Topband: Slightly OT - amplifier noise

[Dave Cuthbert]

Mike, thank and these are great data points you've provided.

Questions:
Was the MFJ-1026 PHASE control set near zero? That could account for the
low AUX gain.

*Mike's data*
FT1000MP MK-V

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

12 to 14 dB increase in the noise floor on 10 meters when the MFJ unit was
turned on

*MFJ-1026 MAIN measured noise and calculations*
Sherwood Engineering measured the FT-1000 MP MKV Field at -133 dBm in 500
Hz BW. This is -160 dBm in 1 Hz, or 2.2 nV/Hz^0.5.

The 12 to 14 dB increase in the noise floor with the MFJ-1026 activated
tells us its output noise power is ~13 dB above -133 dBm, or -120 dBm in
500 Hz BW. This is -147 dBm, or *10 nV/H^0.5.* Given the measured AUX path
gain of 0.5 dB this tells us the MFJ-1026 AUX input-referred noise is *~9
nV/Hz* on 10 meters. But wait, there's more.

In my March 16 email I said "The MFJ-1025 [calculated] output noise is* 9
nV/Hz^0.5."* But, given the MFJ-1026 measured AUX gain of 0.5 dB (10
meters) when I estimated 14 dB AUX gain (20 meters) is a huge discrepancy.
This points to the phase shifter circuit attenuating the signal. Might the
PHASE control knob be set to 0? This attenuates the AUX path.

*Notes*
The MFJ-1025 and MJF-1026 use the same circuit with the 1026 adding a whip
antenna amplifier.

*References*
https://www.mfjenterprises.com/support.php?productid=MFJ-1026

http://www.sherweng.com/table.html

http://www.sherweng.com/documents/TermsExplainedSherwoodTableofReceiverPerformance-RevF.pdf


    Dave KH6AQ

On Tue, Mar 17, 2020 at 4:07 AM Michael Tope <W4EF at dellroy.com> wrote:

> 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
> > _________________
> > Searchable Archives: http://www.contesting.com/_topband - Topband
> > Reflector
>
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