Right, Greg. We're talking about VERY strong signals here.
For example: This is why dynamic range is so important to 6m contesters.
Signals on 6m during intense sporadic-E (Es) openings can be incredibly
strong, due both to the shorter ionospheric path for this kind of propagation
and to the high efficiency of Es clouds as radio wave reflectors. (Es clouds
tend to be both intensely ionized and very planar, like a reflecting mirror as
compared to a refracting atmospheric layer as in F2 propagation.) Signals of
40 dB to 50 dB over S9 are fairly common on 6m during a hot contest Es
opening.
But readable signals on 6m can also be very weak, as the atmospheric noise
floor is significantly lower on this band than on the HF bands, and copying a
signal producing only a half-microvolt at the receiver front end is not
unrealistic in the absence of man-made RFI or thunderstorms.
When you're talking about trying to copy this 0.5 uv S1 signal in the presence
of 40-over-9 signals 5 and 10 kHz away (again, a common real-world scenario on
6m during a contest), you're looking at a range of 40 dB + ( 8 S-units x 5
dB/S-unit ) = 80 dB.
Just as a reality factor, my Kenwood TS-2000 has been measured by ARRL Labs to
have a 5 kHz 3rd-order IMD dynamic range at 50.1 MHz of a scintillating 66 dB.
OUCH! This isn't even close. A 25-over-9 signal would still push the envelope
of the receiver's IMD dynamic range, and signals of that strength are all over
the band on 6m during the June contest. I can tell you from experience that
using a receiver with 66 dB @ 5 kHz DR3 is painful. You really can't hear weak
signals if strong signals are within 10 kHz or more of your operating
frequency.
Which is why I'm awaiting my K3 with bated breath. Too bad it won't be here in
time for the ARRL June VHF, but I'm still holding out some faint hope for the
July CQWW VHF. I'm in the first-production-run queue. ;-)
Bill / W5WVO
FISCHER,GREG wrote:
Don,
I don't think the concern here is with S9 signals, but
rather much stronger ones. Here is a clip from Erics
article talking about roofing filters......
If you only use the 2.7 kHz stock filter for CW or data
operation you will be significantly desensed once signals
within that filter's bandwidth exceed about S9+25.
Hope that helps...
73
Greg
AB7R
On Wed, 16 May 2007 14:23:54 -0700 (PDT)
Don Rasmussen <[EMAIL PROTECTED]> wrote:
The 756pro has no roofing filters, yet there is little
or no AGC pumping when listening to an S2 signal with
the DSP filter set to 500hz and a S9 signal is within
the 15khz first hardware filter. I'm hoping the K3 can
do at least this well without switching in a roofing
filter of less than 2.7khz.
The TenTec Argo V will pump somewhat in the same
scenario and pump very hard in a contest, at times
swamping the receiver. It's a great radio for a quiet
afternoon or contesting with modest antennas.
The TS480Sat will pump dramatically and you need to
leave the 500hz filter in line anytime there is an S9
signal within the 2.8khz ssb filter. The 500hz filter
is superb for selectivity but adds a very high level
of background noise when the bands are not quiet.
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