Try 29.6 MHz x 15 = 444.000 MHz
Neil - WA6KLA DCFluX wrote: > > 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 > > > > 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/
