Exactly! This is very important to get a deep noise null. Mike W0BTU
On Sat, Mar 14, 2020, 1:55 PM Michael Tope <w...@dellroy.com> wrote: > Another pitfall that you have to be careful to avoid with the MFJ-1026 > is making sure the level of the QRN you are trying null that is coming > out of the sense antenna isn't much lower in amplitude than what is > coming out of the main TX antenna (I am talking about levels measured > directly from each antenna without the MFJ1026 in the line). For > example, if you are getting -100dBm of QRM from the main antenna and > only -110dBm from the sense antenna, then you could have a problem. If > it is the case that the QRN level from the sense antenna is lower, then > you have to use extra gain from the preamp in the sense antenna path or > you have to attenuate on the main antenna path to get the two QRN > voltages matched so they'll cancel effectively. Either way, you'll > degrade the thermal noise performance. In the former case the sense > channel will sum-in excess thermal noise, and in the latter case the > attenuation added in front of the J310 in the main channel will degrade > its effective noise figure. > > 73, Mike W4EF................. > > On 3/14/2020 10:46 AM, Michael Tope wrote: > > Steve, > > > > Couple of questions to rule out gain distribution issues: > > > > 1) On the sense antenna input do you have the pre-amp turned on or off? > > 2) Is your sense antenna intrinsically inefficient like a beverage or > > K9AY loop or is it something efficient like a tribander up reasonably > > high? > > 3) Is your sense antenna intrinsically efficient when you include the > > feed line loss (I assume the feedline loss on your main TX antenna is > > very low)? > > > > 73, Mike W4EF............ > > > > On 3/14/2020 9:40 AM, n2ica...@gmail.com wrote: > >> This discussion of Hi Z amplifiers has been quite interesting. > >> Excellent information from W1FV and K7TJR. > >> > >> I have an RX amp question, related but slightly off-topic. > >> > >> I have an MFJ-1025/1026 noise canceler. I like to use it on the > >> higher HF bands to cancel power line QRN. The noise is typically > >> S3-S4, but I want it down to S0 to hear the bottom layer of stations. > >> My sense antenna works fine, and the QRN is canceled. However, the > >> MFJ-1025 amplifier noise is quite significant, often negating the QRN > >> cancellation. The MFJ amplifiers are J310's. Any recommendations for > >> something quieter ? > >> > >> 73, > >> Steve, N2IC > >> > >> _________________ > >> Searchable Archives: http://www.contesting.com/_topband - Topband > >> Reflector > > > > _________________ > > Searchable Archives: http://www.contesting.com/_topband - Topband > > Reflector > > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector