Steve, The document in question is on the repeater-builder WEB site, not the files area for this group.
There were two documents produced by Motorola , each dealing with specific antenna combinations allowing the use of 2 antennas to cover areas of VHF lowband. One dealt with using two Motorola series fed base loaded antennas to cover two 400 KHz segments of low band seperated by at least 4 MHz. The other dealt with adding a series fed base loaded antenna to an installation that used a ball mount quarter-wave whip. There was also a ball mount quarter-wave whip with a matching section in the coax feed which allowed for extra wide bandwidth operation. Motorola had one and Antenna Specialists had a similar version. The standard base loaded antenna usually covers no more than a 1 MHz segment of lowband before the SWR goes above 2:1. I have often seen cases where the programming of radios well exceeded the abilities of the antenna system, however one has to remember that if the usage was localized, such as on a fire scene, the only thing that mattered was if you could be heard by everyone in the immediate area, often no more than a half a mile. I ran into a situation where county A's dispatch was a 46 MHz frequency and the adjoining country used 33 MHz channels. In this situation the usage for units from county A when in county B was only mutual aid on the scene operations. Thus the radios from county A were programmed for county B's operations channels but the antenna was left as a 46 MHz base loaded whip. The reflected power was almost equal to the foward power on county B's ops channels but from the perspective of the users things worked fine. Diplex packages were eventually installed as the vehicles were changed out, but it took quite a long time to get past the initial setup. The standard package "diplex kit" for two Motorola Spectrum base loaded antennas consists of a PL-259 T-connector, a UHF barrell, 4 PL-259s with reducers, and a length of coax. Two Spectrum base loads are required as well. The critical length of coax is from the NMO mount to the T-connector. The length of coax from the output of the T-connector to the radio was not critical. When the antennas are properly mounted and tuned for the individual segments of low band, the combination usually does not add any more than 1-2 watts in reflected power within the resonant segments. The chart in the document was developed specifically for the Spectrum series series fed LB antenna. THE MEASUREMENTS IN THE CHART WILL NOT WORK WITH SHUNT FED COILS! I have used many of these packages on fire/ems and police vehicles and they work very well. The Spectrum antennas can get rather picky about the ground plane. As long as the individual antennas are able to be tuned to resonance the combination works. And the standard operating ranges of the Spectrum series antenna are 29.7-36 MHz, 36-42 MHz, 42-50 MHz, and 66-88MHz, there are no other coil ranges . Milt N3LTQ ----- Original Message ----- From: "steven_schultz92" <steven.schult...@gmail.com> To: <Repeater-Builder@yahoogroups.com> Sent: Friday, September 11, 2009 1:42 PM 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 > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > > -------------------------------------------------------------------------------- No virus found in this incoming message. Checked by AVG - www.avg.com Version: 8.5.392 / Virus Database: 270.13.90/2361 - Release Date: 09/10/09 18:12:00
