Transmitter combiners are not always "high loss, avoid like the plague" kind of thing. 10 channels at 250 Khz spacing on 800 does give you quite a hit but it is still better than the alternative.
The cavity on each transmitter must give at least 10 db of attenuation at the other frequencies in the combined system. So for close spaced frequencies the cavity insertion loss must be increased in order to obtain the proper skirt selectivity of the cavity so that 10 db can be met. With wider spacing there is much less insertion loss needed to obtain the required isolation. Also as frequency comes down, UHF or VHF the selectivity of the cavity gets better and less insertion loss is required for that same 10 db needed for isolation. So don't overlook transmitter combining as a high loss thing. It may not be. When spacing gets real close then hybrid combiners are needed and losses really get high there especially with several channels. These are quite common on 900 MHz and 220 MHz ACSB systems where channel spacing is very close. Sometimes combined channels can be split up with 2 antennas putting half the channels on each antenna and staggering their frequencies so you can use minimum loss. Also sometimes receive channels can be on those same antennas with 1 or 2 receiver multicouplers and appropriate filters. It all depends on what frequencies are involved. In regard to the two repeaters on one antenna, Jeff summed it up very well. 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Jamey Wright > Sent: Thursday, February 21, 2008 12:12 PM > To: [email protected] > Subject: RE: [Repeater-Builder] Two Repeaters, One Antenna > > I agree with Ken. On an 800 Mhz system I maintain, 100 watts into the > combiner yields about 16 watts at the antenna. This is a 10 channel > combiner with 250khz spacing and 320 ft of 1 5/8 Heliax. We have actually > installed a second antenna and feedline and have plans to split the > combiner > but just haven't gotten around to it yet. Kinda sucks but there's always > a > warm spot in the room in the winter. > > Jamey Wright > Systems Analyst/EDACS Administrator > Morgan County EMCD 911 > Decatur, AL > 256-552-0911 > > > -----Original Message----- > > From: [email protected] [mailto:Repeater- > > [EMAIL PROTECTED] On Behalf Of Ken Arck > > Sent: Thursday, February 21, 2008 10:51 AM > > To: [email protected] > > Subject: Re: [Repeater-Builder] Two Repeaters, One Antenna > > > > At 08:43 AM 2/21/2008, you wrote: > > > > >It can be done. I used to work with a UHF commercial system which > > >combined multiple repeaters to a common antenna. The cans were built > > >& tuned by TxRx; rather expensive in terms of money and insertion loss. > > > > > > <-----One comment for what it's worth. Transmitter combiners are > > generally quite lossy. And that loss can increase to prohibitive > > amounts the closer in frequency adjacent ports are. > > > > In other words, there is a point of diminishing returns IMHO > > > > Ken > > ------------------------------------------------------------------------ > -- > > ---- > > President and CTO - Arcom Communications > > Makers of repeater controllers and accessories. > > http://www.arcomcontrollers.com/ > > Authorized Dealers for Kenwood and Telewave and > > we offer complete repeater packages! > > AH6LE/R - IRLP Node 3000 > > http://www.irlp.net > > "We don't just make 'em. We use 'em!" > > > > > > > > > > > > > > Yahoo! Groups Links > > > > > > > > > > > > > > > > Yahoo! Groups Links > > >
