RG-11. When connecting two units together, themutual coupling affects the overall resonant frequency, lowering it by 300-400 Khz. The best way tocompensate for this is to tune each element independently to be that much above your desired frequency. Whenyou 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 theelements and throw the resonance off. All coax ends should have their exposed sections covered in siliconesealer 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 withcaulk to act as a radome and prevent water from detuning the elements during rainfall.

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 baysare employed, the pattern will be strongest along the horizon, with very little radiation up or down. Fall-off insignal 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 0.672 0.99 0 0.993 1.56 +1.9 1.24


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4 2.13 +3.2 1.465 2.71 +4.3 1.646 3.3 +5.1 1.81Power gain is in each polarization.

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

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Commercial-Style CP Dipole FM