Re: Motorola Diplex Antenna Manual >> Scott Zimmerman wrote: >> I would LOVE to know some of the theory behind >> this method. I was hoping to use this on a remote >> base antenna with 'Station' type antennas, but I >> don't think that will work since it clearly states >> that "Only standard base-loaded antennas are used"
Looks like a straight forward scheme to isolate two ports with odd wave-length coax stubs. The paper says only one of the stubs may be extended an extra half wave-length. I suspect the restriction to base loaded coils forces each of the antenna feed points to retain in forced physical hardware something close to their (hopefully) 50 ohm drive impedance at the F-center tuned frequency. That relatively low Z value would hopefully be 1/4 line transformed into a relatively high Z (impedance) value back at the T-Connector. Some type of ensured feed-point decoupling requirement might be involved/required. >> Comments? Suggestions? Theory? I think we went through that already... > "men...@..." <men...@...> wrote: > The Motorola document is based on the use of the Spectrun > base loaded antennas sold by Mother. The Spectrum antenna > is a series coil arrangement, not a shunt fed or tapped > coil; this is very important! I don't remember seeing any paperwork on the Spectrum Base Loaded Antennas... Out here on the west coast, "Mother" is a Cookie Company (with decent but hard Oatmeal cookies). I would expect the series coil antenna to be something similar to a 3/4 wave or electrically shortened antenna system and the shunt/tapped coils to be more 1/2 and 5/8 wave. Why would a series coil antenna be a requirement? > The chart works quite well for the Spectrum antennas and > will probably work for any other series fed LB coil. It > will not work for any antenna that is shunt fed as myself > and several others found out when trying to make two > non-Motorola antennas work on a fire engine. On Low Band with Utility Vehicles, I've found a number of reasons why certain low band antennas don't work well. Depends on each situation and I've recently had an install where only a shunt fed antenna would work. > The maker of the Untenna antennas told me once that they > could be combined in the same way but the method was different; > IIRC the antenna to T cables were to be quarterwaves but > were for the opposing frequency. Never tried it and that > was a long time ago and no notes to back up my memory. > Another document exists that details using a ball mount > full length whip and a Spectrum series fed LB base load > in the same shared configuration. > Milt > N3LTQ Quickly looking at a Spectrum Antenna data sheet, I might suspect they are trying to simulate/emulate/achieve a "no ground plane" halve-wave operation for proper feed-line decoupling. got to go, back later... s.
