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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