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In a message dated 12/2/01 6:46:19 PM Eastern Standard Time,
owner-secc@contesting.com writes:
<< Hi Ken,
> Also, for those of you who own FT1000MPs, has anyone made tests
> on the change in signal to noise ratio on 160 meters with the
> radio receive front end in the "tuned" and non "tuned" position
> menu settings? I made some tests this morning listening to local
The overall selectivity of a receiving system determines the noise
window, not the front end gain. The only exception is when
overloading and clipping in stages with noise, which would be
devastating to working anyone of any strength at any time!
Changing from a 500HZ filter to a 250 Hz filter of the same shape
factor will improve CW signal S/N by exactly 3dB when it is limited
by broad noise.
Adding a 3kHz filter in front of the receiver instead of a 300kHz filter
won't change a thing, if you are using 500Hz filters in the IF.
Removing gain, assuming nothing is saturating with noise and the
receiver is quieter than the noise floor of the antenna, will make no
change at all in S/N.
> line noise and a friend 30 miles away. With the additional gain
> of the front end device inserted the line noise is S7 and the
> station is S9.A S/N of 12DB. When the Front end amplifier is
> removed the line noise decreases to S4 and the signal of the
> other station is S8. A S/N of 24db. This is an effective S/N
> improvement of 12 db with the front end removed on 160.
The test is not really meaningful. The FT1000MP, like virtually all
other receivers, is nowhere near 6dB per S unit as you move down
on the scale. It also does not have good resolution. As a matter of
fact, most receivers are "designed" to be 5dB per S unit, but few of
them come close below mid-scale on the meters.
My FT1000D is about 5dB per S unit near S-9, and gradually
changes to about 1dB per S unit at S-3. My FT1000D is about a
10dB change from S-6 down to S-1!!!!
It is not useful to use S meters to measure dB changes, unless we
are positive we are in a linear and properly calibrated part of the
scale! No matter what we do, we should always use a step
attenuator and set the S reading to the same amount, and read the
change in the attenuation needed to produce the same exact S
meter reading. Or we could use the step attenuator to calibrate the
meter, understanding it will hold true ONLY for one set of
conditions.
73, Tom W8JI
W8JI@contesting.com
>>
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