Thanks Mark and John for your response. I can appreciate using the Comet CF-360B duplexer as a quick solution, John. Thanks.
Mark, I can also see the matching bandwidth being better with shorter transmission line. But my concern is that 1/4 wavelength transmission line sections seem to be very inferior to the 1/2 wavelength transmission in achieving a good match for both antennas at their respective resonances. I will review my simulations and see if match bandwidth is adversely affected by the 1/2 wavelength transmission line. I will try to include cable properties. I wanted to know if I was way off base in my look at things and you guys have helped. Steve WB8WGY On Fri, Sep 11, 2009 at 4:16 PM, JOHN MACKEY <jmac...@usa.net> wrote: > > > I did this same thing but went about it by a different direction. > > I mounted the 52 mhz antenna and the 29 mhz antenna on the roof of > my Ford Explorer. I ran the coaxes down and connected them both to a > Comet 10meter/6meter diplexer and plugged the common end of the diplexer > into my radio. > > Look for the "CF-360B": > http://www.cometantenna.com/products.php?CatID=1&famID=6&childID=0 > > Usually I like to "roll my own" but I was in a hurry to make this > installation > work, so I just bought the comet diplexer. > > > ------ Original Message ------ > Received: Fri, 11 Sep 2009 10:45:43 AM PDT > From: "steven_schultz92" > <steven.schult...@gmail.com<steven.schultz.m%40gmail.com> > > > To: Repeater-Builder@yahoogroups.com <Repeater-Builder%40yahoogroups.com> > Subject: [Repeater-Builder] Diplex antenna installation using coaxial cable > for 10M and 6 M > > > 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 > > > > > > > > >
