Not to be picking nits, but the 5K preamp might be useful on 160M (for
example) if you're using small, multiturn loops for diversity antennas.
That's stretching things just a smidgen, maybe...
And Robert, if you want to tweak your system gain for the *BEST* dynamic
range per band, calculate how much attenuation you need for 160, 80 and
40M then build a simple bandswitched attenuator box to go in the 5000A's
RX1 loop. It would take a 2-pole, 3-position rotary switch, maybe 2 or 3
USD worth of resistors, (if you buy good ones) an aluminum hobby box and
a couple of BNC connectors. Perhaps $20-$30 with all new parts.
http://www.m0ukd.com/Calculators/Attenuator_Calculator/T_Attenuator_Resistor_Calculator.php
But before you rush out to build it, ask yourself how many times an
extra 5 or 10dB of dynamic range on the low bands would make the
difference between doing something exceptional and not doing it.
My .02,
Jim N7CXI
On 1/19/2013 5:52 PM, Robert Costa, KB6QXM wrote:
Brian,
I like the procedure, but I have a 5000, so I have no attenuator to adjust. Any
suggestion?
73,
Robert
KB6QXM
"Ham Radio Open Conversation"
Yahoo group owner/moderator
----- Reply message -----
From: "Brian Lloyd" <br...@lloyd.com>
To: "Robert Costa, KB6QXM" <kb6...@yahoo.com>
Cc: "Ray percival" <percival....@gmail.com>, <flexradio@flex-radio.biz>
Subject: [Flexradio] Additional preamp on 40m and 30M...
Date: Sat, Jan 19, 2013 4:39 pm
On Sat, Jan 19, 2013 at 11:56 AM, Robert Costa, KB6QXM <kb6...@yahoo.com> wrote:
But considering that the radio is Software Defined, does that mean this will
work on all of the Flex radios? 1.5k, 3k and 5k?
This is where you have to understand the HARDWARE in your software defined
radio. Each of the flex radios have different attenuator and preamp
capabilities. The 5000 is the simplest with just a single-gain preamp that can
be switched in and out. It has no attenuator. The 3000 has an attenuator
position and two preamp gain levels. If I recall properly, the 1500 has two
attenuator and two preamp gain levels. (I am doing this from memory and could
be wrong so don't string me up -- it doesn't matter for the sake of this
discussion.)
On the lower bands (lower meaning lower in frequency) the atmospheric noise
level is high enough that there is NO reason to ever turn on the preamp or at
least not to the high-gain position. So, PowerSDR does not give you all the
preamp options that are available in the radio on those lower bands because
turning on the preamp would compromise the performance of the radio. Yes, those
positions are there ON SOME OF THE RADIOS but they will only make the radio
perform worse so you don't want to use them. Hence PowerSDR does not offer you
the option. The dynamic range on the 5000 is much better and it has more
headroom to accommodate very strong signal so it does not have or need an
attenuator position.
So there is nothing magic or secret that Flex is hiding from you. They are
doing that to preserve the dynamic range of the receiver. Yes, you probably can
get the preamp into the high-gain position but that is only going to reduce the
dynamic range of the radio, thus making it work WORSE. You want to be making
more judicious use of the ATTENUATOR position in order to get best performance
from your 3000 and 1500.
Here is a technique for setting the attenuator/preamp to get the best dynamic
range for a given band. Select a dummy load instead of the antenna. Measure the
noise floor of the receiver without the preamp. Switch back to your antenna.
Now select the attenuator position that yields an increase in the noise floor
by about 5-10dB over the dummy load. That will let you hear the weakest signals
while maintaining the greatest dynamic range.
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