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Topband: 46 radials now on vertical/FT1000MP S/N observation

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Subject: Topband: 46 radials now on vertical/FT1000MP S/N observation
From: (Tom Rauch)
Date: Sun, 2 Dec 2001 18:44:12 -0500
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 

73, Tom W8JI 

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