Topband: Slightly OT - amplifier noise

[Mike Waters]

Exactly! This is very important to get a deep noise null.

Mike W0BTU

On Sat, Mar 14, 2020, 1:55 PM Michael Tope <W4EF at 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, n2icarrl at 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
> >>
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