While it is basically true of how isolators are used, a "low end combiner" is not defined by how many isolator stages it employs. Most systems need around 65 db or so of isolation between transmitters. A common combination is a two stage isolator that gives around 60 db of isolation and a pass cavity following that. The pass cavity does dual purpose. It provides harmonic filtering for the isolators and also provides the "switch" between each leg in the combiner. Generally about 10 db is needed to provide the switch function. This gives enough isolation so that one leg does not load the other at the star junction or combine point.
A single isolator could also be used followed by 2 or 3 pass cavities (or notch cavities) to provide the additional isolation required. If the spacing is far enough apart less cavities may be needed for the required isolation between transmitters. When TX-RX systems first started in business they did not manufacture isolators. Most of their combiner designs had only cavities and no isolators. If close spacing was needed they would buy isolators from another source. But there were many combiners sold with no isolators at all, however that required additional cavities to get the needed isolation. Not using any isolators in a combiner has the risk of a close in (frequency) from another transmitter getting thru the pass band of the combiner filters and into the transmitter to mix and generate intermod. However with pass band cavities any IM products would also be attenuated by the pass cavities. This is one reason that a pass/notch, or straight notch, duplexer is not a good choice for combining two transmitters. A typical pass/notch duplexer, while being able to provide good isolation between the two transmitters, provides little protection from other transmitters that may be on the same site or near by. A pass/notch duplexer doesn't have much pass band protection outside of the two frequencies that it is tuned to. The pass band is a pseudo pass band and not nearly that of a regular pass band type cavity. Usually the pass band protection in the band between the two frequencies is quite good but on either side of the pair the skirts are very broad and not very deep. An additional pass cavity or an isolator on each transmitter is a good thing to have even though the duplexer provides the needed isolation between transmitters. A triple stage isolator is generally employed where a known mix is going to result in interference problems. It provides additional attenuation of the offending signal keeping it out of the transmitter where IM could be generated. It is not a pre-requisite of close spaced transmitters in a combiner. 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of DCFluX > Sent: Thursday, October 25, 2007 10:40 AM > To: [email protected] > Subject: Re: [Repeater-Builder] Re: duplexer isolation and receiver noise > bud > > Single isolators are meant for transmitter protection from reflected > power and antenna failure and low end transmitter combiners. > > Dual stage is more tailored for intermod reduction and wide bandwidth > low distortion (TV) applications. > > Triple stage is more tailored for a close spaced high power single > antenna transmitter combiner. > > Also take into consideration that the tuning caps in a circulator are > a weak point, and can fail at the worst possible moment. I've had one > circulator that has been rebuilt 10 times. > > Also the circulator changes tuning with tempreture. Not really a > problem with intermitant use, but in commercial service the circulator > is usually tuned 0.5 to 1 MHz low and drifts on to frequency during > the first 15 minutes of use. > > On 10/25/07, Steve S. Bosshard (NU5D) <[EMAIL PROTECTED]> wrote: > > Can't an isolator do both, provide a constant 50 Ohm load to the > > transmitter, and offer 30 (single junction) to 60 (dual junction) of > > isolation from signals travelling from the antenna to the transmitter > > for mixing. If isolators were just to provide a constant load, why are > > there dual junction isolators ? > > > > Steve NU5D > > > > > > > > Yahoo! Groups Links > > >
