First: not all of the noise the transmitter is creating is on the intended frequency. A prime example is a certain (nameless) repeater controller has a 15kHz whine in the audio path due to bad capacitors in the negitave supply section.
This whine gets into the exciter along with the normal "inteligence" and the built in audio filters attenuate it say 30dB. but then you hit that into a preemphasis network which has a gain or 6dB per octave and before you know it this 15kHz is 20dB better than it started and your mixing with the carrier and creating interference every 15kHz on either side of your intended carrier. A phenomenon called "Picket Fencing" when viewed on a spectrum analyzer. Second: Without a narrow front end out of band signals can easily overload the IF section of a reciever and create undesired results. Example, tune a FM radio to 100.1 FM and drive within 300 feet of a transmitter on 94.7 with a ERP of 47,000 watts. Your going to get the 94.7 station even with the radio off! Helicals are great, small little filters. But they are lossy because of their small size so you need to follow with a Amplifier, This is already done in the front end assembly, MASTR-II VHF uses 5 helicals and then an amplifier and another 2 helicals, if I remember correctly. Just the other day I was doing an experiment using the helicals out of a 42-50 split lowband rig as an IF filter for a CADCO 375 television reciever. It was crazy enough and it actually worked, but I forgot about the 30dB of loss so my picture was all muddy and dim. but I was able to pull in channel 33 from Las Vegas over the two local channel 34s and my antenna isn't even pointed that way! It is more desired to run a duplexer that is narrow pass only instead of pass reject, because a BpBr duplexer is still passing a lot of the undesired signals into the reciever only at -6 to -12dB below normal, take a look at the curves for a Wacom BpBr set and you will see this. BTW, I would stay away from using 146.76 for a repeater at all costs if you want to put up a box. Sure it is the Number 1 pair in america, but I did some math the other day when presented with a radioshack HTX-202 that was desensing its self on that channel. 3.579545 X 41 Sure that don't mean much now, but stop and look at how many items use a 3.58 crystal. Happy Hours Testin' --Matt On Thu, 17 Mar 2005 05:17:43 -0000, Laryn Lohman <[EMAIL PROTECTED]> wrote: > > > --- In [email protected], Eric Lemmon <[EMAIL PROTECTED]> > wrote: > It also makes a big difference if your receiver has a helical > > resonator filter in the front end or not. > > Eric, I'm not disputing the statement above, just want to learn. > > Looking at the specs on a Mastr II base station receiver, I note that > the Maximum Frequency Separation for the 150.8-174mc version for full > specifications is 1.0mc. I read that also as +or- 500kc. The 3db > Degradation Separation is 1.8mc. So if I'm thinking right here the > response of this receiver at +or- 600kc is only down maybe 1db or so. > I don't see this receiver rejecting much energy at all at the TX > frequency. 99% of needed isolation is provided by the duplexer. So > why is a helical front end such a big advantage for a repeater > receiver? > > Laryn K8TVZ > > > Yahoo! Groups Links > > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
