FM Transmitting Antenna Page

The following design is being offered in the public domain for the benefit of those who wish to do their part in the defense of freedom, justice and the American way by exercising freedom of the airwaves.This is a very effective antenna system, for getting the most out of a small transmitter.

FCC WARNING - Many small transmitters are capable of exceeding Part 15 of the Rules for low-power broadcasting when connected to this antenna. Please be aware that it is possible to use this equipment in a manner that can place you in violation of federal laws. Before you construct this antenna, you should familiarize yourself with FCC rules with regard to radio transmission devices, and be aware of the boundary between legal operation and illegal, unauthorized operation. It is up to you to assume legal responsibility for your actions as you use this equipment.

The antenna system about to be described here is a superset of the classic "J-Pole" design. The basic J-Pole has a gain comparable to a dipole - about 2.5DBi. The author wanted something better, so he investigated several types of antennas. An omnidirectional pattern was desired. Also, minimal skyward radiation and minimal radiation back to the building on which the antenna is installed was a requirement. A vertical design addressed these concerns. The radiation pattern of this antenna approximates that of a doughnut lying on its side. Maximal coverage of populated areas is achieved in this way. The gain of the antenna described herein is about 8.8DBi. Switching to this antenna has much the same effect as tripling or quadrupling transmitter power.

The 4-bay J-Pole antenna, nearing completion, just after the wooden mast received a coat of black epoxy paint. Note circular phasing stubs, which are non-radiating 1/2-wave transmission lines. The entire radiating element set is formed of one continuous piece of 10 AWG aluminum wire, measured and bent at the precise 1/2-wave intervals. Light weight was a major engineering requirement for this project, so laminated wooden strips and aluminum wire were chosen as the main materials. This antenna's support is tapered thinner at the top end. The design was successful, since the bottom of the antenna may be easily lifted and the antenna held out horizontally in front of the person, without bending or snapping in half. Such a strength to weight ratio makes it possible for a single man to raise this 30 foot antenna, nailed to the end of a 12 foot 2x3 without assistance. The entire antenna, by itself, weighs just about 8 pounds. Earlier designs became too heavy after more than 2 bays were included.

Detail of the matching transformer assembly and phasing stub assembly. The matching system is of the classic "J" variety, functioning much like an autotransformer voltage step-up at R.F. frequencies. The phasing stub at right is a half-wave delay line -- essentially, a half-wave antenna, folded back on itself so it does not radiate. (See "Phasing Stub Operating Principles" later in this document.) The spacers for the phasing stubs are 1-1/4" cut pieces of computer card cage guides. Snap-in holes to snugly wrap around the 10 AWG aluminum wire were formed in the plastic by melting through with the tip of a hot soldering iron.

Detail of a choke balun, or current balun. Unbalanced antenna currents naturally flow back on the outside of coax feeder lines; this choke presents a high resistance to those currents, effectively blocking antenna currents from carrying back down the outside of the feeder and causing various potential problems with SWR, loss of power at the antenna, etc. Two of these baluns are placed 1/2 wave apart for very effective suppression. Note the wooden slider assembly made from furniture-grade Ash wood and a hardwood dowel. The photo at right shows the sandwiched PC boards that form the sliding contacts which connect the feeder line.

A detail shot of the top section radiator and how it's supported by a pair of lightweight wooden dowels.

Updated Photos

Two views of the finished installation on a third-story rooftop. Due to the severe battering hurricanes, the number of guy wires was increased to 16. The antenna height was increased by 13-1/2 feet. The active radiating element area is approximately 23 feet long (between the horizontal phasing "rings," which lends a little perspective to how massive this antenna is). The RG-11 feeder coax has low radiation in this installation and antenna performance is several orders of magnitude better than prior 2- and 3-bay J-Poles the author previously constructed from copper pipe. Note the two choke baluns just above the first guy cables. The top of this antenna is roughly 82 feet above ground on a 1253' AMSL elevation.

A carefully-constructed antenna can make all the difference between good coverage and enjoyable listening for your audience, and no listeners at all. The antenna project described here has been tested and has shown that a dramatic increase in signal strength can be obtained over more conventional antennas, just by increasing antenna gain. I leave these designs in the public domain for those hearty technical people who wish to build a better station than what they could get by buying a kit. Use them to further the goal of free radio broadcasting and the American way!