Wireless data links and other emerging forms of low-power communication have created a market for compact indoor and outdoor antennas in the 150 MHz to 1 GHz range. Concern over RF exposure from cellphones and hand=held transcievers has kindled an aggressive search for safer UHF radiators. The discpole antenna design may provide for both of these applications. The discpole is a capacitively loaded, off-center-feed (OCF) dipole proportioned such that a 50 Ω feedpoint occurrs at the junction of the element and one loading disc. (see Figure 2) Functionally, the discpole provides the following electrical and mechanical characteristics. 
Figure 1 
Figure 2 Discpole evolution The discpole is compared and contrasted with other designs in Figure 3. When conventional, center-fed, halfwave dipole (A) is reconfigured as an end-fed vertical, it typically takes the form of either a sleeve antenna (B) or a ground plane using bent radials in resonant counterpoise (C). In an alternate form, the ground plane may be implemented as a 50 Ω OCF dipole by lengthening the upper element to approximately 0.32 λ and shortening the radials (D). To modify the OCF dipole for capacitive loading, a disc may substitute for the radial wires to form a single-disc loaded vertical (E). The true discpole is formed by installing a second disc at the top of the radiator (F). Installing a second disc of equal proportions reduces disc diameter and shortens radiator length, and at the same time redistributes RF current to the center of the element. 
Figure 3 Efficiency Despite its small physical size, the discpole is an efficient RF radiator. At least two factors contribute to this efficiency. First, although monopole-like in appearence, the discpole functions as a balanced dipole radiator with an evenly distributed current loop forming at mid-element. (See Figure 4) This prevents pattern tilting and afforts maximum radiation efficiency on the horizon. Second, although the discpole has a lower mid-point impedance than a dipole, there are no lumped constants or mechanical breaks in series with the radiator to increase the percentage of Ωic loss. As a result, high efficiency is maintained. 
Figure 4 VSWR and bandwidth A properly constructed discpole normally exhibits a voltage standing wave ratio (VSWR) of 1:1 at its intended frequency of resonance. At 150 MHz, the discpole's 1.5:1 VSWR banswidth was measured at 7.5%, yielding a useful operating span of approximately 10 MHz. Normally, one would expect a 50% reduction in element length to constrict bandwidth severly. But, the narrowing effect of shortening the element is countered by broad-banding effects of decreasing system Q. Frequency scaling Although the discone's overall size determines resonant frequency, the ratio of disc size to radiator length sets the feedpoint impedance. Small discs paired with long element will yield a higher feedpoint impedance, and a large disc paired with a short element will yield a lower impedance. The formulas below were developed for scaling 50 Ω designs and should provide accurate sizing approximations as high as 500 MHz. Minor adjustments may be required for variations in tubing diameter and mechanical structure: Disc diameter in inches = (730+0.7•FMHZ) ÷ FMHZ Element height in inches = 2800 ÷ FMHZ Multiple resonances occur with the discpole, but these do not fall in a direct harmonic relationship with the antenna's fundamental frequency. Harmonic detuning is a likely result of disproportionate capacitive loading and inappropriete disc positioning at overtone frequencies. Feed system When the discpole is proportioned for a 50 Ω feed, no external matching is required. However, because the antenna is fed with unbalanced line at a point approximately 45 degrees off center, a coaxial choke must be installed at the feedpoint. This prevents unwanted detuning and reduces common-mode currents from propagating down the line and radiating. A minimum of 900 Ωs reactance normally is provided to ensure adequate isolation. The required inductance may be calculated using the formula: LµH = 900 ÷ (2•PI•FMHZ) For low-power operation as high as 250 MHz, a choke may be wound from 0.1" Teflon cable (RG-316) and secured to a 3/8" OD PVC form. From 250-500 MHz, an air-wound choke made from mini-hardline, such as RG-402 or semirigid Beldon 1671A will prove more satisfactory. At frequencies higer than 600 MHz, the effectiveness of air-wound chokes diminishes rapidly, and other more appropriete UHF decoupling techniques are recommended. To provide adequate element isolation when mounting, the firat 1/10 λ of antenna's support mast should consist of an RF insulating material such as hollow fibergalss stock of microwave tested PVC pipe. Conclusion To date, discpoles have been constructed sucessfully and tested at operating frequencies from 50 to 900 MHz. Over this frequency range, field tests suggest that the discpole offers a compact and effiecient alternative to traditional low-gain base-station designs. For example, range trials at 150 MHz have shown the the discpole may be substituted for a halfwave dipole, sleeve dipole, voltage-fed halfwave stick, J-pole, ground plane or radial decoupled OCF without measurable penalty. The discpole also promises to provide significant advantages in hand-held applications in the 800-950 MHz band and above, where symetrical current distribution on a elevated radiator is advantageous and where chassis-borne radiation may constitute a medical liability for manufacturers. References 1. Provisional U.S. patent protection for the discpole antenna has been applied for. 2. Rick Littlefield, "The two Meter Discpole Antenna," Communications Quarterly, Summer 1996. About the Author
This is the information about Rick Littlefied, K1BQT, currently on WWW.QRZ.COM which was last modified 2005-02-12.
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