This particular commentary seems somewhat inaccurate in saying signal to noise cannot be improved...as even GE offered a factory preamp for the Mastr II in VHF and UHF. The sensitivity spec improved....They DID NOT suggest using it in a station environment as THEIR preamp overloads very easily in Site conditions.... As it bolted into the front housing ( just inside the antenna jack) of the receiver, it sure was not offered to correct cable loss. :-) .. This would seem to suggest that a preamp can, and will help ... but as it is subjected to site issues, it needs to be appropriately designed. This is mentioned repeatedly in the hyperlinked documents and various other sites.
The noise figure of a stock Mastr II receiving system can definitely be improved upon with a preamp, as can other receivers designed in that time period.... if done correctly ! Many modern LMR type receivers may NOT improve much, especially if they already have sub .2uv sensitivity to start with. In that case a preamp would likely cause more harm than good unless tower mounted at the antenna (and in that case would be to overcome losses in the feedline as noted below by Dave). Doug KD8B At 07:39 AM 8/19/2009, you wrote: > > >Mike, > >Pre-amps are fine if you need to reduce feedline and connector loss, >for those lucky few that have antennas way up on commercial towers >and have significant loss. Otherwise, nada. Signal to noise is not >improved and can be effected negatively. Pre-amps can also be easily >overloaded by pager and other nearby signals. Lastly, the pre-amps >are physically sensitive devices when it comes to EMP...IMHO. If >you want to improved receive capture area or coverage think >satellite receivers. > >73, >dave >wa3gin > >----- Original Message ----- >From: <mailto:[email protected]>Mike Morris WA6ILQ >To: <mailto:[email protected]>[email protected] >Sent: Wednesday, August 19, 2009 4:43 AM >Subject: Re: [Repeater-Builder] ENHANCED RECEIVE > > > >At 07:47 PM 08/18/09, you wrote: > >I have heard of repeater owners using pre-amps on the receive side > >of the duplexer and adding 1 pass-reject cavity after the preamp and > >placing a pre-amp on the pass reject cavity to enhance more receive. > > > >Does this work or is it a myth? > > > >Artie > >k2aau >Depends on if you have enough headroom in the duplexer and enough >system isolation. > ><<http://www.repeater-builder.com/rbtip/preamps.html>http://www.repeater-builder.com/rbtip/preamps.html> > >While this is on 900MHz the theory and comments are just as applicable >on 2m, 220 and 440. ><<http://www.repeater-builder.com/tech-info/speaking-of-preamps.html>http://www.repeater-builder.com/tech-info/speaking-of-preamps.html> > > > >
