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Details the construction of a Copper Cactus Dual-Band Super J-Pole Antenna, providing specific measurements for 1/2-inch copper tubing sections, including a 57-1/2-inch long section and a 19-inch short section, along with a 42-inch piece of 3/16-inch or 1/4-inch soft copper tubing for the matching stub. It covers soldering techniques for copper fittings, drilling an SO-239 panel mount coaxial fitting, and securing feed point connections with stainless steel adjustable band clamps. The resource specifies materials such as Schedule M 1/2-inch copper tubing, various copper fittings, a hardwood dowel or Fiberglas rod for insulation, and #14 stranded copper wire for the feed point. The guide simplifies the J-pole feed point by using an SO-239 fitting with an elongated mounting hole and band clamps, noting an optimal feed point distance of approximately 3 inches above the crossbar for proper impedance matching. It recommends a 4-turn coax choke, 5 inches in diameter, placed within 3 to 4 inches of the feed point for 2-meter operation to mitigate RF on the feedline. The project emphasizes weather sealing with silicon or butyl rubber compound and clear lacquer for durability and appearance.

VE1ZAC's analysis details the performance of **MFJ927** and **SGC239** autotuners with portable HF vertical antennas, specifically comparing 31 ft and 43 ft configurations. The resource originated from challenges encountered during a Maritime QSO Party roving operation, necessitating a lightweight and easily deployable antenna system. Target bands for the contest included 80, 40, 20, 15, and 10 meters, with a maximum power handling of 100 W CW. The author utilized a 30-foot carbon fiber push-up pole to support a vertical wire element, noting its 2 lb weight and reliability. EZNEC modeling was employed to predict performance, showing favorable results for a 30-foot vertical with elevated radials, particularly on 40 and 20 meters. Feedpoint impedance measurements, taken with an AIM4170C, are presented for various HF bands, both with and without a 41-foot RG6 stub designed to reduce reactance on 80 and 20 meters. The stub significantly improved matching on these bands, easing the tuner's workload. Operational tests revealed issues with the MFJ927's reliability during contest setup, leading to reliance on the K3's internal tuner. The SGC239, tested post-contest, performed flawlessly. A detailed side-by-side comparison covers mechanical aspects, connection options, power bias, impedance range, board quality, and documentation. Modifications to the MFJ927, including a new aluminum case, white paint for heat reduction, and upgraded impedance-measuring resistors, are also described.

This project documents the construction of a coaxial 50 MHz notch filter to eliminate inter-band interference between 50 and 70 MHz transceivers. Using RG-213 coax and based on quarter-wave stubs, the filter achieved a 44 dB attenuation at 50.060 MHz while maintaining low insertion loss on 70 MHz. A dual-stub design broadened the notch response and minimized attenuation on 70 MHz to 0.2 dB. Fine-tuned using an FA-NWT network tester and Elecraft XG3 signal source, the filter effectively resolved interference for seamless dual-band operation.

A 5/8-wavelength vertical antenna for 2-meter FM operation is detailed, focusing on eliminating loading coils by utilizing a series inductor to cancel capacitive reactance at the feed point, thereby presenting a 50-ohm impedance match. The design illustrates three basic configurations, including a method employing a short-circuited coaxial stub for inductance, as implemented by K4LPQ. An alternative design is presented where the center conductor of the stub is extended one-quarter wavelength, creating a signal-frequency short and allowing for an insulated wire stub to develop the required series inductance. The article provides electrical theory and mechanical considerations for building the antenna, emphasizing the adjustment of stub length for proper impedance matching. This technical documentation is intended for amateur radio operators interested in homebrewing VHF antennas, offering practical insights into impedance matching techniques for vertical radiators.

This article explores the role of velocity factor (VF) in calculating stub lengths for VHF/UHF Baluns. It clarifies misconceptions about VF's relevance, distinguishing between coaxial cable interior fields and external stub fields. Practical examples, such as the Pawsey Stub and Coaxial Cable Balun, are analyzed alongside experimental findings. The results reveal that traditional VF adjustments are unnecessary for stubs with external fields but critical for internal coaxial applications. Historical and theoretical insights provide a comprehensive perspective for antenna enthusiasts and designers.

The article discusses the construction of a UHF band-stop stub filter to protect an APRS receiver from potential damage during a balloon launch. The author, who communicates using a 441 MHz transmitter, needed to ensure that the RTL-SDR dongle receiving at 144 MHz wouldn't be damaged by the transmissions. The solution involved creating a quarter-wavelength open stub filter using coaxial cable, which attenuates the 441 MHz signal while allowing the 144 MHz signal to pass through. The filter's design is based on the principles of constructive and destructive interference, with careful measurement and trimming to achieve the desired frequency response. The final filter provided 34.8 dB of insertion loss at 441 MHz and minimal loss at 144 MHz, effectively protecting the receiver.

Constructing a **J-Pole antenna** from 450 Ohm ladder line for 2-meter (144 MHz) and 70-centimeter (440 MHz) operation involves specific calculations and assembly steps. The design, based on an earlier KD6GLF concept using 300 Ohm twinlead, features a ¾ wavelength radiator and a ¼ wavelength matching stub, functioning as an end-fed half-wave antenna without requiring ground radials. It offers a gain of 2.4 dB over isotropic. The resource provides the formulas for determining the lengths of the ¾ wave radiator and ¼ wave stub, incorporating a velocity factor of 91% for 450 Ohm ladder line. For 146 MHz, the radiator measures 55 3/16 inches and the stub 18 3/8 inches. Construction details include cutting a 57¾-inch piece of ladder line, stripping 4 inches from one end, and attaching a 24-inch section of RG58 or RG8X coax with a 3-5 turn RF choke. SWR adjustment is achieved by sliding a shorting bar or by incrementally trimming the elements at a 1:3 ratio (stub to radiator). The goal is a 1:1.1 SWR at 146 MHz, which typically yields 1:1.2 at 446 MHz. The article also discusses power handling, noting that while 10-15 watts is fine, 50 watts may increase SWR to 1:2.1, and advises keeping the antenna away from other objects to prevent coupling and SWR degradation.

This page provides a detailed guide on the J-pole antenna, an end-fed half-wave antenna matched to the feedline by a quarter-wave transmission line stub. It covers the characteristics, construction materials, feeding options, and mounting considerations for optimal performance. The information is useful for hams or amateur radio operators looking to build and set up a J-pole antenna for improved transmission and reception.