All specifications are subject to change without notice or obligation. All rights reserved. Part No. 5022-3001-A

FIGURE 9 The AEA IsoPole antenna

IsoPole^TM
The same twin-5/8 wavelength design shown in Figure 8 is used in the AEA IsoPole antenna, shown in Figure 9. Models of this antenna are produced for the 144-148 MHz, 220-225 MHz and 420-450 MHz amateur bands (the two meter IsoPole can be tuned anywhere between 138 and 174 MHz for commercial applications).

The design objectives for the IsoPoles were challenging:

1. to produce a gain-type omnidirectional vertical antenna with excellent decoupling,
2. capable of accepting full legal power,
3. impedance matched at the factory for complete coverage of the respective commercial & amateur bands,
4. with input coaxial connectors and matching section protected from the weather,
5. all at an attractively low price.

   To reach these objectives, innovation was necessary. For example, the conical shape of the decoupling sleeves was chosen to achieve structural rigidity, good decoupling via a wide mouth diameter, and a simple means for clamping the sleeve to the mast. These mechanical advantages, combined with certain electrical properties of noncylindrical sleeves have led AEA to obtain patent protection for this design.

   The IsoPole is designed to be mounted atop a metallic pipe or mast. The base of the upper element of the antenna contains an insulating section of Delrin?and metal sleeve to slip over the top of the mast. The coaxial connector is located within this sleeve, allowing the coaxial cable (usually RG 8/U) to be brought up inside the mast. The RF connection to the antenna is therefore completely out of the weather. The insulating section houses the L-C tuner, which is pre-adjusted at the factory to provide low SWR over the entire band. The L-C tuner compensates for the slight impedance mismatch introduced by the common (VHF) female SO-239 connector used. For best results, the user should employ a BNC or type N fitting at the transceiver end of the RG 8/U cable.

   While hardly necessary, the user of the antenna, if he so desires, can tune the IsoPole to a specific frequency using an SWR bridge. This is done by lengthening or shortening the upper element (which is constructed of two telescoping sections) for minimum SWR and making a corresponding adjustment of the distance between the feed point and the mouth of the upper decoupling sleeve.

   FIGURE 10 End-driven twin-5/8 antenna with no decoupling

   Other twin-5/8 antennas
   Some manufacturers offer twin-5/8 antennas which are driven at the lower extremity, as shown in Figure 10. A phasing section is used at a point 5/8ths wavelength down from the tip in order to obtain the desired current distribution. The coaxial connector for the feed line, the matching network, the base insulator, and the hardware for mounting the antenna atop a mast are all located at the lower extremity. Antennas of this design have absolutely no decoupling. The same current entering the base of the antenna must spill-over the mast (if metallic) and the outside of the coaxial line. Currents on the mast and coaxial line radiate and create fields which combine with those radiated from the antenna. The resulting radiation pattern is virtually unpredictable, and varies with every installation. Figure 11 shows a superimposition of two radiation patterns, one for an AEA IsoPole , and the other for a non-decoupled twin-5/8 antenna as supplied by its manufacturer. These patterns were measured on an antenna pattern range at a nearby university, using high standards of antenna engineering practice. The patterns were measured in such a way that the effect of the presence of the earth on the two antennas was identical. In each case, a length of 8-1/2 feet of coaxial cable was mounted so as to extend beyond the bottom extremity of the antenna in a straight line. Radiation patterns were recorded in dB on a linear chart recorder, and transferred carefully to polar plots showing relative radiated power vs. vertical angle, to simplify interpretation of the results. Note that the main lobe of the

   FIGURE 11 Radiation patterns of AEA IsoPole compared to that of non-decoupled twin-5/8 antenna

   IsoPole is centered on the horizon, while the main lobe of the nondecoupled antenna is tilted upward, with a corresponding loss of power in the direction of the horizon. This effect was produced by radiation from the currents spilling out on the 8-1/2 feet of coaxial cable. Longer cable runs could be expected to produce much more serious degradation of the radiation pattern, in the form of many sharp lobes caused by the "long-wire" radiation from the feed line phasing in and out with respect to the radiation from the antenna. While there would be a finite chance that radiation from the feed line might combine fortuitously with that from the antenna to create a net gain, this result is improbable. The loss of performance of the nondecoupled antenna will vary with each installation, and can easily be in the range of 3 to 6 dB below its well decoupled counterpart.

   FIGURE 12 Simple RF detector

   How to make an RF detector
   A simple series circuit consisting of a loop of wire, a pilot light and a tuning capacitor, makes an inexpensive and highly effective detector of antenna currents and the spill-over effect.

   Figure 12 shows a typical detector, using a square loop of wire 2 inches on a side. A No. 49, 2 volt, 60 mA Pilot light should be used in order to obtain enough sensitivity for good results with as little as 10 watts of transmitter power. The variable capacitor can be a compression-mica, air dielectric, ceramic, or tubular plastic variety. A dielectric handle of wood or plastic should be glued to the loop in order to prevent the user's hand from detuning the detector or the antenna.

   To tune the detector, bring the loop up close to the antenna while the transmitter is on, with the plane of the loop. Adjust the capacitor with an insulated tuning tool to obtain maximum brightness of the lamp. You may have to back the loop away from the wire to avoid burning out the lamp during tuning. With the suggested capacitor, the loop should be able to tune to any frequency between 140 and 230 MHz.