Do not confuse FST4 with FT4, which has a very different purpose!
FST4 is designed primarily for making weak-signal 2-way QSOs on the LF
and MF bands. T/R periods from 15 s up to 1800 s are available. Longer
sequences provide better sensitivity only if Tx and Rx frequency
instability and channel Doppler spread are small enough that received
signals will remain phase coherent over periods spanning several
transmitted symbols.  Generally speaking, Rx and Tx frequency changes
during the transmission and channel Doppler spread should each be
small compared to the symbol keying rate. (See <<SLOW_TAB,Table 7>> in
the Protocol Specifications section.) For example, the keying rate
for FST4-1800 is 0.089 Baud, so successful operation requires frequency
stability to be better than (and Doppler Spread less than) around 0.05 Hz.

Operation with FST4 is similar to that with other _WSJT-X_ modes. Most
on-screen controls, auto-sequencing, and other features behave in
familiar ways.  However, operating conventions on the 2200 and 630 m
bands have made some additional user controls desirable.  Spin boxes
labeled *F Low* and *F High* set lower and upper frequency limits used
by the FST4 decoder, and these limits are marked by dark green
angle-bracket symbols *< >* on the Wide Graph frequency scale:

image::FST4_Decoding_Limits.png[align="center"]

{empty} +

image::FST4_center.png[align="center"]

It's best to keep the decoding range fairly small, since QRM and
transmissions in other modes or sequence lengths will slow down the
decoding process (and of course will be undecodable).  By checking 
*Single decode* on the *File | Settings | General* tab, you can
further limit the decoding range to the setting of *F Tol* on
either side of *Rx Freq*.

A noise blanker can be enabled by setting the *NB* percentage to a
non-zero value. (This control is located just above the input level
thermometer.) The *NB* setting determines how many of the
largest-amplitude samples will be blanked (zeroed) before the data is
submitted to the decoder. Most users find that settings between 0% (no
blanking) and 10% work best. If the noise blanker percentage is set to
-1%, then the decoder will try 0, 5, 10, 15, and 20 % in
succession. Similarly, a setting of -2% causes the decoder to loop
over blanking percentages 0, 2, 4, ... 20 %. To save time, the
multiple blanking percentages triggered by negative *NB* settings are
tried only for signal candidates located near (within +/- 20 Hz) of
the *Rx* frequency setting.

.Open a sample Wave File:

- Select *FST4* on the *Mode* menu. Set *T/R* to 60 s and *Decode | Deep*.
- Set *NB* (noise blanker) to 0%.
- Set up the Wide Graph display with settings appropriate for the FST4-60 mode. 
For example, try *Bins/Pixel* 2 and *N Avg* 4. Set the *Start* frequency and the width of 
the Wide Graph to include the frequency range that you want to decode. For this
example, make sure that *Start* is less than 1000 Hz and that the Wide Graph extends to above 1400 Hz.
- Set *F Low* 1000, *F High* 1400. These settings define the decoder's frequency search range.
- Open a sample Wave file using *File | Open* and select the file
...\save\samples\FST4+FST4W\210115_0058.wav. After _WSJT-X_ has processed the file you should see something similar to the following screen shot:

image::FST4-1.png[align="left"]
