Topband: Slightly OT - amplifier noise

[Michael Tope]

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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