Topband: Slightly OT - amplifier noise

[Michael Tope]

I just happen to have a spare MFJ-1026 at home. I did a real quick test 
using my FT1000MP MK-V while powering the MFJ-1026 from an Astron 
supply. Similar to your results, Steve, I got about a 12 to 14 dB 
increase in the noise floor on 10 meters when the MFJ unit was turned on 
and connected to the MK-V's main antenna input versus when the MFJ unit 
was powered off. This suggests you would have to resort to having 
low-noise pre-amps ahead of each of the MFJ-102x two signal paths and 
the right amount of attenuation after the MFJ-102x's combined output to 
work in your particular very low noise situation.

Since the main antenna path is normally bypassed for transmit, to put a 
pre-amp ahead of the main antenna path you would have to either move 
that MFJ-102x out of the transmit path by putting it in a receive-path 
breakout-loop that is common on many modern transceivers (e.g. between 
RX_OUT and RX_IN jacks), or you would have to have add a dedicated T/R 
bypass scheme for the external main path pre-amp.

FWIW, I also made some gain measurement on my MFJ-1026 (both main and 
sense path gain controls fully clockwise):

Main Antenna Path Gain:
10 Meters: 4dB
15 Meters: 3dB

Sense Antenna Path Gain (sense-path pre-amp on and internal jumpers set 
for maximum gain):
10 Meters: 0.5dB
15 Meters: 1.6dB

Difference between having Sense Path Pre-Amp On vs Off:
10 Meters: 8dB
15 Meters: 9dB

I used an Elecraft XG3 as the signal source and an HP141T to measure 
amplitude.

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




On 3/16/2020 9:47 AM, Steve London wrote:
> This has been a very interesting thread - Thanks for all the input.
>
> Perhaps I have set my expectations too high.
>
> A typical application is on 15 meters, late in the opening, working 
> JA's from here in SW New Mexico. Absent any local QRN, the band is 
> very quiet. Any local noise covers the bottom layer of 5 watt JA's 
> calling me.
>
> I did some more testing with the MFJ-1025. With no antennas connected, 
> the box raises the noise floor about 10 dB, irrespective of the Aux 
> Antenna Gain or the Main Antenna Gain. I haven't yet tried powering 
> from a battery, to make sure the power supply isn't the source. 
> Assuming the J310's are quiet, that leaves the 2N5109 emitter 
> follower, or the back-to-back protection diodes. Might also try 
> disconnecting the RF sense circuit.
>
> 73,
> Steve, N2IC
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