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Spring 2003 Issue |
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HSMM
A
Horizontally Polarized By Neil Sablatzky, K8IT |
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As mentioned in the Spring 2003 column, most HSMM communication will be horizontally polarized to minimize interference to ISM Part 15 users, who predominately use vertical polarization. This month we will discuss how to build a very low-cost Alford Slot antenna suitable for HSMM Part 97 stations. As shown in figure 1A, an Alford Slot antenna is based on a one-half-wavelength vertical slot. By machining a slot into a cylinder (figure 1B), the impedance of the circumference becomes lower than the vertical-slot impedance, which allows current to flow and a set of infinite stack loops to radiate. If the diameter of the cylinder is about one eighth the desired wavelength, the slotted length is adjusted to 0.75 of a wavelength, and the slot width is maintained at 0.02 wavelength, a useful 7–8 dBi horizontally polarized omni-directional pattern results. Figures 2A and 2B illustrate the difference in radiation patterns between the slotted sheet and the cylinder. Empirical testing of several prototypes reveals a stable design. The antenna consists of a length of slotted tubing as shown in figures 3A and 3B. The width and length of the slot, the wall thickness, and the diameter of tubing are all related and will vary slightly. For HSMM use, I decided to use 1/2-inch copper water pipe, which has a nominal OD of 0.625 inch. The slot was cut to be 2.5 mm wide and 88 mm long. The slot was centered in a length of 1/2-inch copper water pipe with an overall length of 188 mm. The slot in the tubing can be cut with a small jeweler’s file. You first will need to drill a hole at either end of the vertical slot. Use a 3/32-inch drill bit. The length of the copper tube above and below the slot is not critical, and the copper tube can be used as a support device. However, you must provide a copper shoprting disk for both the top and bottom end of the vertical slot. Slipping a copper disk in from each end and soldering from the open side around the disk works great. The feed-point impedance of this antenna is approximately 200 ohms. A convenient method of feeding it from 50-ohm coax is to build a 4:1 balun, which consists of a piece of 0.141-inch (UT141) semi-rigid coax with two slits cut along opposite sides of the outer shield. These slits are 29.3 mm long. Next short one of the outer shield sides to the inner conductor at the top of one slit and then solder both outer shields to the mid-point of the copper water pipe vertical slot on opposite sides of the copper water pipe vertical slot.
Now route the UT141 coax inside and down the
tube along the inside wall opposite the vertical slot. By drilling a small
hole in the bottom disk, you can route the coax out the bottom of the
length of copper pipe. Finally, terminate the UT141 coax with your
favorite UHF RF antenna connector. Click here to return to Summer 03 highlights Click here to subscribe to VHF.. _________________ © Copyright 2003, CQ Communications, Inc. All rights reserved. This material may not be reproduced or republished, including posting to a website, in part or in whole, by any means, without the express written permission of the publisher, CQ Communications, Inc. Hyperlinks to this page are permitted. |
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