I do have a band pass filter in front of my receiver now. The site is pretty quiet as it's at my house and I don't have a lot of transmitters around.
A couple of the guys that help me out were saying I should put a 5 to 7 db preamp on it. The real MASTR II one sounds like it might be a good one. Thanks, Vern On Wed, 17 Oct 2007 18:49:02 -0600 Nate Duehr <[EMAIL PROTECTED]> wrote: > > On Oct 17, 2007, at 12:01 PM, [EMAIL PROTECTED] wrote: > >> How can I tell if my MASTR II has a preamp built in? If >> it doesn't what is the best preamp to use? > > The other answer for how to see if it's in there, is >correct. Kinda > hard to explain if you haven't seen a normal one >first... but there's > an extra little board in the "hole" and a tiny RCA >jumper from that > board to the receiver. > > As far as "the best" preamp to use, that's very >dependent on outside > factors -- is your site quiet? Noisy? How much >filtering are you > doing? How much gain are you looking for? > > Example - at one local site, we run without a pre-amp >because the > noise floor from hundreds of transmitters and multiple >broadcast > stations there, is so bad -- that amplification just >brings up more > of the "crud". We've talked about getting aggressive >with the > filtering and adding a light amount of pre-amp at that >site, but it's > not a priority for us right now. > > At another similarly noisy site, on VHF the hams use a >shared antenna > for receive, and the noise is so high there that the >"community" pre- > amp has been removed forever, because it caused nothing >but problems > -- and we're all doing filtering and our own >pre-amplification AFTER > our filters and duplexers, because the repeaters are >literally spread > across the band, and band-pass filtering and >pre-amplification of the > community "feed" is ineffective... it ends up being too >wide. One > repeater is at the bottom of 145 with a 144 input, and >the others are > at the top of 146, with mid 146 inputs. > > It's better to target a specific usable receiver >sensitivity number > that you'd like to see (after knowing what the site >noise-floor looks > like) than trying to work backward into the design from >the pre-amp, > but with that said... pre-amps that have been >successfully used by > many people here include: > > - The stock GE pre-amp (not much gain, but also not too >"unhappy" in > high noise environments) > - Advanced Receiver Research (my favorite, but can be a >little too > "hot" for the MASTR II receivers we use) > - Hamtronics (I don't like them, but others report good >luck and > behavior from them, and they're cheaper than most) > - Angle Linear (Chip's got some nice stuff there, and it >won't be > cheap, but he'll also custom build some pretty nice >setups if you > work with him and answer his questions about your setup >and site. I > keep meaning to try out one of his PHEMPT pre-amps on >one of our > systems to see if we find any reason to use them over >the GaAsFET > ARR's... but haven't had any time to do it yet.) > > Just popping a pre-amp in without measuring useable >sensitivity > first, sometimes works out... but it's far better to >measure and know > how much it helped. > > If you measure, you can then tell if you've over-done it >in the pre- > amp (common when using the ARR... it's pretty hot) and >perhaps you > may want to add a 3 or 6 dB pad behind it to keep from >overloading > the receiver if it's dragging in a lot of extra "stuff". > You can > measure the behavior of your specific receiver as you >lower the > signal (a set of different pads of different values or >one of those > accurate "DF'ing" switchable attenuators is nice during >the testing). > > Remember, if you don't bandpass filter before a pre-amp, >it's going > to "stuff" a lot of off-channel extra signals (and >noise) from other > nearby transmitters -- or even far away ones! -- into >your receiver. > That off-frequency stuff, if strong enough is just going >to make your > receiver overload and may actually perform WORSE than >without the pre- > amplification. > > Another common problem is when people add >pre-amplification and don't > have enough isolation in the duplexer... now your >transmitter is > being "heard" by the receiver where it couldn't hear it >previously... > creating desense or just general "deafness". > > It's all about trade-offs when you start going for the >"theoretical" > receiver limits. Sensitivity versus selectivity, the >same ol' game > whether you're talking about repeaters or any other >weak-signal > station's receiver. > > Maybe some of the pros here can share some of their >pre-amplifier > "secrets". > > -- > Nate Duehr, WY0X > >
