Friday, June 20, 2014

FM Radio Rectangle Super Gainer (Moxon Antenna)


It is not much popular antenna but much old design!! The original name of this antenna is Two Element Driven Arrays. In 1952 Les Moxon published this Genius Design in QST July Issue. It is a rectangular shaped two elements with a closed spacing of 0.18 lambda. The ends of the two element are folded in 90 degree face to face with a critical spacing of each other. This rectangle beam is popular among ham radio operators as Moxon Antenna. Few hams are using it for HF (SW) bands. It is a directional type antenna with a wide angle of 136 degree typical aperture and with a very good band width. (The Radiation pattern is like Kidney shape, and it is the same in reciprocity) . Due to the bend at the ends of each element and due to the critical spacing of each tips it is a capacity loaded, and it yields the wide bandwidth and low SWR levels. It is a low take of angle type of 14 degree or low typically, and it pick ups maximum stations from planes. Therefore I decided to make this antenna in FM Radio band to receive the spectrum of 88 MHz to 108 MHz.

One of my SWL Murali (School Teacher), who is a good listener of MW-SW-FM bands, asked to make an antenna which gives directivity, wide angle and very high gain for his own use. For this purpose I converted the basic design to 3 meter BC band radio use. I made this rectangle beam (Moxon) with 3/8 th aluminum tube. For the critical spacing of each element tips, I made hilum insulators as a prototype. The feed point is connected with a simple cable TV connector called F-Connector which is economical and easily available at the local market.

While testing, if it is pointing to eastern direction it will pick the signals from East and also it will pick the signal from South East and North East due to its wide angle aperture. More over the beauty of this antenna is the two elements will give 9 dBi + gain. Typically this antenna gives 7 to 14 dBi depending upto the accuracy of the construction and it can be a wide angle of 100 degrees to 136 degrees, the F/B ratio can be 30 dB to 40 dB. This antenna should be mounted at least 1 lambda of the operating frequency above the ground level. i.e., 3 meter (10 ft). For excellent performance, the height should be 25 to 30 ft. and the surrounding clearance should be maximum. Do not test this antenna near to any metallic objects and that will reduce its performance drastically.

For receiving FM BC Bands you can connect any good quality and low loss 75 ohms coaxial cable to the driven elements at the middle point of this antenna. No matching is required like balun, gamma, hair pin etc. and therefore no question of matching loss. Refer the following figures for getting specific ideas about the antenna and its construction. A well constructed rectangle beam antenna is equivalent to a four element yagi antenna.

In the next part we will reveal some antenna engineering about rectangle beam for 2 metre ham band operation.

FM bc band - mesurements for 100 Mhz
Fig. 1. FM bc band - mesurements for 100 Mhz. This measurements are for 3/8 Aluminium tube (9.5 mm OD)

FM bc band - mesurements for 100 Mhz
Fig. 2. Rectangle beam (Moxon) plot

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