Regular bodied gray pvc solvent cement for carlon conduit

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Contents
1 Introduction ........................................................................................................................................ 4 2 A Parasitic Lindenblad Antenna ......................................................................................................... 5 Parasitic Elements ............................................................................................................................. 5 Passive Circular Polarizer .................................................................................................................. 6 Impedance Matching .......................................................................................................................... 6 3 Antenna Construction ......................................................................................................................... 7 Overview ............................................................................................................................................ 7 Parts ................................................................................................................................................... 8 Assembly ............................................................................................................................................ 9 4 Radiation Pattern and Gain .............................................................................................................. 13 5
Testing ............................................................................................................................................. 15 Impedance Match ............................................................................................................................. 15 Power Handling ................................................................................................................................ 15 Circularity ......................................................................................................................................... 16 6 Summary .......................................................................................................................................... 16

This paper introduces a novel and considerably simpler way to construct a Lindenblad style antenna. It uses a single dipole, driven-element along with a passive, parasitic, circular-polarizer. The single driven element is designed to provide a 50-ohm load and the circular polarizer is very easy to build. While technically not a “Lindenblad,” the antenna radiation pattern is nearly identical.

This antenna was invented by Nils Lindenblad of the Radio Corporation of America (RCA) around 1940. The antenna uses four, dipole, driven elements that are fed in-phase. The dipoles are canted at 30 degrees from horizontal and positioned equally around a circle of about λ/3 diameter. Figure 1 shows a drawing of the antenna concept. At the time, Lindenblad was working on antennas for the then nascent television industry but the start of World War II delayed further TV broadcasting work.

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After the war, Brown and Woodward, also of RCA, began investigating ways to reduce fading on airplane-to-airport radio links. Airplanes use nominally vertically polarized antennas so using circular polarization on the airport antenna could reduce or eliminate the cross-polarization induced fading that results from the maneuverings of the airplanes. Brown and Woodward decided to try Lindenblad’s earlier TV antenna idea1 and constructed VHF and UHF prototypes. An original Brown and Woodward prototype is shown in the photograph of Figure 2.

The Brown and Woodward design uses tubing for the dipole elements and a 100-ohm open-wire BALUN for each dipole. The actual dipole feed is a coaxial cable that runs through the center of one side of the open-wire line. The four coaxial cables meet at the center hub section of the antenna where they are combined in parallel and fed to another coaxial cable as an impedance matching section to get a good match to 50-ohms. While this design is very clever and in fact worked very well, it would also be quite difficult for the average ham or the author to duplicate.

The goal of this project was to develop an antenna for the 70cm band that would have the same radiation pattern as a right-hand circularly polarized (RHCP) Lindenblad but would be much easier to construct.

2A Parasitic Lindenblad Antenna

Parasitic elements in Yagi-Uda antennas are always in the same plane as the driven element because their purpose is to improve the gain or front-to-back ratio of the antenna.

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The induced current flow in the parasitic elements causes an electro-magnetic field to be generated just as if they had been driven from a feed line. However, the current flow in each parasitic element travels along the path of the conductor, which is at 30 degrees from horizontal, rather than vertically like the driven element. This current flow distribution is exactly like the dipole currents in a traditional Lindenblad and the resulting electro-magnetic field generated from the parasitic elements is circularly polarized just like that of a Lindenblad. The circularly polarized field has both horizontal and vertical components and they are in phase-quadrature.

The overall effect is basically the sum of the radiation patterns of a traditional Lindenblad and a co-located vertical dipole with the same power applied to both. Since the power in the parasitic elements goes equally into vertical and horizontal components, the vertical field component from the parasitic elements is only one-half the magnitude of the field from the driven dipole.

To keep construction simple, it is desirable to provide a good match without requiring additional components or folded elements. This can be done by taking advantage of the stray capacitance at the dipole feed point. This stray capacitance is about 4pF and appears in parallel with the dipole feed. By making the driven dipole a little bit longer than the resonant length, we can introduce a small amount of inductive reactance. Using this

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This antenna was designed to be easy to construct using only hand tools. Most of the construction and materials are not critical and experienced antenna builders should feel free to substitute their own favorite techniques. The few critical dimensions are noted in the text.

Overview

The dipole driven element is made from two pieces of ¾” OD aluminum tubing that are connected with a PVC insert connector. The dipole is mounted to a threaded 12” x 1/2”

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Table 1. Parts list

3/16”

1/8” hole”

Figure 5. Driven dipole assembly

Clean the holes in the dipole with steel wool and apply Ox-Gard™ grease. Carefully thread the machine screws in the holes but do not tighten them. Strip the insulation from one end of the coax and un-braid the shield making a pair of wires. Leave about ¼” insulation on the center conductor. Wrap the center conductor around the #6 screw and the shield around the #8 screw and tighten as shown in Figure 6. Attach the coaxial cable with wire ties to hold it in place.

Figure 8. Ferrites aligned with bottom of driven dipole

After this is done, drill a hole through the top part of the hub and through the aluminum tubing for a #8 machine screw. Screw in the #8 screw to hold the hub in place.

Temporarily attach the 12” PVC riser to a vertical support so that it is perfectly straight. For each parasitic assembly, apply the gray PVC solvent cement around the outside of the ferrule on the end away from the aluminum wire. Carefully insert the ferrule into the ½” hole in the hub about 1/8” and quickly set the angle of the wire to 30-degrees from horizontal by rotating the ferrule. Looking towards the hub, the left hand side of the wire should be up. The aluminum wire should be 3-9/16” from the outer wall of the dipole. This is 4-1/16” from the exact center of the antenna. Allow the PVC cement to dry on all of the parasitic assemblies.

The PVC riser is used to attach the antenna to a mast. A pair of U-bolts can be used for this in the traditional manner. Since the author’s antenna was intended for portable use, a quick connect scheme was used instead. A ½” to 1” PVC conduit adapter was cemented to the end of the 12” riser after cutting off the threaded section. This fits into the top of a portable mast. A hole was drilled through the mast and the PVC adapter near the top of the portable mast and was fitted with a 2” long, #8 stainless steel screw. A #8 stainless thumbscrew is used to secure the antenna to the mast. This arrangement, shown in Figure 10, allows the antenna to be put up and taken down in less than 5 minutes.

The maximum gain predicted by the model is 7.47 dBic at an angle of 3-degrees elevation. Note that the pattern will have more and finer lobes if the antenna is mounted higher above ground and the maximum gain will vary with height and ground quality like any antenna. The predicted azimuth pattern, not shown, is almost perfectly circular with less than 0.1 dB variation.

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5Testing

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A horizontally polarized reference antenna was tested first. The sense antenna was switched from horizontal to vertical. The reference antenna showed about 15 dB difference between horizontal versus vertical polarization. The Parasitic Lindenblad antenna was tested next using the same procedure and showed no measurable difference between the horizontal and vertical polarization of the sense antenna indicating very good circularity.

6Summary

1Circularly Polarized Omnidirectional Antenna, by George H. Brown and O. M. Woodward Jr., RCA Review vol. 8, no. 2, June 1947, pp. 259-269.

2 Ibid. pg 267.