Hello, I am new to this group. A fellow ham recommended I look into this group to answer a question I have. I am also interested in amateur radio equipment design and building so this motivated me to join the group as well.
My question is related to the design of a diplex low band antenna installation. It is described in a Motorola technical publication 68P80100W86-A dated 9/27/85. This note describes how to connect two low band antennas (30 to 50 MHz) to a common coax. A coaxial cable cutting chart is provided in the note. The installation is such that a given length of coax is connected between say a 30 MHz antenna and a "T" connection and another different given length of coax is connected between say a 50 MHz antenna the "T" connection. The third connection of the "T" connection is connected to the radio with an arbitrary length of coax. The cutting chart refers to what they call standard RG-58A/U coax. For a 30MHz and 50MHz antenna installation the chart prescribes a 47 inch length of coax for feeding the 30 MHz antenna and a 100 inch length of coax for feeding the 50 MHz antenna. My ham friend says these are 1/4 wavelengths of transmission line for the opposite band (100 inch is approx 1/4 at 30 MHz and 47 inch is approx 1/4 at 50 MHz). Being an analytical guy I modeled the antenna system in both Ansoft Designer and an antenna modeling program. In Ansoft Designer the antennas were modeled as a series RLC load with 50 ohms at resonance and a Q of 15. Transmission lines were modeled with VF equal to that of RG-58A/U coax. In the antenna modeling program the antennas were modeled as 1/4 verticals with transmission lines having VF=0, antenna spacing of 1 meter, and perfect ground (assuming auto metal). The Motorola note suggests a minimum spacing of 3 feet between antennas. My conclusion was that the values in the Motorola cutting chart for 30 MHz and 50 MHz antenna diplexor are not optimum. Much better results are obtained with transmission line lengths corresponding to 1/2 wavelengths. With 1/2 wavelength transmission line the impedance is repeated. For 1/4 wavelength verticals the out of band impedance is high and with the in band impedance being low the total impedance is governed by the in band antenna. I would like to know what others might know about this 30 MHz and 50 MHz diplexor made of coax and what is the basis for the cutting values in the Motorola note. I tried to find information about this system on the Repeater Builder site but I could not. Thank You Steve Schultz WB8WGY
