Topband: Slightly OT - amplifier noise

[Michael Tope]

Hi Mike,

Yeah, when the off-air noise floor is a lot higher than the thermal 
noise floor from the JFET pre-amps, the imbalance I was alluding to 
wouldn't typically impact the overall noise floor since that is still 
limited by off-air noise. In Steve's particular case, aside from one 
S3-to-S4 source of QRN, it sounds like has an extremely quiet QTH. Under 
those circumstances (especially since we are talking about the higher HF 
bands), the thermal noise from the receiver can start to dominate over 
the off-air noise. That is when getting the off-air QRN levels balanced 
between the two channels (as measured going into the MFJ unit) can matter.

FWIW, I've had the same experience as you nulling out local QRN. In my 
case it is noise picked-up on my 160 meter transmit antenna that I null 
with the MFJ-1026 using a 20ft top-loaded vertical as a sense antenna 
(it's what's left of a 160M RX 4 square that got shredded by the wind 
after some varmints chewed through the guy ropes). I should be so lucky 
as to have a QTH so quiet that the hiss from those JFET preamps is the 
dominate source of noise (that's quiet!).

73, Mike W4EF...............

On 3/14/2020 11:57 AM, Mike Waters wrote:
> Steve,
> I have an MFJ-1025, and I have never experienced your issues. I was going
> to ask some of these questions myself.
>
> I have had very good results indeed with mine for eliminating a single
> source of RFI. Something at your end is not right.
>
> Mike,
> There have been cases where my 580' Beverage antennas have made a superb
> sense antenna. I usually use the W7IUV preamp with the output somewhat
> attenuated. Mostly, with the signal antenna being my 75m dipole (with
> severe RFI) and a amplified Beverage being the sense antenna.
>
> Every QTH and noise is different, of course. What works for someone else
> might not work for you.
>
> 73, Mike
> W0BTU
>
> On Sat, Mar 14, 2020, 12:46 PM Michael Tope <W4EF at dellroy.com> wrote:
>
>> 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:
>>> 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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